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VMware Application Consistent Snapshots - Real Need?

 
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roddy_pimentel
Occasional Visitor

VMware Application Consistent Snapshots - Real Need?

Hello All,

A little background I guess.  I have been running my Nimble for about 9 months now and it has been running very champion like.  I have had no complaints whatsoever.  I do have a question though.

I'm currently doing application consistent snapshots of all our datastores.  Which does take its fair amount of resources...

Lately I have been asking myself if I truly do need it.   Other than Exchange and SQL I see no reason to have it.  But even then, should I be bothering with it at all is the question.

Crash consistent, should suffice as I really only care that I can bring the application back up, how much data loss are we really speaking about here? If any...

Your thoughts are appreciated...

8 REPLIES
wen35
Trusted Contributor

Re: VMWare Application Consistent Snapshots - Real Need?

hey Roddy, good question.  it really depends on what application you are running inside the guest - if it's a web server of some sort or one that that does not require proper quiescing, then you are fine.  If you have SQL/Exchange, and you are running VMDK, then you should have both vmw + array based snapshot.  We leverage VMware VSS requestor to send request to VSS to quiesce the app, when the VSS writer properly freezes new I/O, then ESX will take the VMW level snapshot...at that point, we will take array based snapshot.  So my recommendation is to skip the vcenter sync for VMs that don't have an application that requires VSS writer.  Hope this helps!

-wen

roddy_pimentel
Occasional Visitor

Re: VMWare Application Consistent Snapshots - Real Need?

Thanks Wen. But isn't the risk of not doing VMware consistent losing only the data that could be written while the snapshot is happening. In that case, isn't that just mere seconds. We don't have any applications that right that quickly anyway.

I was thinking of disabling the VMware consistent altogether.

Roddy Pimentel

Director of IT

Mary Ann Liebert, Inc.

wen35
Trusted Contributor

Re: VMWare Application Consistent Snapshots - Real Need?

correct - one thing to keep in mind...what recovery do you have for exchange/sql?  is it simple recovery mode for sql & circular logging for exchange?  If you are running into issues w/ vmwware snapshots, we'd like to investigate and see how that can be addressed.  Would hate to see surprises when there's a need for restore.  Do you have a support case open already?  I can help taking a look if you supply the case number.

roddy_pimentel
Occasional Visitor

Re: VMWare Application Consistent Snapshots - Real Need?

Mix of full and simple for SQL. Exchange is not circular.

Roddy Pimentel

Director of IT

Mary Ann Liebert, Inc.

amirul93
Advisor

Re: VMWare Application Consistent Snapshots - Real Need?

Hi roddy, it's not only about accepting a little data loss, but you could actually corrupt data in the process, thereby losing all your data. Also, by taking vmware consistent snapshots, you are ensuring the OS is being snapshotted cleanly also, without risk of corrupting the OS also. Thus when you come to recovering the vm (holding the app and data) from a snapshot, you have a better chance of recovery etc without having to repair/chkdsk the OS.

kevin_kumpf
Occasional Visitor

Re: VMware Application Consistent Snapshots - Real Need?

If it's of any value, we've successful restored VM's (even running MS SQL) with zero data or consistency loss from a non-synchronized hardware (Nimble) snapshot.  I'm not saying that I recommend this approach, just that it's possible.  I imagine it partly has to do with how active your SQL/Exchange server is as the VSS provider is flushing those pending transactions to disk before we quiesce.

Our stance at my employer aligns with Wen's comments in that if it's a web/app datastore we don't use any synchronization in our snaps, but those SQL/Exchange VM's that are more critical, we do.

hduong71
Occasional Advisor
Solution

Re: VMware Application Consistent Snapshots - Real Need?

You won't lose all your data if you take an un-quiesce snapshot. Databases have a log and data volume to maintain ACID.

You can learn about ACID here:

http://en.wikipedia.org/wiki/ACID

It's a matter of pay now or pay later like everything in life.

Let's say you take a quiesce snapshot, the VM has to get all it's I/O synced to disk in a consistent manner and the database will be marked with a "clean" flag. If you want to recover later from that snapshot, since everything is in a consistent state, you don't have do anything to recover the VM.

However let's say you take an un-synchronized snapshot. Since the VM didn't quiesce, if you recover from the snapshot, you may have to run a consistency checker in order to recover the VM. The consistency checker may or may not find anything but it is needed to run because the database was not marked as a "clean" backup/snapshot.

This really comes down to your environment. Some people feel that when you have to recover something, you want to do it quickly since it may be an emergency, so this is when you want to run a quiesce snapshot. Some people feel don't like the quiescing since there can be a slight pause and rather have a little longer recovery process.

P.S. You can have a snapshot schedule where it queisce only the Dailys, but not the Hourlys. It's configurable under your schedule.

roddy_pimentel
Occasional Visitor

Re: VMware Application Consistent Snapshots - Real Need?

First of all, thanks all for the great answers.

What I have decided is two divvy up the snapshots on a per Datastore Basis. Right now I am snapping multiple DataStores at the same time and causes some slowdowns.

The datastores that have no Exchange or SQL will get just crash consistent. The ones that have them will get VMWare integrated.

Thanks again.