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Analysis of Sparse VMDK File

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I did some analysis of the layout of a VMDK file. The graph below shows this analysis: x-axis is the distance in the vmdk file for two logically adjacent blocks(sectors). For for example is block 33 is at offset 3467 and  block 34 is at offset 3468 then the distance in the vmdk file is 3468-3467=1.  The Y-axis is log to the base 2 of the frequency of the distance.

Distance (in VMDK file) of logically adjacent blocks

The graph shows that most logically adjacent blocks have adjacent offsets in the vmdk file. This file had a grain size of 128, so you would expect that only 2/128 sectors would have non-adjacent neighbours (1/64). That is roughly the case. The rest of the distances are clustered around multiples of 128 (as one would expect?). I am not sure if this has to do with NTFS allocating blocks in some sequence or VMW workstation expanding the disk in a particular way.

The vmdk was a windows vmdk 4GB disk with windows XP. Very little use after installing the base OS. Installed a couple of packages like Flex Builder.

Another interesting plot would be to plot the actual distance on the physical disk on the host. Thats next.

Written by RS

December 4, 2008 at 2:46 pm

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