29 09 2010
Explanation of VMware Snapshot Files
For this explanation I’ll use my current SharePoint deployment testing environment. I had installed Windows Server 2008R2 then created a series of snapshots whilst I preceded to promote to Domain Controller (dcpromo), installed SQL Server 2008, PerformancePoint Server 2008 and finally SharePoint Server 2007.
I had pre-grown my virtual hard disk to approx 50GB prior, and the VM has 2GBs of RAM.
Here is the file structure of this particular VMs directory:
In this article, I am only interested (and will be looking at) files that start “SharePointVM-“, that is, the name of my virtual machine immediately followed by a hyphen. This is an easy way to look out for files created by a Snapshot. The rest of the files are standard VMware files and should be documented further on the VMware website.
First point of attention is the file SharePointDevVM-flat.vmdk which is the size 52GB. This is the main virtual hard disk, the starting point of all the snapshots.
You’ll also spot 5 pairs of “SnapshotX” files, each with a vmem file and a vmsn file. The VMem files are all the same size, as they describe the virtual “RAM” of the virtual machine at the point the snapshot was taken. This is sort of like hibernation files used by Windows, when it saves the running state to disk. These snapshots are also accompanied by vmsn files which contain further configuration and machine state details. These two files are required in combination in order to power on the virtual machine at the saved state.
There are also five VMDK or virtual hard disk files, each one describing the “delta” or difference since the last snapshot was made. VMware called this a “re-do” log file.
Let’s have a look at the VMDK files to see what’s been going on since each snapshot.
The first is around 9GB. This is my flat install of Server 2008 R2.
The second is 335MB. This was just after I ran dcpromo.
The third is 4.7GB. This was after the SQL Server 2008 R2 install.
The fourth is 560MB, and this was from the MOSS install, and finally…
The fifth is 124MB, from after the PerformancePoint install.
Apart from the first file weighing in at 9GB, the rest are only slightly relative to the data installed, and as time goes on and more snapshots made, this may prove inaccurate, as the VMDK from the snapshot also stores what files from the original VMDK were deleted. This is described in the VMSN file (may need confirming!).
If I find out any more through my trawls, I’ll be sure to post it here!
This information is provided on an informatory basis as a result of my own curiosity and research. No warranty is provided here and all information is provided “as-is”. Full product support from VMware should be sought for any technical queries, and I disclaim all liability from any data loss incurred through reading this information. Whatever you do, take a backup!!
Futher information can be found at: http://www.vmware.com/support/ws55/doc/ws_learning_files_in_a_vm.html