2024-04 SCA you like?

This month, I must thank one of my readers who simply asked, “Roy, can you do a newsletter about the Shared Communications Area (SCA) for me please?” Naturally, I replied (after getting two beers from him of course!) So now I wish to delve into the inner workings of the Coupling Facility (CF) and the SCA…

Warning: Scary Stuff Ahead!

In a non-datasharing world, a -DISPLAY GROUP shows this sort of output:                                       

                  CURRENT FUNCTION LEVEL(V13R1M504)          
                  PROTOCOL LEVEL(2)                          
                  GROUP ATTACH NAME(....)                    
DB2          SUB                     DB2    SYSTEM    IRLM    
-------- --- ---- --------  -------- ------ --------  ----   --------
........   0 DD10 -DD10     ACTIVE   131504 S0W1      IDD1   DD10IRLM
SPT01 INLINE LENGTH:        32138                                   
*** END DISPLAY OF GROUP(........)                                  

If that is what you see in your production system, dear reader, then this newsletter is, sadly, not for you!

Hopefully your output actually looks like my little test system:

DSN7100I  -SD10 DSN7GCMD                                             
                  CURRENT FUNCTION LEVEL(V13R1M504)                 
                  HIGHEST ACTIVATED FUNCTION LEVEL(V13R1M504)       
                  HIGHEST POSSIBLE FUNCTION LEVEL(V13R1M504)        
                  PROTOCOL LEVEL(2)                                 
                  GROUP ATTACH NAME(SD1 )                           
DB2          SUB                     DB2    SYSTEM    IRLM          
-------- --- ---- --------  -------- ------ --------  ----   --------
MEMSD10    1 SD10 -SD10     ACTIVE   131504 S0W1      JD10   SD10IRLM
MEMSD11    2 SD11 -SD11     ACTIVE   131504 S0W1      JD11   SD11IRLM
SCA   STRUCTURE SIZE:    16384 KB, STATUS= AC,   SCA IN USE:     3 %
LOCK1 STRUCTURE SIZE:    16384 KB                                   
NUMBER  LOCK ENTRIES:     4194304                                   
NUMBER  LIST ENTRIES:       16354, LIST ENTRIES  IN USE:          81
SPT01 INLINE LENGTH:        32138                                    
*** END DISPLAY OF GROUP(GSD10C11)                                  

The interesting stuff is from the “SCA   STRUCTURE SIZE:” line down to the “NUMBER  LIST ENTRIES:” line.

Off We Go!

To understand what an SCA is, we need to backtrack a second and first discuss what a CF is?

What is Inside?

The Coupling Facility is de facto *the* central part of data sharing, it contains three objects:

  1. A lock structure called LOCK1, where all the table locks using hashes live, and Record Lock Elements (RLE),
  2. A list structure that is actually the SCA, which contains a bunch of stuff I will go into later,
  3. The Group Buffer pools. Technically, you can run without these but then why are you data sharing if you are not sharing data?

Lock Structure Details

It is called LOCK1 and the system lock manager (SLM) uses the lock structure to control shared Db2 resources like tablespaces and pages and can enable concurrent access to these. It is split internally into two parts: The first part is the Lock Table Entry (LTE), and the second part is a list of update locks normally actually called the Record List Entry (RLE). The default is a 50:50 split of memory. The size of this structure must be big enough to avoid hash contention, which can be a major performance problem. Do not forget that the IRLM reserves 10% of the RLEs for “must complete” processes so you never actually get to use them all!

List Structure Details

This is really the SCA and it contains Member names, BSDS names, Data Base Exception Table (DBET) statuses and recovery information. Typically, at installation time, you pick a number for the INITSIZE (first allocated size of the SCA) from a list of 16 MB , 32 MB, 64 MB or 128 MB. Each of these INITSIZEs then has a SIZE which is typically twice the INITSIZE size as a maximum limit.


IBM write quite happily “Running out of SCA space can cause Db2 to fail” – I can change that to “does” not “can”!

Double Trouble?

The LOCK1 and the SCA do *not* have to be duplexed but it is very highly recommended to do so, otherwise, you have a single point of failure which defeats the whole point of going data sharing, really.

Death by DBET

The DBET data is, strangely enough, the thing that can easily kill ya!

How So?

Imagine you have the brilliant idea of using COPY YES indexes as you have tested a few and seen that RECOVER INDEX is quicker, better, faster than REBUILD or DROP/CREATE for the critical indexes at your shop.

What Happened Next?

So how do you enable COPY YES at the index level? Just a simple ALTER INDEX xxx.yyy COPY YES is all it takes. But *what*, dear friends, does this ALTER do under the covers? It sets the INDEX to ICOPY status – „Not too bad“, you say as the index is fully available, „just wait until the next COPY and that status is then cleared“ – But wait, that is a DBET status! It creates a DBET entry in your SCA… What if you alter 15,000 Indexes to all be COPY YES? Yep – Kiss goodbye to your Db2 sub-system!

Suicide Protection

Some software out there (RealTimeDBAExpert from SEGUS for example!) actually warns you about this and recommends you do this sort of thing in chunks. First the ALTER, then the COPY, one hundred blocks at a time and then the next batch etc.

and then if you do press PF1:

And even then, we have an emergency stop built-in that you can still override but then you must *know* the possible risk involved:

On the other hand, some software is like using SDSF DA when you put a P for “print” by your userid!

Now you know what *not* to do!


Use of the ALLOWAUTOALT(YES) has been discussed for years… on the one hand it automagically adds storage if you are running out, which is a good thing, *but* it also allows other competing systems to decrease storage which can then lead to you running out of storage and losing this Db2 sub-system… nasty, nasty!

Operator command time!

/f <yourirlm>,STATUS,STOR

Gives me:

PC: YES  LTEW:  2 LTE:     4M RLE:   16354  RLEUSE:      18        
BB PVT:  1266M  AB PVT (MEMLIMIT):   2160M                         
CSA USE: ACNT:     0K  AHWM:     0K  CUR:  2541K  HWM:  6122K      
        ABOVE 16M:    64   2541K     BELOW 16M:     0      0K      
        AB CUR:               0K     AB HWM:               0K      
PVT USE:   BB CUR:  5684K        AB CUR:     5M                    
           BB HWM:    18M        AB HWM:     5M                    
ACCNT    T-1     2      4M    T-2     1      1M    T-3     2      8K
PROC     WRK    14     70K    SRB     5      5K    OTH     4      4K
MISC     VAR    41   7549K    N-V    22    565K    FIX     1     24K

This maps pretty nicely to the – DISPLAY GROUP I just did again:

                  CURRENT FUNCTION LEVEL(V13R1M504)                 
                  HIGHEST ACTIVATED FUNCTION LEVEL(V13R1M504)       
                  HIGHEST POSSIBLE FUNCTION LEVEL(V13R1M504)        
                  PROTOCOL LEVEL(2)                                 
                  GROUP ATTACH NAME(SD1 )                           
DB2          SUB                     DB2    SYSTEM    IRLM          
-------- --- ---- --------  -------- ------ --------  ----   --------
MEMSD10    1 SD10 -SD10     ACTIVE   131504 S0W1      JD10   SD10IRLM
MEMSD11    2 SD11 -SD11     ACTIVE   131504 S0W1      JD11   SD11IRLM
SCA   STRUCTURE SIZE:    16384 KB, STATUS= AC,   SCA IN USE:     3 %
LOCK1 STRUCTURE SIZE:    16384 KB                                   
NUMBER  LOCK ENTRIES:     4194304                                   
NUMBER  LIST ENTRIES:       16354, LIST ENTRIES  IN USE:          18
SPT01 INLINE LENGTH:        32138                                   

Looking at this line in the Modify output:     

PC: YES  LTEW:  2 LTE:     4M RLE:   16354  RLEUSE:      18        

I only have two members so my LTEW (LTE Width) is two bytes, LTE is 4M which is my NUMBER LOCK ENTRIES: 4194304 , RLE is 16354 which is my NUMBER LIST ENTRIES: 16354 and RLEUSE is 18 which is my LIST ENTRIES IN USE: 18. A perfect match – if only they had agreed on a naming convention! ! !

You can also see that my SCA STATUS is AC for ACTIVE and the SCA IN USE is a measly 3%, so no worries for me today!

Finally, you can see that we have 50:50 split as the SCA and the LOCK1 are the same size: 16 MB.

One more line of interest:                  

BB PVT:  1266M  AB PVT (MEMLIMIT):   2160M       

Here you can see the calculated MEMLIMIT for the IRLM, which for my test system is very low, but you should check that the number is good for your site as the range is now much bigger!

Panic Time?

