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

If you specify SHRLEVEL CHANGE, the applications can read from and write to the index,
table space or partition that is being checked. The phases of processing inside CHECK
INDEX differ in SHRLEVEL REFERENCE and SHRLEVEL CHANGE. In the case of
SHRLEVEL REFERENCE, DB2 unloads the index entries, sorts the index entries, and scans
the data to validate index entries. On the other hand, SHRLEVEL CHANGE first goes to the
table space data set and unloads the data and sorts the index keys and merges (if the index is
non-partitioned), and then scans the index to validate.
CHECK INDEX with option SHRLEVEL CHANGE performs the following actions:
► Issues QUIESCE WRITE YES for the specified object and all of its indexes (this drains the
writers so no updates are allowed for a while)
► Invokes DFSMSdss at the data set level to copy the table space and all indexes to shadow
data sets
Note: Online CHECK INDEX will use FlashCopy V2 if available, otherwise concurrent
copy will be used, this can elongate the read-only time.
► Allows write access to the specified object and indexes
► Runs CHECK INDEX on the shadow data sets
Tip: FlashCopy V2 is needed since it supports data set copy (FlashCopy V1 works only at
the volume level). Another advantage is that the source and target of a FlashCopy V2 can
be a different logical subsystem within the Enterprise Storage System.
Index processing with SHRLEVEL CHANGE
SHRLEVEL CHANGE has a new architecture. SHRLEVEL REFERENCE unload phase
unloads keys from the index and the check phase compares sorted keys against the data via
tablespace scan. SHRLEVEL CHANGE unload phase does a table space scan extracting
keys and the check phase compares sorted keys against the keys in the index.
CHECK INDEX in V8 can provide parallel processing like REBUILD INDEX, which minimizes
elapsed time for checking the data and indexes. If you specify more than one index to check
and the SHRLEVEL CHANGE option, CHECK INDEX checks the indexes in parallel unless
constrained by available memory or sort work files. Checking indexes in parallel reduces the
elapsed time for a CHECK INDEX job by sorting the index keys and checking multiple indexes
in parallel, rather than sequentially.
It is important for parallel processing to specify appropriate values in the SORTDEVT and
SORTNUM parameter. SORTDEVT specifies the device type for temporary data sets that are
to be dynamically allocated by the DFSORT. SORTNUM can specify the number of temporary
data sets dynamically allocated by the sort program (DFSORT). Example 6-1 is a sample JCL
of CHECK INDEX.
Example 6-1 Sample JCL of CHECK INDEX SHRLEVEL CHANGE
//CHKIDXB EXEC PGM=DSNUTILB,REGION=4096K,PARM='SSTR,CHKINDX1'
//SYSPRINT DD SYSOUT=A
//SYSUDUMP DD SYSOUT=A
//UTPRINT DD SYSOUT=A
//DSNTRACE DD SYSOUT=A
//SYSUT1 DD UNIT=SYSDA,SPACE=(CYL,(5,2)),VOL=SER=SCR03
//SYSOUT DD UNIT=SYSDA,SPACE=(CYL,(5,2)),VOL=SER=SCR03
//SORTLIB DD DISP=SHR,DSN=SYS1.SORTLIB
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