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VMWare 4/Netapp: When you delete a VM snapshot..

Posted on 2012-03-19
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When you delete a VM snapshot (we are on VMWare 4 and have Netapp iscsi storage) - is that a write intensive operation?  Or is it just a matter of removing a pointer?
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Question by:amigan_99
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Andrew Hancock (VMware vExpert / EE MVE) earned 500 total points
ID: 37738349
It can be Read and Write Intensive as it merges the snapshot to the parent disk.

It's not the same as a NetApp Filer Snapshot.

A snapshot is NOT a backup of a VM; that is a gross misconception.  

A snap shot is a way to preserve a point in time when the VM was running OK before making changes. A snapshot is NOT a way to get a static copy of a VM before making changes.  When you take a snapshot of a VM what happens is that a delta file gets created and the original VMDK file gets converted to a Read-Only file.  There is an active link between the original VMDK file and the new delta file.  Anything that gets written to the VM actually gets written to the delta file.   The correct way to use a snapshot is when you want to make some change to a VM like adding a new app or a patch; something that might damage the guest OS. After you apply the patch or make the change and it’s stable, you should really go into snapshot manager and delete the snapshot which will commit the changes to the original VM, delete the snap, and make the VMDK file RW. The official stance is that you really shouldn’t have more than one snap at a time and that you should not leave them out there for long periods of time. Adding more snaps and leaving them there a long time degrades the performance of the VM.  If the patch or whatever goes badly or for some reason you need to get back to the original unmodified VM, that’s possible as well.  

I highly recommend reading these 2 articles on VMware Virtual Machine Snapshots:

Understanding Snapshots - http://kb.vmware.com/kb/1015180
Snaphot Best Practices - http://kb.vmware.com/kb/1025279

Also check out the following Snapshot Articles by Eric Siebert

Pt.1- http://is.gd/Lajg4p
Pt.2- http://is.gd/NdKQWC
Pt.3- http://is.gd/tp2vEK
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by:Alan_White
ID: 37738368
It would depent.

Typically when you take a snaphot, the original vmdk is frozen and all changes are written to a newly created snapshot file.

When you come to delete it, typically you want to commit the changes (made since the snapshot was taken) back into the vmdk.  Therefore this is write intensive.

If on the other hand you want to roll back to the snapshot and then delete the "forward" changes, it will be less write intensive.
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Depending upon the size of the snapshot, how many snapshots, how long VM has been running on a delta (snapshot) disk, how fast the datastore, SATA or SAS disks, RAID type, it could take many hours, to the longest I've seen 4 days to merge the disks.
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by:paulsolov
ID: 37738538
I concur with @hanccocka, don't get your vmware and Netapp snapshots confused.  

SMVI can do both (while making a backup so it can quiesce the VM using vmware snapshots prior to taking Netapp snapshots)  The load and read/write is minimal since it's only for a short period of time.
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by:amigan_99
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This answer is so awesome I wish I could give more points!  Thank you.
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