I would start to get seriously sweaty if the SCA IN USE ever got near 70%.


I would not use ALLOWAUTOALT(YES) and allocate 256MB or, if forced, I would go with INITSIZE 128 MB and SIZE 256 MB and then ALLOWAUTOALT(YES).

Lock it Down!

The Max Storage for locks is between 2,048 MB and 16,384 PB with a default of 2,160 MB that I have in the PVT (MEMLIMIT) output above.

Locks per table(space) (NUMLKTS) is 0 to 104,857,600 with a default of 2,000 in Db2 12 and 5,000 in Db2 13. If this number is exceeded then lock escalation takes place unless it is zero in which case there is no lock escalation. As IBM nicely put it “Do not set the value to 0, because it can cause the IRLM to experience storage shortages”.

Locks per user (NUMLKUS) is also from 0 to 104,857,600 with a default of 10,000 in Db2 12 and 20,000 in Db2 13. 0 means no limit. IBM do not recommend 0 or a large number here unless it is really required to run an application. Db2 assumes that each lock is approximately 540 bytes of memory. Here you can also drain your IRLM until it runs out of ACCOUNT T-1 storage space:


This is not a message you ever want to see on the master console! Just do the math on your locks and your memory size!

The Bachelor Problem – Fear of Commitment

You must get the developers to COMMIT or not use row-level locking everywhere.

SCA DBET Calculation

The lock size for each DBET entry is approximately 1,864 bytes from my tests in Db2 12 FL 510. How did I determine that, you ask?

What I did, was multiple ALTER INDEX aaa.bbb COPY YES SQLs until the „SCA in use“ percentage changed from 4% to 5%, and then I kept doing ALTERs and DISPLAYs until the percentage changed from 5% to 6%. It took me exactly 90 ALTERs and with an SCA size of 16,384KB, that gives me a size of around 1,864 bytes per DBET entry. Use this as a Rule Of Thumb for your DBET inducing ALTERs! It gives me a hard limit of about 6,000 Alters.

I hope you found this little discourse into the world of the Coupling Facility and SCA useful!


Roy Boxwell

2024-03 I am fine, up to a certain DEGREE

This month is all about going parallel! In the Db2 world, we have had the ability to run SQLs using parallel processing for decades. It started off a bit wobbly and most people didn’t use it, or even like it, but these days it is extremely useful for certain cases.

Sort Yourself Out

Sort is very important here. If you can do a parallel sort with only one extra parallel task it will halve your elapsed time… if you can add more tasks it takes even less elapsed. Naturally, you do not save on CPU here, in fact it will probably “cost” more, but you are trading CPU for elapsed time.

Ground Rules

IBM states in the documentation that parallel processing is “Only for partitioned objects” but then they mention that even non-partitioned objects can benefit, as the access to the non-clustering index and the data can be done in parallel… which is not helpful if you only have clustering index access, of course!

Bells and Whistles!

There are quite a few things to adjust and play with on the road to parallel processing!

No Way!

If you declare your cursor as WITH HOLD and with isolation level RR or RS then there is *no* CPU parallelism allowed at all but you can still get parallel sorts.


First up is CDSSRDEF (CURRENT DEGREE) where the IBM recommendation must be read:

CURRENT DEGREE field (CDSSRDEF subsystem parameter)

The CDSSRDEF subsystem parameter determines the default value that is to be used for the CURRENT DEGREE special register. The default value is used when a degree is not explicitly set in the SQL statement SET CURRENT DEGREE.

Acceptable values: 1, ANY

Default: 1

Update: option 30 on panel DSNTIPB


1 Specifies that when a query is dynamically prepared, the execution of that query will not use parallelism. If this value is specified, Db2 does not use any optimization hints for parallelism.

ANY Specifies that when a query is dynamically prepared, the execution of that query can involve parallelism.

Recommendation: In almost all situations, accept the default value of 1. You should use parallelism selectively where it provides value, rather than globally. Although parallelism can provide a substantial reduction in elapsed time for some queries with only a modest overhead in processing time, parallelism does not always provide the intended benefit. For some queries, and in many other situations, query parallelism does not provide an improvement, or it uses too many resources. If you are using nearly all of your CPU, I/O, or storage resources, parallelism is more likely to cause degradation of performance. Use parallelism only where it is most likely to provide benefits.

The ZPARM INDEX_IO_PARALLELISM should get an honorable mention here as it was there right up until Db2 11 and it is, in fact, still within the Db2 12 and 13 indexes at the end of the installation guide pdf, but it has thrown off its mortal coil…


Specifies whether I/O parallelism is enabled for index insertion.

Acceptable values: YES, NO

Default: YES


Security parameter: No

YES I/O parallelism is enabled for index processing. I/O parallelism allows concurrent insert operations on multiple indexes and can reduce I/O wait time when many indexes are defined in a table.

NO I/O parallelism is disabled for index processing.

Naturally, YES is what it should be set to!

To round out the dearly departed, there was also this one:

PARALLELISM EFFICIENCY field (PARA_EFF subsystem parameter)

Controls the efficiency that DB2 assumes for parallelism when DB2 chooses an access path. Valid values are integers 0 – 100. The integer represents a percentage efficiency.

It came in Db2 9 with PM16020 and started life with a default value of 100 before getting a default change to 50 in Db2 10. It got deprecated in Db2 12 and was removed in Db2 13.

Then PARAMDEG with a handy Top Tip:

MAX DEGREE field (PARAMDEG subsystem parameter)

The PARAMDEG subsystem parameter specifies the maximum degree of parallelism for a parallel group. When you specify a non-zero value for this parameter, you limit the degree of parallelism so that Db2 cannot create too many parallel tasks that use virtual storage.

Acceptable values: 0 – 254

Default: 0

Update: option 30 on panel DSNTIPB


0 Specifies no limit to the maximum degree of parallelism that Db2 chooses based on the cost estimate for the query and the system configuration, in particular the number of processors online. Db2 counts both general purpose and zIIP processors equally, and applies further adjustment to determine the degree to use.

1 – 254 Specifies the maximum degree of parallelism that Db2 uses. When optimization hints for parallelism are used, the value of the PARAMDEG subsystem parameter does not limit the degree of parallelism at bind time. However, the value of the PARAMDEG subsystem parameter is enforced at execution time. So, if the value of the PARAMDEG subsystem parameter is lower than the degree of parallelism that is specified at bind time, the degree of parallelism is reduced at execution time.

Tip: For systems with more than two zIIP processors configured, use the number of zIIP processors as the starting value, and then adjust as needed for your response time requirements.

Basically, set this value to be one or two times the number of online available CPUs but take into account the ZiiPs.

Then its new DPSI baby brother:


The PARAMDEG_DPSI subsystem parameter specifies the maximum degree of parallelism that you can specify for a parallel group in which a data partitioned secondary index (DPSI) is used to drive parallelism.

A DPSI is a non-partitioning index that is physically partitioned according to the partitioning scheme of the table. When you specify a value of greater than 0 for this parameter, you limit the degree of parallelism for DPSIs so that Db2 does not create too many parallel tasks that use virtual storage.

Acceptable values: 0-254, DISABLE

Default: 0

Update: option 30 on panel DSNTIPB


Data sharing scope: All members use the same setting

0 Specifies that Db2 uses the value that is specified for the PARAMDEG subsystem parameter, instead of PARAMDEG_DPSI, to control the degree of parallelism when DPSI is used to drive parallelism. This is the default value for the field.

1 Specifies that Db2 creates multiple child tasks but works on one task at a time when DPSI is used to drive parallelism.

2-254 Specifies that Db2 creates multiple child tasks and works concurrently on the tasks that are specified. The number of specified tasks may be larger or smaller than the number of tasks as specified in PARAMDEG. When PARAMDEG is set to 1, the rest of the query does not have any parallelism.

DISABLE Specifies that Db2 does not use DPSI to drive parallelism. Parallelism might still occur for the query if PARAMDEG is greater than 1.

This is for fine-tuning the DPSI use case. Remember, you can have 4096 partitions and so it could well be that a query goes crazy if it sees the ability to go massively parallel. Here you can limit, or even inhibit, that from happening. Only use if you have been bitten by a rogue SQL!

Then we get to Utility parallel processing which is not for all Utilities – but REORG TABLESPACE and COPY are there!

MAX UTILS PARALLELISM field (PARAMDEG_UTIL subsystem parameter)

The PARAMDEG_UTIL subsystem parameter specifies the maximum number of parallel subtasks for some utilities.

