In 5.0, we improved on Storage vMotion by doing a Storage vMotion operation in a single pass rather than multiple iterative copy passes. We then recursively went through one or more copy passes until the number of changed blocks was small enough to allow us to switch the running VM to the destination datastore using the Fast Suspend/Resume operation. CBT keeps track of which disk blocks changed after the initial copy. I wrote about this on the vSphere Storage blog some time back. This is very reminiscent of the Storage vMotion mechanism we had in vSphere 4.0 which used a feature called Changed Block Tracking (CBT).

We retried this process for a defined number of iterations, but we have had situations where, due to the amount of I/O in flight, we could never successfully consolidate the snapshot chain and helpers. If it was outside the acceptable time-frame, then we repeated the process (new helper snapshot while we consolidated original helper snapshot) until the helper could be committed to the base disk within the acceptable time-frame.

If the time to consolidate the helper is within a certain time-frame (12 seconds), we stunned the VM and consolidated the helper snapshot into the base disk. It could be that this helper snapshot has grown considerably during the consolidate operation. Once the original chain is consolidated, we then did a calculation to see how long it would take to consolidate the helper snapshot. What we did previously is used a helper snapshot, and redirected all the new I/Os to this helper snapshot while we consolidated the original chain. In fact, there are situations where the snapshot consolidation operation could even fail if there is too much I/O. Many of you will be aware of the fact that when they VM is very busy, snapshot consolidation may need to go through multiple iterations before we can successfully complete the consolidation/roll-up operation. It seems that we have made a major improvement to the way we do snapshot consolidation in vSphere 6.0. This is something I only learnt about very recently, and something I was unaware of.

Snapshot consolidation