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Hyper-V snapshots

Posted on 2015-02-18
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Last Modified: 2015-02-27
Before doing some work on a Hyper-V VM, I want to make sure I can reverse the change should something happens. I know this can be done with snapshot but I wonder if they are really worth it. The .AVHD file can occupy a significant amount of space and it won't give it back until the VM is restarted. Also, the merging process makes me nervous.  Sometimes I find it easier to just shutdown the machine, copy the .VHD file, restart the VM, do the work on it and if nothing bad happens, I simply delete the copy after a few days. Am I causing an unnecessary headache by proceeding like this? The Hyper-V host runs on W2k8 R2.


Thank you!
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Question by:Alan Dala
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Cliff Galiher earned 500 total points
ID: 40617939
It really depends on the workload. Taking a checkpoint *or* putting an old VHD back in place for a DC, as an example, would have equally bad consequences.

Checkpoints tend to be faster, and if you have large AVHDs, that's usually a good sign that you are letting them linger too long. Having a checkpoint around for weeks is terrible for performance, but replacing a VHD with a two-week old copy would be equally as bad for *most* workloads.

In short, checkpoints are not intended to replace real backups, but do make a nice supplemental tool for quick low-risk maintenance tasks and *don't* require taking the VM offline for an extended period, unlike copying a VHD. I use them regularly and safely for things like monthly patches, minor app changes, or other things that a quick rollback would be nicer than a full BMR should the task go sideways. But they don't linger beyond 48 hours. Ever.
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by:Alan Dala
ID: 40636593
Hi and thank you for your answer.

Patches are the reasons why I'm looking into using 'snapshots'. The problem is that my VMs run on W2k8 R2  so there is no 'live' option to  consolidate snapshots. The idea of turning off the servers in order for the snapshots to consolidate, it's a real concern. It doesn't seem to work when you 'shut down'. It needs a hard shutdown which is not ideal to say the least. It looks like 2012 has the ability to consolidate snapshots while live but I'm still not sure of what's going on in the background. Is the performance affected in any way? Is this transparent to the users?


Thank you!
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by:Cliff Galiher
ID: 40636606
Yes, you need yo shut down the VM to merge in 2008 R2. That would be the reason I said you don't need to shut down "for an extended period" as copying a VHD requires. I didn't say the maintenance window was zero.

You should not have to "hard" turn off a VM though. Shutting down works. If it isn't, you have some other problem.

Yes, 2012 supports live merges.

Yes, there is a performance penalty during the merge.

So yes, that means end users may notice the performance slowdown during the merge.

As I said, this isn't a replacement for backups, nor does it replace proper maintenance planning in regards to staff communications, maintenance windows, and finding times of least impact.
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