PARAMDEG_UTIL affects the following utilities:








Acceptable values: 0 – 32767

Default: 99

Update: option 34 on panel DSNTIPB


0 No additional constraint is placed on the maximum degree of parallelism in a utility.

1 – 32767 Specifies the maximum number of parallel subtasks for all affected utilities.

Interesting default huh? In Db2 11 it was actually zero!

LOAD must get a special mention here, as back in Db2 11 it got a new keyword PARALLEL
( nnn ) which enabled parallel loading from a *single* input file (Initially not for PBGs but then that was allowed in Db2 12). This little chestnut has often been forgotten.

Further into the guts of REORG is this little one:


The REORG_LIST_PROCESSING subsystem parameter specifies the default setting for the PARALLEL option of the Db2 REORG TABLESPACE utility.

Acceptable values: PARALLEL, SERIAL


Update: option 37 on panel DSNTIPB


PARALLEL The default value PARALLEL specifies that the REORG TABLESPACE utility is to use a default PARALLEL YES option when the PARALLEL keyword is not specified in the utility control statement. The PARALLEL YES option specifies that the REORG TABLESPACE utility is to process all partitions that are specified in the input LISTDEF statement in a single execution of the utility.

SERIAL Specifies that the REORG TABLESPACE utility is to use a default PARALLEL NO option when the PARALLEL keyword is not specified in the utility control statement. The PARALLEL NO option specifies that each partition that is specified in the input LISTDEF statement is to be processed in a separate execution of the utility.

I would also happily stick with the default unless you have experienced serious problems with VTS or some such.

Don’t Forget the Bufferpool, Stupid!

BUFFERPOOLs play a major role. The setting of VPSEQT (Virtual bufferpool sequential steal threshold) and VPPSEQT (Virtual bufferpool parallel sequential threshold) might well both have to be raised. Remember that the VPPSEQT is a percentage of the VPSEQT available pages. The bufferpool size itself might well have to be raised (VPSIZE) if you do not see an improvement in the degree of parallelism.

All Ready?

So, you have set, checked, reviewed, changed the ZPARMS and are ready to go?

Where’s the ON Switch?

For Static SQL just bind or rebind with DEGREE(ANY), for dynamic SQL issue a


CDSSRDEF Zparm has the default for this parameter.

If you bind with isolation level CS, then also try and make sure you use CURRENTDATA(NO) as well. This helps performance anyway and also aids Db2 in working with ambiguous cursors. Explicit read-only is always better!

For sorts, where the big elapsed time gains come from, make sure you have sufficiently sized work files allocated! Here, even the WITH HOLD and isolation RR or RS can benefit.


Always at least one problem isn’t there? If you do DEGREE(ANY) you can expect the EDMPOOL usage to go up between 50% and 70% due to run-time structures. Check your SYSPACKAGE AVGSIZE before and after the BIND/REBIND if you are worried. Always monitor this pool and make sure it is correctly sized for your workload!

Naturally, the CPU might well go up but you should see a good drop in elapsed times and, as far as sort is concerned, you might actually manage a successful parallel sort in a normally constrained sub-system!

Where’s the OFF Switch?

For Static SQL just bind or rebind with DEGREE(1), for dynamic SQL issue a


This is also the default value but what if someone changed your default?

If you shrink the VPPSEQT to 0 that will disable all parallel access for objects in that bufferpool.

Insert rows in the resource limit tables – Not recommended as this is a lot of work!

Are You Running in Parallel then? EXPLAIN is Your Friend!

There are a few “access patterns” that allow CP parallelism and they are all documented in the Managing Performance book – “Checklist of query restrictions for query CP parallelism” table.

How do You Check?

The columns of interest in the PLAN_TABLE are the ACCESS_DEGREE and JOIN_DEGREE. If either of these two is not NULL then you are using, or hoping to use, parallel processing! The moment you have more than one table then four other columns become interesting: ACCESS_PGROUP_ID, JOIN_PGROUP_ID, SORTN_PGROUP_ID and SORTC_PGROUP_ID. PGROUP is short for PARALLEL GROUP and for a given PLANNO step they all have the same number. Finally, the PARALLELISM_MODE reflects which type you are using which, these days, can *only* be ‘C’ for Query CP parallelism (Came in DB2 V4). It used to also have the values ‘I’ for parallel I/O operations (Came in DB2 V3)  and ‘X’ for Sysplex query parallelism (Came in DB2 V5) but they were deprecated in Db2 9 and are now both dead and buried!

Which SQLs are Best for Parallel Processing?

SQLs that are I/O intensive and scan lots of pages while returning just a few rows, SQLs that have lots of aggregate functions, and naturally SQLs that require Sort – all of these are good candidates.

Trial it DUMMY!

Best thing to do is a trial rebind to a dummy collection of all your SQL in a sandbox system with production statistics copied over and with parallel processing enabled (Do not forget the bufferpools!). This will then quickly reveal which queries could indeed benefit from going parallel and enable you to activate it only at the package/SQL level that you require in production.

Apply and Test

Once you have your candidate list, and I hope it is not that long, you can simply enable it all in production and do a live run through and review. First with EXPLAIN and then really live.

Remember that it is not good for *all* SQLs but it can really help when it hits the spot!

My Favorite Table Ever!

Db2 11 introduced an automatic five level control system for parallel queries:

Level 1 OK: Query runs with planned parallel degree

Level 2 Mild warning: Reduce parallel degree by ¼

Level 3 Moderate warning: Reduce parallel degree by ½ or to degree 2

Level 4 Severe warning: Reduce to sequential run

Level 5 Melt down: Reduce to sequential run

Level 5 is sub-optimal!

The original table is here: (Do a search for „Melt“)


How to Tame the Beast?

For static SQL, just bind/rebind with DEGREE(1) to switch off or DEGREE(ANY) to switch on.

For dynamic SQL, if you cannot add the SET CURRENT DEGREE = ‘ANY’ and RLF tables do not work for you then the only way is to assign the tables in question to their own bufferpools and set the VPPSEQT to a value, or leave at default 50, and for *all* other bufferpools set the VPPSEQT to 0 which switches off parallel processing.

Another way of handling dynamic, but a bit over the top for my taste, is a new data-sharing member where the ZPARM CDSSRDEF is set to ‘ANY’. Then any dynamic work that should be allowed to go parallel is simply routed to just this member.

Whaddya all think?

Going to start testing out parallel processing anytime soon?

I’d love to hear from you!


Roy Boxwell


One of my readers mentioned that the primary reason they use parallelism is to increase zIIP offload which offers not just elapsed reduction but also saves the cost of the cpu as well.

Naturally, if you get this, then you are really laughing all the way to the bank!

2024-01 Happy New SQLCODE!

Aren’t I supposed to be wishing Happy New Year there? Not this month! One of my readers asked me a question about SQLCODEs and it opened up a quite fascinating can of worms!

What is the SQLCODE?

Anyone reading this blog should already know exactly what the SQLCODE is, however, for those of you out there using Google in 2045, the definition is, at least in COBOL:

   05 SQLCAID     PIC X(8).
   05 SQLCABC     PIC S9(9) COMP-5.
   05 SQLCODE     PIC S9(9) COMP-5.
   05 SQLERRM.
      49 SQLERRML PIC S9(4) COMP-5.
      49 SQLERRMC PIC X(70).
   05 SQLERRP     PIC X(8).
                  PIC S9(9) COMP-5.
   05 SQLWARN.
      10 SQLWARN0 PIC X.
      10 SQLWARN1 PIC X.
      10 SQLWARN2 PIC X.
      10 SQLWARN3 PIC X.
      10 SQLWARN4 PIC X.
      10 SQLWARN5 PIC X.
      10 SQLWARN6 PIC X.
      10 SQLWARN7 PIC X.
   05 SQLEXT.
      10 SQLWARN8 PIC X.
      10 SQLWARN9 PIC X.
      10 SQLWARNA PIC X.
      10 SQLSTATE PIC X(5).

The third field within the SQLCA is the SQLCODE, which is a four-byte signed integer and it is filled after every SQL call that a program/transaction makes.

All Clear So Far?

So far so good! In the documentation from IBM is this paragraph about how to handle and process SQLCODEs:


Db2 returns the following codes in SQLCODE:

• If SQLCODE = 0, execution was successful.

• If SQLCODE > 0, execution was successful with a warning.

• If SQLCODE < 0, execution was not successful.

SQLCODE 100 indicates that no data was found.

The meaning of SQLCODEs, other than 0 and 100, varies with the particular product implementing SQL.

Db2 Application Programming and SQL Guide

So, every programmer I have ever talked to checks if the SQLCODE is 0 – Green! Everything is fine, if the SQLCODE is negative – Bad message and ROLLBACK, if the SQLCODE is +100 – End of cursor or not found by direct select/update/delete – normally 100% Ok, everything else issues a warning and is naturally very dependent on the application and business logic!

