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

cache is governed by the DSNZPARM EDMDBDC parameter, with a size between 5 MB and
2 GB.
There is little overhead in below the bar storage to support this new DBD pool above the bar.
So, make sure you adequately size the EDMDBDC parameter in order to avoid continually
loading DBDs from disk.
Storage for plans and packages
Plans and packages (SKCT, CT, SKPT, and PT) and the authorization cache are the only
control blocks that are stored in the EDM pool that remains below the bar. The DSNZPARM is
EDMPOOL as before. As other DB2 structures have moved above the bar in the DBM1
address space, more space may become available below the bar to accommodate more
plans and packages in the EDM pool.
Remember that if you plan to use the current V7 size of the DSNZPARM EDMPOOL
parameter in V8, the EDM pool may be over-allocated, since the DBDs (and potentially also
the dynamic statement cache) no longer reside in that storage area in V8. However, we
suggest you monitor your EDM pool performance before making any changes. Plans and
packages are bigger in DB2 V8 and consume some of the space in the EDM pool that was
vacated by the DBDs. You can monitor the health of the EDM pool by using the pertinent DB2
PE report fields monitoring. If SKPT and SKCT I/O are to be eliminated, oversizing the EDM
pool and monitoring SKPT and SKCT requests are best.
The recommendations when monitoring are:
► Failure due to EDM pool full should be 0
► Request not found in EDM pool < 1 to 5%
► CT + PT < 80% of pool
Global dynamic statement cache
In V7, if global dynamic statement caching is active (DSNZPARM CACHEDYN=YES
parameter), the statements can be cached in a data space (if the DSNZPARM EDMDSPAC
parameter is also >0), or in the normal EDM pool. In V8, cached, dynamic SQL statements
are always cached in the EDM statement cache pool above the 2 GB bar. The maximum EDM
Statement Cache size, specified by the DSNZPARM EDMSTMTC parameter, is 1 GB.
The default value for EDM STATEMENT CACHE is taken from the EDMPOOL STORAGE
SIZE field on panel DSNTIPC, or 5000 K, whichever is larger. The maximum size is 1048576
KB.
Storage for the dynamic statement cache is always allocated (at least 5 MB), and is no longer
related to whether CACHEDYN is YES or NO. This is because the DSNZPARM CACHEDYN
parameter can be changed online in V8, and DB2 needs an initial size to be allocated at
startup time to allow the size to be changed online.
Although the EDM Statement Cache pool is located above the bar, its size has a real impact
on storage below the bar. It impacts the Statement Cache Blocks pool, which contains various
control blocks for dynamic SQL statements that are cached in the EDM Statement Cache
pool above the bar. It impacts the dynamic SQL pools (local cache) below the bar that are
used for statement execution and locally cached statements resulting from
KEEPDYNAMIC(YES). The storage consumed below the bar is not only dependent on the
size of the EDM Statement Cache above the bar, but is also dependent on the number of SQL
statements in the cache.
Note that the PTF for APAR PQ96772 further relieves virtual storage constraint for systems
that are very active with dynamic statement caching by moving statement control blocks
above the bar.
Chapter 4. DB2 subsystem performance 143