With both dynamically expanding and differencing virtual hard disks – the following pattern is used when writing data:
- Check to see if data has already been written to this area of the disk
- If yes – write new data to the same location as the old data
- If no – create new blocks on the disk, and record the new block location in the block allocation table for the virtual hard disk
Depending on the write patterns experience by a virtual hard disk – these blocks can end up being poorly aligned, or “fragmented”. You can check the fragmentation level of a virtual machines virtual hard disks by running the following command in PowerShell:
get-vm “VM Name” | Get-VMHardDiskDrive | get-vhd | Select path, FragmentationPercentage
If you believe this is causing a performance problem in your situation – there are a number of ways to defragment a virtual hard disk. But the easiest way is to perform a storage migration – which will re-layout the blocks in a linear fashion.