Storage changes in Db2 12
Db2 12 introduces changes to the storage configuration for a Db2 for z/OS installation.
database services address space (ssnmDBM1) MEMLIMT increased to 19T
In job DSNTIJMV, which defines Db2 to the z/OS operating system, the default MEMLIMIT setting for the ssiddatabase services address space (ssnmDBM1) is increased from 4 TB to 19 TB, to support the new 16 TB limit for all buffer pool storage.
MEMLIMIT specifies the limit on the total size of usable virtual storage above the bar in a single address space.
New requirements for 32KB work files
For migration to Db2 12, you might need to increase your 32KB work file allocations for work file data sets and work file buffer pools. If you did not define 32KB work files in previous Db2 releases, you might need to define 32KB work files in Db2 12. To implement improvements in sort processing and performance, Db2 12 uses more 32KB work files than previous Db2 releases. For sorting a very large number of records, Db2 12 might require 32KB work files, even though previous releases did not use them.
When Db2 12 uses 32KB work files for sorting a very large number of records, and there is an insufficient amount of 32KB work file space, Db2 12 cannot use work files of a smaller size. The operation that requires the sort fails with an error that indicates that there is not enough work file space.
Changed behavior for PGSTEAL(NONE) buffer pools
Db2 12 introduces the following changes to management of PGSTEAL(NONE) buffer pools:
- In Db2 12, Db2 keeps all pages for open objects that are assigned to PGSTEAL(NONE) buffer pools in memory. When the buffer pool is allocated, Db2 creates an overflow area within the VPSIZE of the buffer pool. The overflow area is used if the buffer pool is not large enough to contain all pages for all open assigned objects. Page stealing can also occur in the overflow area. LRU chains are maintained for the overflow area, but buffer stealing in the overflow area uses the FIFO page-stealing algorithm.
In Db2 12, if you specify PGSTEAL(NONE) with FRAMESIZE(2G) in an ALTER BUFFERPOOL command, when Db2 allocates the buffer pools, it temporarily changes the page-stealing algorithm to PGSTEAL(LRU) and issues message DSNB549I.In Db2 11, FRAMESIZE(2G) was allowed with PGSTEAL(NONE), but this combination might waste real storage and lead to storage shortages.
For more information about this change, see Storage release incompatibilities in Db2 12.