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

If you have not measured the amount of storage per thread, here are some rough estimates:
► Low-end: 200 to 400 KB - Static
► Mid-range: 500 KB to 2 M - Static/Dynamic
► High-end: 3 MB to 10 MB or higher - Dynamic, heavy sort, parallelism applied, long
running persistent thread (WFI, CICS Protected Entry Threads with many packages with
RELEASE(DEALLOCATE) or (RELEASE(COMMIT) with CONTSTOR=NO).
These ranges are based on historical observations of typical SQL statements utilized in
customer production environments, the low-end of the range representing static SQL vs. the
high-end representing dynamic SQL-based workloads. The estimation should be verified and
modified appropriately through the actual measurement using the above formulas.
4.3 Monitoring DBM1 storage
Despite the fact that DB2 V8 provides many enhancements to relieve virtual storage
constraints in DB2, it is still important you proactively monitor virtual storage usage within
DB2.
It is also important for you to monitor storage usage above the bar. Within the z/OS
implementation of the z/Architecture, there is no expanded storage. Paging goes directly to
disk, and then you will obviously suffer the performance impact of writing to and reading from
disk. DB2 should rarely page to disk. So, it is more important than ever now to monitor the
number of real frames in use and ensure auxiliary storage is never used.
DB2 provides two new messages to help you not overallocate your buffer pools relative to the
amount of real storage available. This is important as buffer pools can now scale to extremely
large sizes, constrained only by the physical memory limits of the machine (64-bit allows for
16 exabytes of addressability).
► DSNB536I: This warning message indicates the total buffer pool virtual storage
requirement of this DB2 subsystem exceeds the size of real storage of the z/OS image. To
relieve this situation you can either allocate more real storage to the z/OS image or, if
there is not enough real storage to hold the buffers, then the number of buffers needs to be
reduced.
► DSNB610I: This warning message indicates that a request to increase the size of the
buffer pool will exceed twice the real storage available to the z/OS image, or the normal
allocation of a buffer pool not previously used will cause an aggregate size which exceeds
the real storage. Either request is then limited to 8 MB (2000 pages for 4 KB, 1000 pages
for 8 KB, 500 pages for 16 KB, and 250 pages for 32 KB).
DB2 still monitors Short on Storage (SOS) conditions below the bar in the DBM1 address
space by monitoring a storage cushion within the DBM1 address space. When the head room
is less than this storage cushion value, DB2 attempts to contract and reclaim free memory
segments which have been fragmented. This work can impact DB2 performance if DB2 is
continually in this state.
DB2 V8 introduces a number of new counters to IFCID 225 to support DB2 V8. In addition the
IFCID 225 is now available via IFI READS to V8 and V7 (APAR PQ85764), this makes its
contents immediately available to online monitors instead of the last interval values.
The statistics gathered represent a point-in-time view of storage used in the DBM1 address
space at the time the record is cut. They do not record information about storage utilization in
any other address space. IFCID 225 records are cut at statistics intervals whenever the DB2
Statistics class 6 trace is active.
Chapter 4. DB2 subsystem performance 157