What’s Wrong With This Picture?

Well, sometimes zero is not really zero… I kid you not, dear readers! The sharp-eyed amongst you, will have noticed the last bytes of the SQLCA contain the SQLSTATE as five characters. Going back to the documentation:

An advantage to using the SQLCODE field is that it can provide more specific information than the SQLSTATE. Many of the SQLCODEs have associated tokens in the SQLCA that indicate, for example, which object incurred an SQL error. However, an SQL standard application uses only SQLSTATE.

So, it still seems fine, but then it turns out that SQLCODE 0 can have non-all zero SQLSTATEs!

New in the Documentation

At least for a few people this is a bit of a shock. From the SQLCODE 000 documentation:


  • 00000 for unqualified successful execution.
  • 01003, 01004, 01503, 01504, 01505, 01506, 01507, 01517, or 01524 for successful execution with warning.

Say What?

Yep, this is simply stating that you have got an SQLCODE 0 but up to nine different SQLSTATEs are possible… This is not good! Most error handling is pretty bad, but now having to theoretically add SQLSTATE into the mix makes it even worse!

What are the Bad Guys Then?

01003 Null values were eliminated from the argument of an aggregate function.

01004 The value of a string was truncated when assigned to another string data type with a shorter length.

01503 The number of result columns is larger than the number of variables provided.

01504 The UPDATE or DELETE statement does not include a WHERE clause.

01505 The statement was not executed because it is unacceptable in this environment.

01506 An adjustment was made to a DATE or TIMESTAMP value to correct an invalid date resulting from an arithmetic operation.

01507 One or more non-zero digits were eliminated from the fractional part of a number used as the operand of a multiply or divide operation.

01517 A character that could not be converted was replaced with a substitute character.

01524 The result of an aggregate function does not include the null values that were caused by evaluating the arithmetic expression implied by the column of the view.

Not Good!

From this list the 01004, 01503, 01506 and especially 01517 just jump right out and scream at you! Here in Europe, we have a right to have our names or addresses correctly written and, in Germany with all the umlauts, it can get difficult if you then have a 01517 but SQLCODE 0 result!

I hope you don’t find this newsletter too unsettling as, after all, Db2 and SQL normally works fine, but I do think that these SQLSTATEs should really have warranted a positive SQLCODE when they were first created…

What do you all think?


Roy Boxwell


One of my readers wonders how practicle these „errors“ are. A good point, and so here is a nice and easy recreate for the 01003 problem:

CREATE TABLE ROY1 (KEY1   CHAR(8) NOT NULL,                 
                   VALUE1 INTEGER ,                         
                   VALUE2 INTEGER )                         
INSERT INTO ROY1 (KEY1)               VALUES ('A') ;        
INSERT INTO ROY1 (KEY1)               VALUES ('AA') ;       
INSERT INTO ROY1 (KEY1,VALUE1)        VALUES ('B', 1) ;     
INSERT INTO ROY1 (KEY1,VALUE1)        VALUES ('C', 2) ;     
SELECT * FROM ROY1                                          
CREATE VIEW ROYVIEW1 AS                                     
 FROM ROY1)                                                 
SELECT * FROM ROYVIEW1                                      

This set of SQL ends up with these outputs:

KEY1           VALUE1       VALUE2    
A         -----------  -----------    
AA        -----------  -----------    
B                   1  -----------    
BB                  1            1    
C                   2  -----------    
KEY1           VALUE1       VALUE2    
C                   2            2    

    AVGVAL1      AVGVAL2                                                
          1            1                                                
DSNT400I SQLCODE = 000,  SUCCESSFUL EXECUTION                           
DSNT418I SQLSTATE   = 01003 SQLSTATE RETURN CODE                        
DSNT416I SQLERRD    = 0 0  0  -1  0  0 SQL DIAGNOSTIC INFORMATION       
DSNT416I SQLERRD    = X'00000000'  X'00000000'  X'00000000'  X'FFFFFFFF'
         X'00000000'  X'00000000' SQL DIAGNOSTIC INFORMATION
DSNT417I SQLWARN0-5 = W,,W,,, SQL WARNINGS                              
DSNT417I SQLWARN6-A = ,,,,   SQL WARNINGS                               
DSNE610I NUMBER OF ROWS DISPLAYED IS 1                                  

This is not so good if you ask me…

2023-09 Toronto visit

Ahhh! What a great time I just had in sunny ol‘ Toronto. I was extremely happy to be back at the Central Canada Db2 Users Group (CCDUG) meeting for 2023 held at the BMO Institute For Learning. It has always been a fantastic venue for presenters and attendees, and this year we had the added bonus of a „Coyote“ warning… fun fun fun!

The CCDUG is a two day, three track mini-IDUG if you like, and the organizers added onto the second day an AppDev track as well. So you had Db2 for z/OS, Db2 for LUW, IMS and AppDev – A very nice mix of things indeed!

Just the Facts, Ma’am!

I am mainly a z/OS style of guy (old school!) and so went for the z/OS track of course! It all started, as normal, with a keynote from Michael Kwok that was all about „accidental discoveries“ and how these often have had a tremendous impact on the world. Examples included Penicillin and Microwave cookers. It also included Db2!

He highlighted the environmental factors required for innovation to succeed:

01 Have an open mind

02 Use Strategy as a filter

03 Commit

Open Minded?

For the first point, Penicillin, having an open mind was important when Alexander Fleming, in September 1928, noticed the effect that a mold had on his old petri dishes. It was having an open mind that enabled him to really see the very first antibiotic in action. Sadly, it took another 14 years before it was actually used in a pure form and yet another year before mass production started in the second world war.

Strategy is Everything!

Using Strategy as a filter was highlighted by looking at how Bill Gates took advantage of the CP/M creators Digital Research people’s failure to meet and agree a deal with IBM to discuss a new OS for the IBM 5150 PC. IBM then contacted Microsoft, who agreed to write PC-DOS which evolved into MS-DOS which basically started the modern PC world we all now know today.

Commit is Not Just a Db2 Verb!

Commit was all about the invention of Microwave Cookers! Percy Spencer working at Raytheon in 1945 was working with Radar sets using high energy magnetrons (A British invention, again in the second world war) and he noticed that a chocolate bar in his pocket melted. It took an awful lot of commitment to eventually get the product to market and it first appeared in 1947 as the „Radarange“ at $5000, 3kw and was water-cooled!

Oh oh! IBM at Work…

Into these three ideas comes IBM! Back in the 60’s and 70’s, data centers started storing more and more data and Tedd Codd wrote down the rules of the Relational Data Model in his seminal paper „A Relational Model of Data for Large Shared Data Banks“ in June 1970. Now back in those days, IBM was not exactly well known as being Open Minded, Strategic or Committed!

Then, a few extraordinary individuals pulled it all together to actually get what was called System R, the granddaddy of all Relational Databases, into existence, starting in 1974 and got its first customer in 1977. It still took another six years to actually arrive at DB2 in 1983, and ten years later, we got DB2 for LUW as well. The actual GA date of DB2 1.1 was the 2nd of April 1985.

So, basically, Db2 was an accidental discovery!

Then it Begins!

The sessions at CCDUG then all kicked off, so here is my list of the ones I either attended or read up afterwards (naturally, I cannot attend every session!) All are linked directly to the CCDUG website and I thank them for allowing public access!

Database Administration Enhancements of Db2 13 for z/OS from Robert Catterall. An excellent run through everything you need to know about Db2 13 including more PROFILE support. This is probably one of the most under-used Db2 features out there.

Db2 for z/OS 101: Buffer Pools and Group Buffer Pools from Tori Felt and Keziah Knopp. Was a highly entertaining 101 level intro to the complex world of Buffer Pools, Managers and disk usage within Db2 – Highly recommended as these little details easily get lost nowadays!

It’s AI Jim, but not as we know it! from me. Obviously, a fantastic presentation all about AI – truth or hype? 😄

Analysis of DEADLOCK IFCID 172 in SMF with no special tool and low cost from Xiaodong Ma. This was all about reading the bits and bytes from the IFCID to find *all* the blockers and waiters – not just the first two you get in the DSNT375I messages.

Who is afraid of DDF? from Toine Michielse was all about controlling your DDF workload simply and easily. Use of PROFILE is also recommended …

End of Day One

Then we had a very pleasant evening with the Vendor Expo and prize draws before retiring to the bar to drink some cold beverages and play pool!

Next day, bright-eyed and bushy-tailed we started off again.

