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[Xen-users] Feature request: snapshot again

To: Xen-users@xxxxxxxxxxxxxxxxxxx
Subject: [Xen-users] Feature request: snapshot again
From: Alain Barthe <ab266061@xxxxxxxxx>
Date: Mon, 24 Aug 2009 16:28:32 +0200
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Hello everybody,

Many requests for Xen are: we need a snapshot mechanism like VMWare. Xen does not provide the feature that allows this kind of snapshot.

With VMWare snapshot, you can do a snapshot on an alive VM, just by clicking a button or issuing a command. The VM is kept alive as nothing happened.
When a VM is reverted from a snapshot, it runs at the same state it was when the snapshot occured (except network connections, I suppose).

To do this, VMWare takes a snapshot of both the memory and the disks. That is the point missed by xen: there is no way to synchronise a kind of suspend with a disk snapshot.

This should be easy to implement, by reusing the live migration feature. We need to have 2 new xm commands:

- snap_and_pause: this function dump the memory of the running VM in a file, using the way the live migration does (instead of sending data through the network, they are stored in a file). After that, the VM is left in a paused state.

- start_from_snap: a VM is created from a file previously created by the previous command, as a VM magically appears on the target dom0 when you live-migrate a VM.

With those 2 commands, the snapshot becomes trivial to script:
- run "snap_and_pause" on the VM
- take a disk snapshot by the appropiate way depending your setup (LVM snapshots, etc)
- unpause the VM

To revert to a snapshot:
- switch the disk snapshot with the original
- run "start_from_snap".

What are your opinions about this? Is it feasable? Have I missed some point?

Several month ago, I heard about a patch on the live migration function to store the memory in a file instead of sending it through the network. I didn't retreive the thread.
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