Q. Are there any things to consider when using fixed-size virtual hard disk (VHD) files and virtual machine (VM) snapshots?

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October 08, 2008 Q. Are there any things to consider when using fixed-size virtual hard disk (VHD) files and virtual machine (VM) snapshots? A Web Exclusive from FAQ for Windows John Savill Virtual Systems InstantDoc #100512 FAQ for Windows	Email this Article Printer Friendly Reader Comments Digg This Del.icio.us RSS
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A. Very much so. Consider what happens when you take a snapshot of a VM. You save the memory, configuration, supporting process information, then create a differencing disk, to which all future writes are written instead of the original volume. The creation of the differencing disk for future writes is the issue. Let's say you create a fixed virtual hard disk. The instant you create a snapshot, that fixed size virtual hard disk is no longer written to. Instead all future writes will be made to the new dynamically expanding differencing virtual hard disk. The use of this dynamically expanding differencing disk removes the advantage of the fixed virtual hard disk and all the unused space in the fixed virtual hard disk will never be used unless you delete the snapshots (which causes data to be merged into the original virtual hard disk).

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