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Tuesday, August 27, 2013

Independent disks in a Virtual machine

When we add a new disk in  a VM hosted in Vmware ESXi, we can choose whether the disk should be Independent or not. If we choose the disk to be independent, it is not included in snapshots. If you browse the datastore after taking a snapshot of the machine, you will not see any delta disks related to the independent disks. There are two types of independent disks:

Persistant: The data written to the disk is retained after we powercycle the machine. It is like any normal disk that we add to a machine, only difference is that we cannot return to a point in time for data in that disk

Non-persistant: Data is deleted when we powercycle the VM. I have tried it on a Windows VM and the disk is listed as unallocated space each time I powercycle the VM, thereby deleting all data saved in it. I had to initialize the disk from the disk management console and format it  as a drive before using it again. Interestingly, if we restart the windows OS the disk and the data that it contains is retained.This could be because when we restart the OS, the VM power state is not changed. It will still be ON

Note: While taking snapshot of VMs with independent disks, make sure that the option 'Snapshot the Vitual machine's memory' is not selected. If this option is selected, snapshot operation will fail with an error "cannot take memory snapshot, since the virtual machine is configured with independent disks"

This is because snapshots are used to revert to a point-in-time state of the VM, and the persistant attribute of disks prevent it from doing so. The solution is to deselect the options to snapshot the VM memory,quiesce option or to ensure that the disks are not of type 'independent'

Reference:
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1007532

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