Securing Db2 for z/OS Data: Encryption and Much More from Robert Catteral which contained a ton of great information all about encrypting and securing your data and communications – Great stuff!

An Audit a day keeps the lawyers at bay! from my colleague Ulf Heinrich listed out all the requirements for auditing a Db2 system from the auditor’s perspective. Including real-world examples and ending with a „how it looks in Zowe“ bonus preview!

Db2 for z/OS Administrative and Developer tools strategy: Customer feedback, roadmap, and future direction from Sowmya Kameswaran was all about the ongoing project to unify the user experience in Db2. This includes the IBM Unified Management Server for z/OS, IBM Db2 Administration Foundation for z/OS, IBM Db2 DevOps Experience for z/OS, IBM Query Workload Tuner for z/OS , IBM Db2 Analytics Accelerator Administration services for z/OS , Zowe and IBM Db2 for z/OS Developer Extension! A ton of stuff here!

Partition By Range (PBR) and Relative Page Number (RPN) explained from Frank Rhodes drilled down into the nitty gritty details of PBG, PBR and the seven-byte RID!

Db2 For z/OS and Unicode – What you need to know from Chris Crone went into details about what exactly you need to know about UNICODE and how it affects you, your data, and your SQL applications! COLLATION_KEY is the really good one here!

Db2 for z/OS System Profile Monitoring: Overview and Db2 13 Enhancements from Mark Rader contained all the great new stuff in Db2 13 regarding PROFILE usage. Once again well worth a read as it is the best way to control DDF. Slide 15 is the highlight for me!

On the Application development front there were also a whole bunch of great presentations:

Db2 SQL and SQL PL – A Journey Through Db2 12 and 13 Functions Levels from Chris Crone where he runs through the new SQL and SQLPL stuff from Db2 12 before showing all the new stuff in Db2 13 as well.

Db2 for z/OS: REST and Hybrid Cloud from Tori Felt and Keziah Knopp was a huge presentation all about RESTful APIs and all the fun and games involved with them… Covered here was all the stuff on GET, POST, PUT etc. etc., also JSON, z/OS Connect et al!

Db2 Hot topics from Progressive Insurance from Bob Vargo and Dustin Ratliff. They discussed the challenges of how to get SQL Data Insights up and running and then got into FTB and PROFILE usage again (I love PROFILEs, have I mentioned this before?)

Explain explained from Toine Michielse. Does what it says on the side of the can! He shows, and explains, all the available access paths so that everyone is „on the same page“ afterwards.

Bringing Db2 for z/OS-Based Applications Into the Modern Age from Robert Catterall where he showed the modern new way to access the data using REST services with plus and minus points of them. Then he went on to highlight the „new features“ in Db2 for z/OS like System and Business time or XML columns and tables and finished off with a review of the advantages that SQL PL and Native SQL Procedures can give you.

DB2 SQL – go beyond the usual – My current TOP 40 SQL tips, tricks, and opinions from Brian Laube where he discussed all of the tricks and secrets he has learned over the years. Very worthy of a good read!

That’s all folks! I did not even mention all of the LUW or the IMS ones – Feel free to look for them as well if you are interested.

As always, a fantastic conference and I am already looking forward to the next one in 2024!


Roy Boxwell

2023-05: ZPARMs never stop changing part II

This month, I want to go through some of the absolutely most important ZPARMs that control how your Db2 systems behave in a very significant manner. All of the following ZPARMs have a performance impact of some sort. We are always trying to squeeze the last drop of performance out of our Db2 sub-systems, aren’t we?

Db2 13 and Some Db2 12 Updates Ahead!

Since this Newsletter topic first came out, in March 2022, out of the ten ZPARMs listed *five* have got new defaults! I have highlighted all these changed defaults. I have also added three new „Usual Suspects“ to the list of ZPARMs that must be checked…

Starting with the Easy Stuff…

CACHEDYN. YES/NO, default YES. Should always be set to YES – unless you do not care about saving dynamic SQL performance. Back a few decades ago, the recommendation was to have this set to NO as default! Hard to believe that these days, where most shops have 80% – 90% dynamic SQL during the day!

Now we Get to the Numerics!

OUTBUFF. 400 – 400,000, default 102,400. This is *extremely* important and you really should set it to the highest possible value you can afford in real memory! As a minimum, it should be 102,400 KB (100MB). This is the buffer that Db2 uses to write log records before they are „really“ written to disk. The larger the buffer, the greater the chance that, in case of a ROLLBACK, the data required is in the buffer and not on disk.

Skeletons in the Closet?

EDM_SKELETON_POOL. 5,120 – 4,194,304, default 81,920. This is one of my personal favorites, (I wrote a newsletter solely on this a few years ago). I personally recommend at least 150,000 KB and actually even more if you can back it with real memory. Just like OUTBUFF, pour your memory in here but keep an eye on paging! If Db2 starts to page, you are in serious trouble! Raising this can really help with keeping your DSC in control.

DBDs are Getting Bigger…

EDMDBDC. 5,000 – 4,194,304, default 40,960. The DBD Cache is getting more and more important as, due to UTS usage, the size of DBDs is increasing all the time.

DSC is Always Too Small!

EDMSTMTC. 5,000 – 4,194,304, default 113,386. The EDM Statement Cache (really the Dynamic Statement Cache) is where Db2 keeps a copy of the prepared statements that have been executed. So when the exact same SQL statement with the exact same set of flags and qualifiers is executed, Db2 can avoid the full prepare and just re-execute the statement. This is basically a no-brainer and should be set to at least 122,880 KB. Even up to 2TB is perfectly OK. Remember: A read from here is *much* faster than a full prepare, so you get a very quick ROI and great value for the memory invested! Keep raising the value until your flushing rates for DSC drop down to just 100’s per hour, if you can! Remember to cross check with the EDM_SKELETON_POOL ZPARM as well. It always takes two to Tango…

How Many SQLs?

MAXKEEPD. 0 – 204,800, default 5,000. The Max Kept Dyn Stmts parameter is how many prepared SQLs to keep past commit or rollback. It should be set to a minimum of 8,000 or so. Raising this might well cause a large memory demand in the ssidDBM1 address space so care must be taken.

RIDs Keep Getting Longer…

MAXRBLK. 0, 128 – 2,000,000, default 1,000,000. RID POOL SIZE is the maximum amount of memory to be available for RID Block entries. It should be at least 1,000,000 and, if you can, push it to the maximum of 2,000,000. Unless you want to switch off all RID Block access plans, in which case you set it to zero – Obviously not really recommended!

Sorts Always Need More Space

MAXSORT_IN_MEMORY. 1000 to SRTPOOL. Default 2000. The maximum in-memory sort size is the largest available space to complete ORDER BY, GROUP BY or both SQL Clauses. Remember that this is per thread, so you must have enough memory for lots of these in parallel. The number should be between 1,000 and 2,000, but whatever value you choose, it must be less than or equal to the SRTPOOL size.

Sparse or Pair-wise Access?

MXDTCACH. 0 – 512, default 20. Max data caching is the maximum size of the sparse index or pair-wise join data cache in megabytes. If you do not use sparse index, pair-wise join, or you are not a data warehouse shop, then you can leave this at its default. Otherwise, set it to be 41 MB or higher. If it is a data warehouse subsystem, then you could set this as high as 512 MB. (This ZPARM replaced the short-lived SJMXPOOL, by the way.)

Sort Node Expansion

SRTPOOL. 240 – 128,000, default 20,000. SORT POOL SIZE is the available memory that is needed for the sort pool. IFCID 96 can really help you size this parameter. Remember that the number of sort nodes leapt up from 32,000 in Db2 11 to 512,000 nodes for non-parallelism sorts and 128,000 nodes for a sort within a parallel child task in Db2 12. This means raising this ZPARM can have an even greater positive effect than before.

The Three New Guys on the Block!

To the MAX!

DSMAX used to be around 20,000 and can now be between 1 – 400,000. Remember that you will never actually reach this maximum limit as it is 31-bit memory-constrained.

Thrashing Around…

NPGTHRSH. Valid values are 0 or 1 – 2147483647. Default up to Db2 11 was 0, from Db2 12 default is now 1. SAP systems use a default of 10. The big change here, was in Db2 12 when the change from „no statistics ever ran“ of -1 forced the value to be the „optimizer default“ of 501 instead of the real value -1. This is also why the default is now 1 ,so that this ZPARM has a normal use! Setting it to 0 means that the access path chosen will always only be cost based.

Lock ‚em Up and Throw Away the Key!

NUMLKUS. 0 – 104857600, with a default of 20,000. Just be careful raising this value too high, as each lock will take 540 bytes of storage in the IRLM!

