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Vmware Snapshot question

Posted on 2011-02-11
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Last Modified: 2012-05-11
I only have experience using Vmware on a very basic level but have been tasked with putting together a viable backup solution for the VM's.  I was in a discussion with a fellow colleague who said I can simply create snapshots and back these up.  After some research, I now know this is complete false (or so I believe).  So I would like some clarification.

Q. When a snapshot is created, it just creates a smaller 'reference' vmdk and a redolog (or cache file) to save all write changes occuring on the VM.  This suspends the real VM to allow it to backed up if needed.  This is why you can't backup a snap.  So let'sa say you are patching a server - you would snap it, patch the server (the snapshot) and if it all goes well, if you deleted the snapshot, it would commit the changes to the existing VM with no downtime?  If the server dies, you would just 'revert'; back to the origional VM (vmdk) and off you go?

Q.  What exactly is quiesce and how does it function?  From what I've read - it commits everything in memory to write to disk before creating the snap.  Doing this will pause the OS (not sure how long) which this is occuring and all changes are cached until this is done.  Once the memory has been committed to disk, the cache data is pushed back to the VM and the VM (OS functions) are restored.  

Q.  What is a good free method of taking a hotbackup of a live VM for use with a coldsite in the event the ESX host dies.

Thanks!

I

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Question by:GDavis193
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coolsport00 earned 278 total points
ID: 34873005
No, no, no! Snaps are NOT b/u solutions, and that person who told you that doesn't know the implications of snaps or the issues they can cause.

1. Yes, a 'snap' (delta) file is created and when new data is written to the VM, it is stored in this delta file; and yes, you are correct in your assumption about VM patching and snaps. It is best to not keep snaps longer than a day, 2 at most.

2. There is nothing you can use for free that takes b/u's of live VMs. All solutions you have to pay for. There are free solutions, but a. they can only be used with the VM off, and b. is a manual process.

Here is an excellent KB on Snapshots:
http://kb.vmware.com/kb/1015180

Hope that helps.

Regards,
~coolsport00
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by:coolsport00
coolsport00 earned 278 total points
ID: 34873015
Actually, I spoke too soon. The only 'free' method of backing up a live VM is by using vCenter Converter Standalone too. But, it is a manual process.

~coolsport00
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by:coolsport00
coolsport00 earned 278 total points
ID: 34873034
Or, if you have vCenter Server, the built in Converter tool. Also, to answer your quiesce question, should be available in the KB I posted.

~coolsport00
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by:Matt V
Matt V earned 56 total points
ID: 34873044
A snapshot creates a new VMDK file and starts writing to it, thus enabling you to at any point discard all changes from that point on and revert to the original VMDK file.  You also of course have the option of merging those changes back into the original VMDK file (which you should always do once you verify the changes you took the snapshot for are working).

Quiescence is telling the OS to let go of any open and locked resources during the snapshot to ensure you get a valid copy in the restore point.

Done correctly, a snapshot gives you exactly that, a point in time "snapshot" (think pictures) of what the machine looked like that you can revert to if required.

If you have VMware Enterprise licensing, you can use VDR, which is a great block-level backup solution from VMware.
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by:danm66
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http://communities.vmware.com/docs/DOC-8760 has some shell scripts that you could use with cron to do an automatic backup
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by:GDavis193
ID: 34873664
Done correctly, a snapshot gives you exactly that, a point in time "snapshot" (think pictures) of what the machine looked like that you can revert to if required.
So how do you correctly create a snapshot?

Quiescence is telling the OS to let go of any open and locked resources during the snapshot to ensure you get a valid copy in the restore point.
This pauses the OS during this operation correct?  If so - would my users experience a loss of connection (ie, file transfer stop, etc)
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by:coolsport00
coolsport00 earned 278 total points
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1. You right-click on the VM and select Snapshot -> Take Snapshot.

2. No. This is done seamlessly.

~coolsport00
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by:GDavis193
ID: 34873856
Ok, so why wouldn't you just always want to quiesce the file system when creating a snap as it seems like this would be more of a complete snapshot?
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by:coolsport00
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Well, there is no setting for quiescing (that I'm aware of). I personally can't think of reason to not quiesce. I know there's a setting in VM backup solutions (Veeam, e.g.) that you can choose to not check that option. Since quiescing involves stopping process/services (albeit briefly), there could be a reason to not have that option for VM backups, if you do have an app that uses a process/service that needs to run continuously.

~coolsport00
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by:bgoering
bgoering earned 111 total points
ID: 34876149
If a vm has a high write transaction rate it may be very difficult to quiesce before taking the snapshot. This could occur in a SQL or Exchange server environment for example. A non-quiesced snapshot has "crash consistancy" which means drives and/or data may be corrup if you revert to the snapshot. Risk about the same as yanking the power cord out of a physical server. If you quiesce the vm then all disks are certain to complete any I/O, buffers all written to disk, etc. Then the vm is paused very briefly whille the empty delta file is created, then resumed with the originall vmdk as read-only and all updates go to the snapshot file.

Hope this helps
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by:bgoering
bgoering earned 111 total points
ID: 34876153
The reason high transaction rates might make it difficult to quiesce the vm is that the vm isn't paused to allow I/O to complete, instead the server looks for a point of time where all I/O has been completed then issues the pause.
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by:GDavis193
ID: 34876165
Thanks!
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