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Re: [Qemu-devel] Combining -loadvm and -snapshot

From: Markus Armbruster
Subject: Re: [Qemu-devel] Combining -loadvm and -snapshot
Date: Wed, 13 Feb 2019 07:07:44 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux)

Cc'ing the QCOW2 folks.

Drew DeVault <address@hidden> writes:

> I recently ran into an issue where I found I couldn't combine the
> -loadvm and -snapshot flags, nor any conceivable combination of
> alternate approaches like loadvm via the monitor. Independently, both
> options work as expected, but together I get this error:
> qemu-system-x86_64: Device 'virtio0' does not have the requested snapshot 
> 'base'
> The goal here is to resume the VM state from a snapshot, but to prevent
> the guest from persisting writes to the underlying qcow2.
> I started digging into the code to understand this problem more, and I
> was pretty deep in the weeds when I realized what the underlying problem
> probably was and the kind of refactoring necessary to fix it - so I'm
> here to touch base before moving any further.
> I believe this happens because -snapshot creates a temporary qcow2
> overlaid on top of the disk you're using, and this overlay does not have
> any snapshots copied, nor does any of the snapshot reading code (e.g.
> qcow2_snapshot_list or qcow2_snapshot_goto) iterate over backing disks
> to load their snapshots.
> At first I was going to adjust the qcow2 snapshot loading code (those
> two functions in particular) to read through their backends, but I'm a
> little unfamiliar with this code and the refactoring is not minor so I
> would like to get feedback from some of the wiser folks on this mailing
> list before I sink too much time into this.
> Thoughts?
> --
> Drew DeVault

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