Consistency Group snapshots are a snapshot which is IO consistent across volumes within a storage controller. The way it works is that writes in ONTAP are held in NVRAM for all volumes part of group. Once IO is fenced for volumes then a snapshot is created on all volumes and writes to the volumes can occur. This is also why there is a timeout and maximum for this timeout is 20 seconds from time fence is up till process is finished. If CG snapshot process does not finishin within timeout ONTAP will fail operation and resume writes to volumes.
Keep in my a CG cannot span volumes on multiple controllers, it is a group of volumes within a controller. If you have multiple volumes on multiple controllers you end up with a CG per controller.
CG has nothing to do with DB2 and probably adds little value as far as consistency goes. You are already putting DB2 in backup mode by suspending writes so DB2 is consistent, nothing more is needed.
An example when CG would add value is if you had say DB2 database and an application and wanted to get some consistency between them you could include application volume on and snapshot it together with CG, or if you had multiple databases. Another use case is for Oracle ASM. CG use cases are limited and most people think, hey well it sounds better than normal snapshot so let me use it. Dont do this, if you dont have a reason to use CG, dont, that is my advice.
Hope this helps
Keith