Your „Top Ten List“ + Three

These thirteen ZPARMs really influence how your Db2 system works and so must always be checked and changed with great care and attention to detail. Always do a before and after appraisal to see whether or not changing them helped or hindered your system!

If you have any comments, or other ZPARMs you think are also important for performance, feel free to drop me a line!

IDUG 2023 NA

IDUG is nearly upon again. I will be there in Philadelphia at the SEGUS booth and doing a fair bit of moderating as well. Drop on by, have a chat and pick up some of our swag and join me at the „Roy reviews AI with our WorkloadExpert“ PSP on Thursday for a chance to win some cool stuff.

Hope to see some of you there!


Roy Boxwell

2021-05 Soundex and other cool features part 7 for Db2 12

In this, obviously, never ending series of new features, I will roll up all the new ones since my „SOUNDEX and other „cool“ features – Part six All new for Db2 12“ newsletter from 2018-08 where I first covered the new stuff in Db2 12.

What was new in FL100

At that level, a new table function called BLOCKING_THREADS was introduced, which is primarily used by the command DISPLAY BLOCKERS to help in looking for bad guys who are ready to BLOCK your change to FLxxx

FL500 Changed Scalars

Two scalar BiF got changed: TRANSLATE and UPPER can now both use UNI_60 as a locale.

FL500 New Scalars

In FL500, we got a nice bunch of new Scalar functions:

GENERATE_UNIQUE_BINARY (like GENERATE_UNIQUE except it returns a BINARY(16) value)

FL500 Changed Aggregates

One aggregate BiF got updated: ARRAY_AGG can now be used for associative arrays.


It brought in just one new Aggregate BiF, which was the very famous LISTAGG. This is famous, as it was the „proof of concept“ that Agile actually worked and could function in the wild.


Two changes to Scalar BiFs in this release:

GRAPHIC The first argument now accepts numeric data types, including SMALLINT, INTEGER, BIGINT, DECIMAL, REAL, DOUBLE, FLOAT, and DECFLOAT.
VARGRAPHIC The first argument now accepts numeric data types, including SMALLINT, INTEGER, BIGINT, DECIMAL, REAL, DOUBLE, FLOAT, and DECFLOAT.


Nothing new for us in that release!


It brought in a brand new concept for Db2 on z/OS. The concept of „pass-through BiFs“. These are just checked for valid syntax before being passed on down to the Accelerator that would actually execute them. Obviously, you must be careful about their usage!

CUME_DIST (aggregate)
PERCENT_RANK (aggregate)


Brought in a whole bunch of encryption and decryption BiFs:

ENCRYPT_DATAKEY converts a block of plain text to a block of encrypted text using a specified algorithm and key label.


Here IBM came up with a slew of „alternative names“ for existing BiFs and a new method of calling HASH routines. The background here was to make porting to Db2 on z/OS that little bit easier!

CHAR_LENGTH CHARACTER_LENGTH, which returns the length of its argument in the number of string units that are specified
COVAR_POP COVARIANCE or COVAR, which return the population covariance of a set of number pairs
HASH HASH_MD5, HASH_SHA1, or HASH_SHA256, which return the result of applying a hash algorithm to an input argument, depending on the value specified for the second argument for the HASH function:

        0 (default) HASH_MD5 Returns VARBINARY(16) instead of BINARY(16)
        1 HASH_SHA1          Returns VARBINARY(20) instead of BINARY(20)
        2 HASH_SHA256        Returns VARBINARY(32) instead of BINARY(32)

POW POWER, which returns the value of one argument raised to the power of a second argument
RANDOM RAND, which returns a double precision floating-point random number
STRLEFT LEFT, which returns a string that consists of the specified number of leftmost bytes or the specified string units
STRPOS POSSTR, which returns the position of the first occurrence of an argument within another argument
STRRIGHT RIGHT, which returns a string that consists of the specified number of rightmost bytes or specified string units
TO_CLOB CLOB, which returns a CLOB representation of the first argument
TO_TIMESTAMP TIMESTAMP_FORMAT, which returns a timestamp for a character string expression, using a specified format to interpret the string


Here IBM development added another slew of „pass through“ BiFs:

ROUND_TIMESTAMP if invoked with a date expression


None that I have read of yet!


None yet…

Naturally I will be keeping this newsletter up-to-date, as necessary.

Any questions or ideas, do not hesitate to drop me a line,


Roy Boxwell

2019-12 Fun with Db2 12 PBR RPN

I have recently enhanced our SpaceAssuranceExpert (SAX) product to automatically help out if partitions, or partitioned indexes, are getting too big for their boots in a productive system.

SAX – What is it?

The idea behind SAX, is to stop any and all chances of getting a dreaded SQLCODE -904 in production, especially “out of the blue”.

Our idea was, in Db2 12, with the new PBR RPN to do an on-the-fly ALTER to the DSSIZE, just like SAX does now with the SECQTY to avoid running out of extents.


A quick look in the manual tells you that :

it is an immediate ALTER (as long as you make the new DSSIZE larger than the old one!) and there are no package invalidations or REORGS required.

This is fantastic!

So I created a nice little PBR and then ran a horrible Cartesian join SPUFI to flood the first partition with data. This join had a TIME card of (,1) to limit it to one second of CPU before getting an Abend S322.

SAX Monitor

The SAX monitor reacted perfectly and did the TP alter to 513 GB DSSIZE (I had an increment size of 512 as a test), and got an SQLCODE -666 (I just love that SQLCODE…), as the INSERT was still running.

This ALTER was then internally queued to be attempted later, like in any other failure case.

All well and good.

Wham! Nasty errors !

Then I did the same for a DPSI on my PBR RPN… Oh dear!I got a nasty IO Error and then an even nastier ROLLBACK loop, meaning I had to cancel the IRLM to stop Db2… (There is an APAR for this problem PH18977.)

Rollback loop?

The ROLLBACK loop was caused by me choosing to use NOT LOGGED as a tablespace attribute. We have a finite amount of log space, and when the transaction was S322’d after one second of CPU,

the rollback could not find one of the archive logs and then we had to cold start Db2

– Not pretty!

Proper test!

I then wrote a couple of little test programs that actually COMMITted after 5000 inserts and then the ALTERs all worked as designed.

Do I worry too much about extended format and extended accessibility?

Next, I worried about the ominous “extended format and extended addressability“ attributes in the DATACLASS for a PBR RPN, and wondered what would happen if a customer has SAX running and it happily ALTERs a TP to say 6 GB when they can only address 4 GB…

IBM to the rescue!

Luckily for us, Db2 development had thought about this!

  • If you attempt to create a PBR RPN (even with a very small size) and your DATACLASS does not have the two attributes set, you get an error message 00D70008 telling you this detail.

  • If you ALTER an existing tablespace to be a PBR RPN and your DATACLASS does not have the two attributes set, then the ALTER works fine. But remember, this is a pending alter and you *must* do a REORG at the TS level with inline TP level copies. This REORG then fails – also with 00D70008.

So, in other words, SAX cannot hurt you here!

What about PBGs?

After all this we also considered PBGs. They have a limit as well – MAXPARTITIONS in their case. So we added an ability to also ALTER MAXPARTITIONS as well. Here you must be more careful though, as these ALTERs are still immediate *but* they invalidate any referring packages!


If you work with ABIND set to YES all is good as the ALTER comes in, Db2 invalidates your packages and the auto rebind happens so fast that you do not even notice it happening, however, if you work with ABIND NO then any packages, even the package actually running and doing the inserts, will fail! If you are just using dynamic SQL then it is 100% OK otherwise – Buyer beware!


I have opened an Analytics Idea (DB24ZOS-I-1057) to try and get this loophole closed, as I cannot see what access path change could be affected by going from 32 to, say, 36 MAXPARTITIONS.

Db2 keeps adding the parts dynamically and nothing happens then… Go figure… Anyway, if you would like it – Please vote for it!

There are already some nice comments attached to it:

  • DP commented

    this is just a limit in the catalog.  So how could access path be affected?  The actual growth of a partition doesn’t invalidate the package so how just changing the limit do so ?
  • BW commented

    I opened a Case on this asking why packages are being invalidated in this situation and it is still open waiting for a reply.
  • BD commented

    With only impact to catalog, not sure how Access Path would be impacted.  Seems wasteful and counter productive to invalidate packages.

Remember – You never stop learning!

As always I would be pleased to hear from you!

Roy Boxwell
Senior Architect

2019-08 FTB (Fast Traversal Block): Just another TLA?

So we get a new TLA (Three Letter Abbreviation) to try and wrap our heads around in the busy busy world of mainframe databases. This time it comes with a twist, as FTB is actually not “Feed The Beast” sadly, but “Fast Traversal Block”.

