Why are snapshots with memory so slow?

[jedijeff]

Can someone please explain why snapshot with memory are so much slower than without? Snapping a 400gb vm with 8gb a ram without memory is extremely fast. Snap it with memory and it could take 15 minutes at least.

Looking at the datastores in esx, I do not see any data being queued or io pending on the san. I just dont see where the bottleneck would be.

[a_nut_in · Accepted solution]

Hey Jeff,

http://kb.vmware.com/kb/1015180

<snip>

Snapshot creation takes too long when specifying the memory snapshot option.

.....

Memory: If the <memory> flag is 1 or true, a  dump of the internal state of the virtual machine is included in the  snapshot. Memory snapshots take longer to create.

</snip>

So essentially, say you have a fileserver and you try and take a snapshot with memory state when lot's of users are logged in and copying/modifying files. Essentially the Guest OS memory state would keep on changing as new data is read off or written to.

This uses the MS Checkpoint/VSS technology and basically the more writes that happen, the more time the snapshot creation would take,

In the above example, say you next take a memory snapshot when the machine is idle, that would be a lot quicker as essentially, there would be less data that is in memory in a dynamic state.

Hope that helps!

Regards

a

[jedijeff]

Thanks!

[a_nut_in]

No worries