DB2 UDB for z/OS Version 8 Performance Topics - IBM Redbooks

The EXTENDED REGION SIZE (MAX) is the size of the region available to DB2 after MVS
has allocated all common ECSA, ELPA, and ENUC type storage. It is commonly thought that
DB2 has 2 GB available, but this is generally more in the range of 1000 MB - 1600 MB. Most
of the storage DB2 allocates below the bar is here, in MVS subpool 229 or 230 key 7.
The general recommendation for monitoring and tuning DB2 storage is to have approximately
200 MB free for fluctuations in workload. This recommendation is also true for DB2 V8. 200
MB is a good conservative number, but you may think of reducing this if your DB2 subsystem
is small.
The EXTENDED CSA SIZE is the common storage area across all address spaces for a
given LPAR. A large ECSA allocation would be .5 to .8 GB, while a typical allocation would be
.3 to .5 GB.
MVS is very likely to report a different number in EXTENDED REGION SIZE (MAX) by as
much as 100 MB higher (the difference is in MVS overhead that DB2 cannot track.)
When DB2 thinks it is running low on extended virtual storage, DB2 begins a process called
“system contraction”, which attempts to freemain any available segments of storage in fixed
and variable pools back to MVS. (This process can be CPU-intensive and cause latch
contention). If the storage DB2 has reserved based on the number of threads that are allowed
in the system, plus the storage DB2 has reserved for open and close of data sets based on
DSMAX, plus the storage DB2 has reserved for must complete operations (such as ABORT
and COMMIT), is less than DB2’s opinion of what storage is available, then contraction
occurs.
Therefore, it is important to correctly estimate CTHREAD, MAXDBAT and DSMAX, and not
be overly specific in estimating them.
Storage estimates for threads
The following formulas apply to storage between the 16 MB line and the 2 GB bar.
The current formula for the Thread Footprint (TF) is as follows:
( TOTAL VARIABLE STORAGE - TOTAL AGENT SYSTEM STORAGE / TOTAL NUMBER OF ACTIVE USER
THREADS
(153.39 - 3.54) / 500 = 0.2997
This value is also reported in the report as AVERAGE THREAD FOOTPRINT as 0.30 MB.
The fixed amounts of storage within the DBM1 address space do not vary a great deal as
workload increases. They can be considered as a fixed overhead, for example, buffer pools,
EDM pool, buffer pool control blocks and compression dictionaries. The total fixed storage
overhead (F) can be estimated as:
or
TOTAL GETMAINED STORAGE + TOTAL GETMAINED STACK STORAGE + TOTAL FIXED STORAGE + 31 BIT
EXTENDED LOW PRIVATE
599.18 + 26.77 + 0.30 + 24.64 = 650.89 MB
The value V for upper limit variable areas is defined as:
V = R - C - F
where:
► R is the theoretical maximum region size
162 DB2 UDB for z/OS Version 8 Performance Topics