In reality, it is referred to everywhere as:

FIT (Fast Index Traversal)

All clear on this and still with me? Good then I may begin!

The problem with modern indexes

The core problem these days is time… the time it takes to go through the leaf pages down to the actual data page is just too long. The vast amount of data we have forces upon the index a large number of layers which, in turn, forces a lot of non-leaf page accesses.

Index structure

We all know how indexes look in Db2, but remember they all have a root page (which is always technically speaking a non-leaf page) which points, normally, to non-leaf pages which, in turn, point to either more non-leaf pages – so you are getting extra levels in the index here  – and finally they point to a leaf page which actually contains pairs of keys and RIDs so that Db2 can get the data from the table page.

Which level are you?

One of the quirks of Db2, is that the leaf pages are called Level 0 and then the non-leaf pages directly above them are Level 1. The root page above these is then Level 2 and so you have a “two level” index.

So how many fetches for a random data access?

The first thing Db2 does is read the root page, looking at the root page it sees which non-leaf page must be fetched and it repeats this until it lands on the leaf page and then it fetches the data. So in my two level index it takes four fetches to get to the data. Now imagine you have a seven level index? Yep you end up fetching a ton of non-leafs!

The solution? FTB!

In Db2 12, IBM introduced the Fast Traversal Block to keep a copy of the non-leaf data in a new area, separate from the buffer pools.

The major reasons for the expected performance boost, are the facts that the structure is L2 cache-aware and each page is equal in size to one cache line (256 bytes), and I/O is massively reduced for the “other” non-leaf pages.

One size fits all?

The size of the FTB is determined by the new ZPARM INDEX_MEMORY_CONTROL with valid values AUTO, DISABLE or 10 – 200,000 MBs. By default AUTO is on. With this value, the size of the FTB is either 20% the size of all available buffer pools or 10MB, whichever is the highest.

The devil is in the detail

To actually start working with FTBs, you have to wait until the FTB Daemon has found an eligible index. From all of your indexes there will be quite a few candidate indexes. These must follow the following rules:

  1. It cannot be longer than 64 bytes

  2. It cannot be versioned (So the OLDEST_VERSION and CURRENT_VERSION must be the same!)

  3. It cannot contain a TIMESTAMP column with TIMEZONE

  4. It cannot have more than 2,000,000 leaf pages

Once through that selection list, the Daemon starts seeing what is happening to this index, is it a random select? That’s is a good thing! Is it an index only access? That’s even better! A split! Oh dear, that’s very bad…from all this, every two minutes, the daemon decides whether or not to use FTB for this index.

Control is in your hands

This is all well and good, but there are a lot of people who like to see *exactly* what is going on and where! So IBM introduced a new command:


From this you get to see which indexes are being processed and how much space they are using. The Daemon also spits out console messages so you can see what happened and why. Further there is a new Catalog table SYSIBM.SYSINDEXCONTROL where you can micro-manage exactly which indexes to attempt to force into FTB and which ones to exclude. I would try and avoid this option and just let Db2 do the business!


Always! FTB is limited to 10,000 per member which is probably ok! When a MASS DELETE (or TRUNCATE TABLE) happens then the FTB is removed for the related indexes and, even more importantly, is to keep up with current Db2 maintenance! As always with new things, it takes a while to get the birthing pains out of the way!

INCLUDE can kill you!

Finally, remember that the length cannot be greater than 64 bytes. So if you decide to add that one byte character column to the index to get Index Only access, you might then push the index over the edge and up to 65 bytes…

Before ALTERing indexes always check if they are in the FTB (or were!) and check if your alteration will disallow the FTB!

A little SQL to show you what you have

WITH INPUT (NLEVELS, LENGTH, INDEX_NAME) AS                            
       , SUM(CASE D.COLTYPE                                             
             WHEN 'DECIMAL ' THEN                                       
 -- IF , SEPERATOR           SMALLINT( CEILING(( D.LENGTH + 1,0 ) / 2 ))
                             SMALLINT( CEILING(( D.LENGTH + 1.0 ) / 2 ))
             WHEN 'GRAPHIC'  THEN D.LENGTH * 2                          
             WHEN 'VARG'     THEN D.LENGTH * 2                          
             WHEN 'LONGVARG' THEN D.LENGTH * 2                          
             ELSE D.LENGTH                                              
       + SUM(CASE A.PADDED                                              
             WHEN 'Y' THEN 0                                            
               CASE D.COLTYPE                                           
               WHEN 'VARG'     THEN 2                                   
               WHEN 'LONGVARG' THEN 2                                   
               WHEN 'VARCHAR'  THEN 2                                   
               WHEN 'LONGVAR'  THEN 2                                   
               WHEN 'VARBIN'   THEN 2                                   
               WHEN 'DECFLOAT' THEN 2                                   
               ELSE 0                                                   
       + SUM(CASE D.NULLS                                               
             WHEN 'Y' THEN 1                                            
             ELSE 0    
                END) AS LENGTH  
 FROM SYSIBM.SYSINDEXES        A                                       
     ,SYSIBM.SYSKEYS            C                                       
     ,SYSIBM.SYSCOLUMNS         D                                       
     ,SYSIBM.SYSINDEXSPACESTATS E                                       
 WHERE A.UNIQUERULE     <> 'D'              -- NOT DUPLICATE            
   AND D.COLTYPE        <> 'TIMESTZ'        -- NOT TIMEZONE             
   AND A.TBNAME         = D.TBNAME                                      
   AND A.TBCREATOR      = D.TBCREATOR                                   
   AND A.NAME           = C.IXNAME                                      
   AND A.CREATOR        = C.IXCREATOR                                   
   AND A.NAME           = E.NAME                                        
   AND A.CREATOR        = E.CREATOR                                     
   AND C.COLNAME        = D.NAME                                        
 GROUP BY A.NAME, A.CREATOR , E.NLEVELS, A.NLEVELS)                     
 SELECT NLEVELS, LENGTH , INDEX_NAME                                    
 FROM INPUT                                                             
 WHERE LENGTH <= 64   
 WITH UR              

I have limited it to just show you just the eligible unique, non timezone, non-versioned indexes with a length of 64 or less of course!

Blog time

Here’s an excellent write up from John Campbell with even more technical details:


and from Akiko Hoshikawa: a very good IDUG Blog:


As always I would be pleased to hear from you!

Roy Boxwell
Senior Architect

Machine Learning for Db2 z/OS: Artificial Intelligence – Hype or Reality?

Now that the Db2 Optimizer has gone all Artificial Intelligent (AI from now on) on us, I thought it would be interesting to review where we are in this brave new world!

Keeps you out of a BIND

My firm created our BindImpactExpert (BIX) software many moons ago to compare access paths, typically before a program non SQL change or a REBIND after RUNSTATS etc. Just to warn you not to do the REBIND or whatever you were going to do, as it would change the access plan to something you may well not really want to have. This means not just no-brainers like index access to tablespace scan, but also different index usage or table access order changes.

Apples and Oranges

So, what has that got to do with AI?
Well, we used a marketing slogan of trying to compare oranges and apples to try and make it clear :

how tough it really is to try and match SQL statements across different program releases and to compare the results.

ML and AI – The new buzzwords

Now in Db2 12, the optimizer has got machine learning (ML from now on) to help it study which host variables are used the most and how many rows are usually returned by a given query.

Do you “know” your data?

When the optimizer “knows” this info it can of course make much better “guesses” at good repeatable access plans that help everyone! Now the usage of AI and ML always gets me interested because of all the fun you can have with them.

There was recently a great article in my favourite science magazine “New Scientist” all about how to fool image recognition AI:

(subscribe to read the whole article)

Now you should see the connection back to our BindImpactExpert slogan. So much for Oranges and Apples – AI cannot even see that an Orange is an Orange…

this is not good or even perhaps “suboptimal”…

Adversarial Images and the fun you can have with them…

From 2018 this article


Within this article is a great paper all about graffiti changed road signs! Here is a direct link to the article https://arxiv.org/pdf/1707.08945.pdf

Fooling Deep Learning

Another great paper where Deep Learning AIs jump off the deep end:


Disappearing Humans

and finally this joyous article and a link to a You Tube video:


But do not wear this picture as a T-Shirt down the street anytime soon!!!

Would you sit in a driverless car?

What this all boils down to is that I, for sure, will never sit in a driverless car within the next 40 years!

