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Should you de-fragment Virtual Disks?

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Windows de-fragmentation tool or some other commercial alternative, need 5-15% of free disk space, for the tool to be effective. Sometimes it may need more if you have some very large files (like video or database files). Below is the layout of c-drive of may virtual machine. The red segments you see are the fragmented files.

If you have a file with one large segment, for the defrag to be effective it has to move this segment to a free area and copy the rest of the segments with it to make the file contiguous. If there is no place to copy the large extent of a file, then it wont get defragmented.

The best way to de-fragment is to get an empty disk and copy all the files onto the empty disk. So the more free disk you have the better these tools will perform.

Also how you think about de-fragmentation in a virtual disk is very different than how we think about de-fragmentation in a physical world. Take the above disk it is a virtual disk 2GB Max Extent Sparse

The disk was full and then I extended the disk (with fatVM) and then defragmented one file (you can do that with Mark Russinovich’s Contig Tool http://technet.microsoft.com/en-us/sysinternals/bb897428.aspx). You can see that the files are contiguous (blue) in the extended portion. The original disk clearly requires defragmentation, but without extending it, we would not have been able to get the key database file to be contiguous.

It makes one ask the question whether you really need the traditional way of defrag the virtual disk. It is much faster to extend the disk and/or attach a separate disk and simply copy over all the files and re-place the original disk with the new extended disk.

Another advantage of doing this is that it is much faster than defragging also you can improve the performance of the virtual machine considerably. Also you can take the files which are static (don’t change)  by taking the files in a virtual machine which don’t change and making the base new disk for c-drive a flat file instead of a sparse disk as the sparse disk is not really saving you anything once you get full. If you have a parent which is flat and then a child which is sparse you get the best of both worlds.

In my limited experience instead of defrag, do the following

  • create a new flat disk, copy all the files from C: to the new disk
  • make the new disk your c: drive
  • create a clone of the base disk (which by definition is sparse)
  • extend the sparse disk

Your virtual machine’s performance will be significantly improved.

One Response

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  1. Never thought about doing it this way. Very clever!

    Diwant Vaidya

    February 25, 2010 at 11:56 pm


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