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VMWare Snapshot Performance Hit?

Posted on 2015-01-14
Medium Priority
Last Modified: 2015-01-16
I am using snapshots for VMs before I run Windows updates on them, as these past few months have been really bad for Microsoft updates.  Lots of potential for RGEs.  Anyway.  I am wondering, in a scenario where you have a couple (2 or 3) of VMs running on a single physical 7200 RPM hard drive, how bad of a performance hit will these VMs have if I leave them running on a snapshot for a few days?  IE no more than a week.  I just want to leave some time to suss out any problems these updates may have before I delete the snapshot.

Do you think a week is fine for hard drive IO?

Question by:CnicNV
LVL 25

Assisted Solution

by:Mohammed Khawaja
Mohammed Khawaja earned 600 total points
ID: 40549306
The answer is it all depends.  It depends on the following:

1.  How much IO each VM requires
2.  How much data is read and written on each VM (i.e. how busy are they)
3.  The number of VMs
4.  Amount of memory on each host

I usually run few VMs on my laptop and as long as the VMs are not very busy, it is not a problem.  I usually do delete the snapshots after I am done my testing.  You will not know in your environment until you try it.
LVL 125

Accepted Solution

Andrew Hancock (VMware vExpert / EE MVE^2) earned 800 total points
ID: 40549482
To be expected I'm afraid, one good reasons why I recommend *NOT* using Snapshots! and the biggest issue is to come, because your VM is running slowly, updates will take forever to process, and when you have decided the updates have worked, the snapshot size can be as large as 1GB per hour, running, that when you come to DELETE ALL the snapshot, the merge can also take many hours to compele!

Whilst a VM writes to a snapshot disk (child) performance will always be bad, no matter what underlying datastore you have! (I've never found it depends, and I've used the fastest datastores in the world, and snapshots still have a huge performance hit! and a single SATA 7,200rpm disk....wow, that's going to be slow! - I think you've only got 40 IOPS there, if the I/O was not virtualised!)

You will also find that Veeam will not support your environment, and it would be too slow for Veeam Backup and Replication, (which uses the Storage API - snapshots) to backup the VMs, and all storage vendors use the same routines and snapshots!

If you search on all my responses to Snapshot Questions, in the last 4-5 years, I've said the same, snapshots are evil, rubbish, do not use them, they cause more issues than they cure!

See my EE Article

HOW TO: VMware Snapshots :- Be Patient


Do not use them! and rather than use a snapshot, take a FULL CLONE as a backup before you start running updates or use a third party vendor backup tool.

If the updates go wrong, destroy the current machine, rename the CLONE (in the inventory), and restart it - done!

If the updated VM is successful - delete the CLONE.

SAFER, Faster than Evil Snapshots!

Assisted Solution

by:Dawid Fusek
Dawid Fusek earned 600 total points
ID: 40549987
@ Andy
"If you search on all my responses to Snapshot Questions, in the last 4-5 years, I've said the same, snapshots are evil, rubbish, do not use them, they cause more issues than they cure!"

You rock Andy !!

VMware snapshots on single SATA disk with only single VM may as Andy suggest get You with dramatic performance issues, especially after Windows Update's where lots of system files are changed, so it's not an good idea at all, I use snapshots in my environment but on SSD storage or good cached ZFS storage and sometimes some VM's are running so slow that there are need to delete a snapshots, what sometimes take really more than 16 hours !!!! (so it also a risky and performance issue for whole storage VM's).

If this is not a very big VM, cloning it is a good backup, sure not so fast as snapshot but it's safer and better solution from performance point of view.

So how bad it may be (if windows updates will come to these 3 VM's with snapshot/snapshots)... hmm, as Andy suggest very very bad, even they may get a BSOD's and apps crash because of problems of accessing a disk/storage, and also your SATA disk may fail if it's not a high performance higher price disk (it happens to me 2 times on cheap USB 5k4 hdd's).

So summarizing snapshots of 3 VM's on a single SATA disks for more than some hours are not a good idea, even for some hours if you add a data or just make a windows update it's a bad devil idea :P at all).

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Expert Comment

by:Dawid Fusek
ID: 40552467
additionally CnicNV,

to give you more real life example, I have a 2 presales on my VI, and they are installing/testing/learning and making a workshops of some BI software (fat software with SQL, Java, .NET), they of course using a snapshots (they have such permission in my VI), and if my storage start goes into performance problems... I look firstly for them and their snapshots (and other snapshots of course) because these VMware poor feature really can kill the performance and act as evil to a storage, VM and whole VI performance.

And sometimes deleting these snapshots took more than 16h !!! :((

best regards

Author Closing Comment

ID: 40553504
Ok thanks for the feedback, it seems almost unanimous.  

Too bad really, its seems like in theory Snapshots would be fantastic.  Ahh well, I will use them with great caution in future.  I have already deleted the snap for one of the VM since the Microsoft RGEs did not cause any issues.  I will delete the other VMs snapshot today, since it too seems to be ok.

Thanks again.

Expert Comment

by:Dawid Fusek
ID: 40553656
good, we gave You some point mate,

the theory of VMware snapshots is fare more brutal than marketing materials, it's one of feature that still can be improved by a vendor.

Also I have to add that here, that if You some day will have a storage based on ZFS there is a possibility to use a ZFS snapshots that are far more efficient, but much more difficult to manage (from cli), not working per VM if used with NFS (working per VM when used with iSCSI as a RDM to single VM) and require a storage server based on ZFS.

good luck mate.

kind regards

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