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

4.10 New CI size
To preserve the SECLABEL values that are in the table, you need to execute the SQL in
separate tasks with the various unique seclabels in the table, and include the predicate:
WHERE SECLABEL_COLUMN = GETVARIABLE(SYSTEM.SECLABEL)
An alternative is to include another SET clause in the SQL UPDATE statement in order to
preserve the SECLABLE values already in the table.
UPDATE PAYROLL
SET SALARY = SALARY * 1.1,
SECLABEL_COLUMN = SECLABEL_COLUMN
This technique would require write-down authority for the user running the job. If there are
any rows in the table that dominate (not equivalent) the user's seclabel, then they are not
updated.
In summary, although MLS Row Level Security can simplify the separation in an application, it
is not transparent. You need to review how your application INSERTs and UPDATEs the data,
as these processes probably need to change in order to manage and maintain the
SECLABEL values in the data. There are similar considerations for batch processes and
some utilities. Utilities may also take a little longer to execute. Processes for managing and
controlling which processes have the write-down authority must also be developed.
With V7, DB2 only uses the VSAM Control Interval (CI) size of 4 KB. If the page size is 8, 16
or 32 KB, DB2 treats the page as a chain of 2, 4, or 8 CIs and requires VSAM to only allocate
CIs in 4 KB blocks.
DB2 V8 introduces support for VSAM CI sizes of 8, 16, and 32 KB, activated by the new
DSNZPARM parameter, DSVCI, in panel DSNTIP7. This is valid for both user-defined and
DB2 STOGROUP-defined table spaces. Index spaces only use 4 KB pages.
Once you have activated the new CI sizes (in NFM), all new table spaces are allocated by
DB2 with a CI corresponding to the page size. The table spaces already existing at the time of
migration to DB2 V8 are later converted by the first execution of LOAD REPLACE or REORG.
When using DB2 user-defined objects (USING VCAT), it is your responsibility to define the
underlying VSAM cluster with the correct CISIZE.
DB2 uses VSAM linear data sets, (LDS), which only uses a CI size which is a multiple of 4
KB, and no CIDF. CIs are written by VSAM in physical blocks which try to use the best fit
within the CI, and keep track occupancy decent.
Now, consider Table 4-12, which describes the existing relationship between DB2 V7 page
sizes and VSAM CI sizes assuming a 3390 disk.
Table 4-12 DB2 V7 - Sizes in KB (usable track size 48 KB for 3390)
DB2 page size VSAM CI size VSAM physical
block size
Blocks per track DB2 pages per
track
4 4 4 12 12
8 4 4 12 6
16 4 4 12 3
32 4 4 12 1.5
Chapter 4. DB2 subsystem performance 199