Shadow data sets for CHECK LOB

When you execute the CHECK LOB utility with the SHRLEVEL CHANGE option, the utility uses shadow data sets.

If a table space, partition, or index resides in Db2-managed data sets and shadow data sets do not already exist when you execute CHECK LOB, Db2 creates the shadow data sets. At the end of CHECK LOB processing, the Db2-managed shadow data sets are deleted.

For user-managed data sets, DFSMSdss can create or scratch and recreate the required shadow data sets as needed. When the CHECK LOB utility completes the processing of user-managed data sets, the shadow data sets are not automatically scratched.

If you have not changed the value of FREEPAGE or PCTFREE on the CREATE TABLESPACE statement, the amount of required space for a shadow data set is comparable to the amount of required space for the original data set.

If you do not want the shadow data sets to be allocated in the same storage class as the production data sets, set the UTIL_TEMP_STORCLAS system parameter to specify the storage class for the shadow data sets.

Shadow data set names

Each shadow data set must have the following name: 

catname.DSNDBx.dbname.psname.y000z.Lnnn

In the preceding name, the variables have the following meanings:

variable
meaning
catname
The VSAM catalog name or alias
x
C or D
dbname
Database name
psname
Table space name or index name
y
I or J
z
1 or 2
Lnnn
Partition identifier. Use one of the following values:
  • A001 through A999 for partitions 1 through 999
  • B000 through B999 for partitions 1000 through 1999
  • C000 through C999 for partitions 2000 through 2999
  • D000 through D999 for partitions 3000 through 3999
  • E000 through E996 for partitions 4000 through 4096
To determine the names of existing data sets, execute one of the following queries against the SYSTABLEPART or SYSINDEXPART catalog tables: Begin general-use programming interface information.
SELECT DBNAME, TSNAME, IPREFIX
   FROM SYSIBM.SYSTABLEPART
   WHERE DBNAME = 'dbname'
    AND TSNAME = 'psname';
SELECT DBNAME, IXNAME, IPREFIX
   FROM SYSIBM.SYSINDEXES X, SYSIBM.SYSINDEXPART Y
   WHERE X.NAME = Y.IXNAME
    AND X.CREATOR = Y.IXCREATOR
    AND X.DBNAME = 'dbname'
    AND X.INDEXSPACE = 'psname';
End general-use programming interface information.

Defining shadow data sets

For a partitioned table space, Db2 returns rows from which you select the row for the partitions that you want to check.

Consider the following actions when you preallocate the data sets:

  • Allocate the shadow data sets according to the rules for user-managed data sets.
  • Define the shadow data sets as LINEAR.
  • Use SHAREOPTIONS(3,3).
  • Define the shadow data sets as EA-enabled if the original table space or index space is EA-enabled.
  • Allocate the shadow data sets on the volumes that are defined in the storage group for the original table space or index space.

If you specify a secondary space quantity, Db2 does not use it. Instead, Db2 uses the SECQTY value for the table space or index space.

Recommendation: Use the MODEL option, which causes the new shadow data set to be created like the original data set. This method is shown in the following example: 
DEFINE CLUSTER +
   (NAME('catname.DSNDBC.dbname.psname.x000z.L001') +
   MODEL('catname.DSNDBC.dbname.psname.y000z.L001')) +
   DATA +
   (NAME('catname.DSNDBD.dbname.psname.x000z.L001') +
   MODEL('catname.DSNDBD.dbname.psname.y000z.L001') )

Creating shadow data sets for indexes

When you preallocate shadow data sets for indexes, create the data sets as follows:

  • Create shadow data sets for the partition of the table space and the corresponding partition in each partitioning index and data-partitioned secondary index.
  • Create a shadow data set for logical partitions of nonpartitioned secondary indexes.

Use the same naming scheme for these index data sets as you use for other data sets that are associated with the base index, except use J0001 instead of I0001.