IBM Storwize V5030 User Manual

Chapter 10. Copy services 495
3. A third write is run to update the database log, which indicates that the database update
completed successfully.
The database ensures the correct ordering of these writes by waiting for each step to
complete before the next step is started. However, if the database log (updates 1 and 3) and
the database (update 2) are on separate volumes, it is possible for the FlashCopy of the
database volume to occur before the FlashCopy of the database log. This sequence can
result in the target volumes seeing writes 1 and 3 but not 2 because the FlashCopy of the
database volume occurred before the write was completed.
In this case, if the database was restarted by using the backup that was made from the
FlashCopy target volumes, the database log indicates that the transaction completed
successfully. In fact, it did not complete successfully because the FlashCopy of the volume
with the database file was started (the bitmap was created) before the write completed to the
volume. Therefore, the transaction is lost and the integrity of the database is in question.
To overcome the issue of dependent writes across volumes and to create a consistent image
of the client data, a FlashCopy operation must be performed on multiple volumes as an
atomic operation. To accomplish this method, the IBM Spectrum Virtualize supports the
concept of
Consistency Groups.
A FlashCopy Consistency Group can contain up to 512 FlashCopy mappings. The maximum
number of FlashCopy mappings that is supported by the IBM Storwize V5000 Gen2 system
V8.1.0 is 4096. FlashCopy commands can then be issued to the FlashCopy Consistency
Group and, therefore, simultaneously for all of the FlashCopy mappings that are defined in
the Consistency Group.
For example, when a FlashCopy start command is issued to the Consistency Group, all of
the FlashCopy mappings in the Consistency Group are started at the same time. This
simultaneous start results in a point-in-time copy that is consistent across all of the FlashCopy
mappings that are contained in the Consistency Group.
Consistency Group with Multiple Target FlashCopy
A Consistency Group aggregates FlashCopy mappings, not volumes. Therefore, where a
source volume has multiple FlashCopy mappings, they can be in the same or separate
Consistency Groups.
If a particular volume is the source volume for multiple FlashCopy mappings, you might want
to create separate Consistency Groups to separate each mapping of the same source
volume. Regardless of whether the source volume with multiple target volumes is in the same
consistency group or in separate consistency groups, the resulting FlashCopy produces
multiple identical copies of the source data.
Maximum configurations
Table 10-1 lists the FlashCopy properties and maximum configurations.
Table 10-1 FlashCopy properties and maximum configurations
FlashCopy property Maximum Comment
FlashCopy targets per source 256 This maximum is the number of FlashCopy
mappings that can exist with the same source
volume.
FlashCopy mappings per system 4096 The number of mappings is no longer limited by
the number of volumes in the system, so the
FlashCopy component limit applies.

IBM Storwize V5030 Specifications

General

Brand	IBM
Model	Storwize V5030
Category	Storage
Language	English