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Re: [Qemu-block] [RFC PATCH 0/2] Increase usability of external snapshot

From: Roman Kagan
Subject: Re: [Qemu-block] [RFC PATCH 0/2] Increase usability of external snapshots
Date: Fri, 2 Mar 2018 19:15:37 +0300
User-agent: Mutt/1.9.2 (2017-12-15)

On Tue, Feb 27, 2018 at 12:56:49PM +0100, Richard Palethorpe wrote:
> Following on from the discussion about creating savevm/loadvm QMP
> equivalents. I decided to take the advice given that we should use external
> snapshots. However reverting to a snapshot currently requires QEMU to be
> restarted with "-incoming".  Restarting QEMU requires a fair bit of
> book keeping to be done by the management application and may take
> measurably more time.

AFAICT "-incoming" is not the only reason for starting QEMU anew.  The
block devices will need to be pointed at different nodes in the backing
chains.  Moreover the snapshot you're reverting to may have been taken
at a point where the VM had different configuration than it has now.

So the management application will need to do a lot of bookkeeping stuff
anyway, and it'll probably have harder time applying all of the
configuration changes to a live QEMU instance.

Is the cost of killing the old QEMU process and starting a new one big
enough to be worth all the hassle?


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