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I have a Windows 2003 server running on VMware Virtual Machine. One of our developers wanted to install a service on it. So I decided to give VM snapshot a try on the machine in case if the developer messes it up. I took a snapshot on the machine including RAM and handed it over to the developer. Couple of days later, after confirming with the developer that everything went well, I decided to delete the snapshot so that all the changes are committed back to the original VMDK file. Unfortunately, the developer reported that some files that he had copied to the machine were missing. This left me wondering whether snapshot is an effective way to use as a system recovery tool at all. Please share your experiences on this.

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4 Answers

up vote 3 down vote accepted

Snapshots are 100% reliable in this scenario. I would be inclined to believe your developer has made a mistake. The snapshot doesn't deal with "files" it deals with low level disk blocks. If there was any problem with the snapshot you would have more serious errors than missing files, more likely you would get partition corruption or disk failure errors (assuming the vmdk could even be mounted).

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"Some files missing" sound like a really serious issue! Have you filed a bug about this? We do not use the snapshot functionality ourselves, but I have never heard about this happening, and I've worked with VMware product for quite some time.

Are you sure he is not trying to shove his own mistake into your shoes?

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I suspect your developer made changes / copies to the current state of the system and not the snapshot. When you deleted the snapshot, you merged it with the previous snapshot but didn't incorporate the current work.

Next time, try making a snapshot of the current state of the VM first, just to make sure.

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As far as my knowledge, in VMWare once you take a snapshot, it freezes the VMDK file(current state) and all the changes you make from that point on are registered in a different file. When you delete the snapshot, you merge the delta file to the original VMDK file. –  BlueGene May 29 '09 at 14:02
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As "20th Century Boy" wrote, snapshots are 100% percent reliable - most of the time. For example if you are running some kind of database most you could get without vss support on windows is a crashconsistant snapshot.

That said, I would rather point at VMware. Are you up to date on patches and hotfixes? I have experianced some inconsistencies depending on the vmware tools and vmware esx versions. Might help to look into this.

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VMware tools (which you should always install inside a VM) has VSS support, so when you take a snapshot of a VM, any application that has VSS writers (such as Exchange or SQL Server) will be in a consistent state. –  rmalayter Apr 5 '12 at 13:43
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