  • Firstly, I believe a true AI does not exist yet and will not exist for at least another 40 years.
  • Secondly, when I see an Orange I see an Orange because I “know” what an Orange looks like! As you have seen AIs have no concept of what an Orange *is* and so can easily see a drill instead or mistake skiers for dogs…
  • Thirdly, I am a programmer and someone somewhere is writing and testing – I hope! – the code… need I say more? Don’t even get me started with Agile or KANBAN.

Artificial Intelligence
extract from © 2019 – „We need to talk, AI“ – Dr. Julia Schneider und Lena Kadriye Ziyal
A Comic Essay on Artificial Intelligence

Room full of CTO’s

Have you heard about the hopefully apocryphal story of a room full of CTO’s at a software conference when they were asked “Would you fly in a computer controlled aircraft?” All their hands went up. Then they were asked “Would you fly if it was your software teams that had written the flight control software?” all their hands went down!

Flying is strangely different

While talking about flying my answer would be different?

Because in the air you have no-one else apart from other highly automated devices anywhere near you! This is the one hope that I can see for the whole driverless car stuff – Driving on the Interstate, Motorway or Autobahn. In the air, the environment is much more rigid. No lights, no crossings, no opposite traffic, no foot traffic, no children chasing a ball across the road etc.

They will come…

I am sure that driverless cars will come soon and I am equally sure that people will die because of them – However, I am also sure that lots of accidents will be avoided as well. After all the AI will always react faster than a human.

With this comes the ethics of AI and ML of course which will also play a major role. If the car has to decide in a crash scenario :

  • who gets hurt, should it protect itself and its cargo above all others?
  • What about a group of playing children or a bus full of nuns?
  • How can it decide who lives and who dies?

Looking on the bright side at least it will not get drunk, or high, and drive! I would also seriously hope that AI does not get road rage!

But even with these “bonus” points, I still have massive doubts and serious worries about the quality of the software and the true image recognition potential of current AIs.

AI everywhere

We were recently asked by a customer to add AI and ML to our SpaceAssuranceExpert (SAX) software. This has been happily running for years, capturing dataset allocations and extent usage in Db2 and making sure they never ran out of space or extents without getting told about it well beforehand.

I had to do a real double take and think “How on earth could AI help here?”

You are approaching 32 Linear Datasets with your segmented tablespace. You have to ALTER and REORG before you use up all the space in the 32nd. Where can AI help here?
We already have extrapolation from current usage…

Don’t get me wrong

I am not against AI and ML…

but I think we *all* need to get a bit more skeptical about what can actually be delivered now.

On the one hand host variable contents and row counts – Yes indeed, but even here outliers will cause grief and then on the other hand the complete and utter science fiction of self-driving cars in our towns and cities.

The money being spent is also amazing, in 2019 it is estimated, by Syncsort,

that the AI budget will be $35.8 Billion 44% more than in 2018.

What do you think?

I am sure that autonomous cars will be a plague upon us – are you?

Roy Boxwell
Senior Architect

PS: I am looking forward to the first Adversarial Images for Db2 z/OS !!!

2019-04 ZOWE for Continuous Delivery – It’s worse than that – he’s tested Jim!

Terrible quote usage I know… But now, with the all new and Agile Db2 12 being picked up and used globally, I think it is time to review a few things that companies and individuals must do, or at least plan, in this “Brave New World” of ours.

To wrap up at the end, I will delve into the idea of using ZOWE to help you test!

This Db2 12 testing checklist, helps to navigate into the Db2 Continuous Delivery world, well supported by Zowe, the modern IBM z/OS GUI

Table of contents – Db2 12-testing-checklist
  1. Cloning
  2. Workload collection
  3. Dynamic & Static SQL storage
  4. IFCID376 & co
  5. EXPLAIN-& Access Path
  6. Execute the SQLs
  7. Db2-12 Test Review
  8. Zowe – Open Source z GUI – for CD

The Db2 12 Testing Checklist

1. Do not be afraid of the clones!

To even begin to start testing Db2 12, you’ll need to start with a complete production clone. I know a lot of people get suddenly scared about this due to audit requirements or, more commonly, space requirements. I mean, who can possibly support a complete 100% data clone of their productive system?

A real clone or a partial clone?

Of course you do not actually need all of your data! All you must have is the Db2 Catalog and Directory, all of the user objects should be DEFINE NO style empty shells. After all, do you really need TBs of productive data to do a test run? Nope! So now we have removed two major problems (Space and Audit) from the list of testing problems.

2. Collect as much workload as you can

You’ll need to gather as much of your current executed SQL as you possibly can. At best 13 months is a good target. “Not possible” I hear you all scream – “Oh yes it is!” say I.

3. Dynamic & Static SQL: Store it cleverly away

You do not need the same SQL statement 12,000,000 times. Just once, but with an execution count. Store the dynamic SQLs and the retrieved Static SQLs away somewhere nice and safe with a COMPRESS YES to save room, and then you can easily get 13 months of data. Another major problem gone from the list of testing problems.

4. Gather your friends close but your enemies closer – IFCID376

Make sure you are running with IFCID 376 always on and everywhere! Test, QA and Prod. The moment it starts spitting out possible “problems”, start trying to track down the causing event (SQL) and verify that it is OK, or get a code change implemented. This only has a little to do with agile Db2 12 but should be on at every shop anyway!

But I don’t get the Dynamic SQL!

You do if you are fast and good enough! If you are permanently snapping the DSC and catching the flushed statements, then you can indeed find out the SQL that caused the IFCID. Another major problem disappears from the list of testing problems.

5. Access Paths a go go

Now it really makes a lot of sense to EXPLAIN all of the SQL that you have before and after the Db2 12 Functional Level (FL) change to see if just flipping to, for example, FL 504 caused access paths to head south… This is something you should all be doing now as well of course. Always validate the access path before being forced to do a REBIND or PREPARE. This way you will not get any nasty surprises… Another major problem vanishes from the list of testing problems.

6. Execute the SQLs

I have one last thing for you…

Take all of the SQL that you have just processed and transform it so it can actually execute.

  1. Logarithmically reduce the execution count and execute all of the SQL while monitoring all of the Db2 KPIs – On the Clone, of course.
  2. Then reset the world back to your starting Clone, do the FL change and then re-execute all the SQL again.
  3. Once finished, compare all of the KPIs – looking for outliers as there has only been the FL change then.

Any differences you see should be just “background noise” but there could also be some nasties in there. This will light them up so that the root problem(s) can be found and corrected way before you actually do the FL change in production.

Well done, you’ve made it!

That’s it! With all these processes set up and fully automated you can actually do repetitive pipeline tests within hours instead of weeks or months or never!

7 – Db2 12 test-Review:

What does this mean for me?


  • You need to get a fast, good, automated Cloning system in place.
  • You need to get a fast, good, automated IFCID 376 system in place.
  • You need to get a fast, good, automated SQL workload collection system in place.
  • You need to get a fast, good, automated Access Path comparison system in place.
  • You need to get a fast, good, automated SQL replay system in place.

… this means :

“ Continuous testing in a continuous development world „…

You might, in fact, need to actually buy some software that does all this for you…

May I introduce you to our newest member of the SEGUS/SEG Family:

Db2 Continuous Delivery Deployment Check

is the software that does all of this, and more!

Check out our website for details of how this software really helps in :

  • Reducing the time to test

  • Enabling you to actually go forward with a good, clear conscience!

Visit our CDDC

CDDC for Db2 z/OS - Continuous Delivery Deployment Check - Agile & Environment simulation, Zowe IBM mainframe GUI

8 – ZOWE – Open Source zUI – to the rescue?

You have probably heard of ZOWE by now, the first open source software on z/OS from IBM, Rocket Software and CA Technologies (A Broadcom company). It was launched last year and is growing very quickly.

Introduction to Zowe:


My firm is also using it now and all future developments will also be ZOWE enabled.

In a nutshell,

it allows users to interact with the Mainframe using a modern GUI.

This is based upon z/OSMF and uses work flows to actually do stuff. You define a “micro service” to do one thing and then you can string as many of these together as you like to get something done like, e.g. Provisioning a Db2 system or Cloning a Db2 subsystem etc.

The IBM web based UI for Db2 z/OS

Zowe at SEGUS and


We will be using ZOWE for two things:

1 – For the Installation and Maintenance of our software at customer sites

ZOWE enables people to do things that normally require “green screen” and we are keenly aware that green screen people are disappearing. If we want the z/OS platform to survive, it must be dragged, kicking and screaming, into the modern world.

2 – For using ZOWE as the front end for the users of our products

The CDDC product that I described in this newsletter will be built around ZOWE, thus expanding its usability in the market of tomorrow.

We are convinced that ZOWE is the way to go – are you?

Roy Boxwell
Senior Architect