Creating consistent copies using FlashCopy consistency groups

When logically related data spans across multiple volumes, FlashCopy® consistency groups can be used to create consistent copies while minimizing application impact. You can indicate consistent backup copies are required for a copy pool by specifying FlashCopy consistency group in the SMS copy pool definition.

When this is specified, a consistency group will be formed during FRBACKUP. As each copy pool volume is processed, the FCFREEZE option will be used to specify that the source volume is to be part of a FlashCopy consistency group. The control unit will put the source volume in an extended long busy (ELB) state, causing I/O activities to be held at the host. When all volumes have been processed, DFSMShsm will issue the DFSMSdss CGCREATED command to thaw all of the frozen volumes and I/O activities will resume on the volumes.

The frozen volumes will remain in extended long busy state until the Consistency Group Created command is received or when the FlashCopy consistency group timer expires. The default timer value is 2 minutes for CKD devices. If the frozen FlashCopy volumes are not already thawed when timer expires, the control unit will reset the extended long busy state and the held I/O activities will resume.

Since it is possible that the FlashCopy consistency group timer might expire before the consistency group has been formed, or that one or more frozen volumes might be thawed unexpectedly by another FlashCopy consistency group operation or by the thaw command, DFSMShsm will always validate the frozen (ELB) state of the source volumes before thawing the volumes. If DFSMShsm detects that the copy pool backup version might not be a consistent backup, it will fail the FRBACKUP operation and invalidate the new copy pool backup version.

Note:

To ensure a consistent copy can be created within the time window, the DFSMShsm FlashCopy consistency group function is intended to be used for small LOG copy pools.
The thaw command is processed by each LSS receiving the command for all frozen FlashCopy volumes residing in the LSS, regardless of which FRBACKUP command (or job) froze them. Therefore, users should avoid creating multiple FlashCopy consistency groups with volumes in the same LSS at the same time. For example, do not run FRBACKUP of multiple copy pools using the FlashCopy consistency group option from multiple hosts concurrently, where the source volumes residing in the same LSS belong to the different copy pools being backed up.