For those that don't want the long story, I'll start with my questions:

Has anyone done a VSS entire volume revert of a large volume (over 8TB) before? And if so, how long should I expect it to take? If it matters, assume the difference between the snapshot and current state is in the 500GB - 1TB worth of change. Does windows set the volume up as inaccessible as it's performing the revert? Is there anyway to tell whether VSS is actually still involved in reverting the volume?

For those interested in the background:

I have a Windows 2012 server that hosts the backups of my VM infrastructure. Power got cut while running a backup job to it and the underlying datastore holding those backups got corrupted. The backup software would no longer work with the repository.

There is a VSS job on that volume that snapshots nightly. So figured no big deal. I mounted the VSS snapshot and started the recovery of the backup repository. Unfortunately, the backup repo is relatively large (over 4TB) and the restore process was looking like it was going to take about four days to finish. I let it run overnight.

Today, I check in on things and the restore of the files is still running fine. But being the patient person that I am, I decided that I didn't want to wait another couple of days and I might as well just revert the entire volume. So, I went in to the properties of the volume, shadow copy config, selected the shadow copy I wanted to revert to and went with Revert.

The Revert dialog came up warning that was irreversible. I said ok. The revert dialog went away a few seconds later and now trying to access the volume, I get an access denied message and properties of the volume shows it as 0 bytes.

It makes a certain amount of sense that Windows would take the volume offline and make it inaccessible as it was doing the revert. But it's been about 30 minutes now and I don't know if it's doing anything or whether at this point the volume is a lost cause. Checking the resource monitor I do see storage activity on the volume that is inaccessible but am unsure of a way to verify if it's activity due to the revert process.

I actually still have the shadow copy of the volume I'm trying to restore mounted (not actively accessing it but the data in the snapshot is still accessible through it). Is it possible that's causing a problem with the revert? I don't want to unmount it if unmounting it won't help the revert. As worst case, I could spend a week or so copying it somewhere else.

So back to my questions:

  • Is this revert behavior normal? Does windows mark the drive as inaccessible as it reverts?
  • Should I expect the revert process to take a long time?
  • Can I tell if Windows is still involved in reverting the volume?
  • Will having the snapshot that is being reverted to mounted cause a problem with the revert?

The volume came back online early this morning. Just in case someone else comes across this:

The revert behavior appears to be normal. Windows will set the volume inaccessible (access denied) and the volume will appear as 0 bytes while in the process of reverting.

The process can take a long time. It took close to 20 hours in my case. Not sure if it's a function of how many changes occurred after the snapshot but would guess that's likely the case. Not going to do a test to verify.

Windows Resource Monitor will show activity on the volume being reverted- seems like there should be a better way to monitor it but resource monitor works. It's what gave me hope that it will still in the process of reverting.

Having the snaphot mounted didn't prevent it from being reverted.

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