Coupling facility structure size allocation
When a new coupling facility structure is allocated for a data sharing group, its size is usually taken from the value of the INITSIZE parameter in the CFRM policy.
After the structure is allocated, you can dynamically change its size with the z/OS® command SETXCF START,ALTER,SIZE=newsize,STRNAME=strname. The new SIZE value cannot be greater than the SIZE value in the CFRM policy, but it can be smaller than the INITSIZE value. If the coupling facility has enough space, z/OS increases (or decreases) the size of the structure to the new size; the INITSIZE value of the policy remains unchanged.
In most cases, DB2® uses the
new SIZE value for any subsequent allocations of the structure instead
of the INITSIZE value in the CFRM policy. Any of the following subsequent
allocations can use the new SIZE value:
- A group buffer pool or SCA is deallocated and then reallocated
- A secondary structure is allocated for a duplexed group buffer pool, SCA, or lock structure, if duplexing is started after the size of the primary structure was dynamically changed
- Any structure is rebuilt with the z/OS command SETXCF START,REBUILD,STRNAME=strname
The new size is recorded across a restart of DB2 and is used for all subsequent allocations
until one of the following events occurs:
- A CFRM policy is started, and the policy has a different INITSIZE value than the size of the structure that was dynamically changed with the SETXCF START,ALTER command
- Another SETXCF START,ALTER command is issued to dynamically change the size of the structure
Exception: If a lock structure is deallocated
and all the members are down, the INITSIZE value is used. This is
a consideration for disaster recovery or for situations where data
sharing groups are cloned. During normal operation of a data sharing
group, you are unlikely to encounter this situation.