[Qemu-devel] [Bug 1184089] Re: [Feature request] loadvm snapshot as read-only

[Michael Coppola <michael.n.coppola@gmail.com>]

Awesome, looking forward to it.  I may be misunderstanding what's
happening under the hood, but at least for me, calling 'loadvm' on a
single snapshot over and over seems to work the first few times and then
immediately blue screens the WinXP guest with PFN_LIST_CORRUPT.  I was
under the assumption that all runtime modifications were being written
back to the image, effectively "corrupting" something (whether it was
changes to the snapshot or the "backing image" causing things to break).

Until then, I've seemed to have found a workaround for the feature
itself.  Instead of creating a snapshot with 'savevm', I can start the
VM with -snapshot and then call:

migrate "exec: gzip -c > snapshot.gz"

in QMP and it saves the live image to a compressed file.  Make sure it's
completed migration before exiting with "info migrate".  Subsequently
loading the snapshot with:

qemu-* <whatever flags> -incoming "exec: gzip -c -d snapshot.gz"
-snapshot

will load the live snapshot and redirect all runtime modifications to a
temp file.  http://www.linux-kvm.org/page/Migration says not to use
-snapshot, but who follows the rules anyways? ;)  It seems to work so
far and things haven't exploded yet.  Running md5sum on the qcow2 image
and gzip snapshot before and after shows no changes to either files.

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https://bugs.launchpad.net/bugs/1184089

Title:
  [Feature request] loadvm snapshot as read-only

Status in QEMU:
  New

Bug description:
  There are many ways to take and manage snapshots in QEMU, but one main
  feature that's missing is the ability to 'loadvm' a LIVE snapshot and
  have all future changes redirected to a temporary file.  This would
  effectively be combining the -loadvm and -snapshot switches and make
  the snapshot read-only.  With this feature, users would be provided a
  "sandbox" and be able to start and restart the same live snapshot
  without corrupting the image in doing so.

  I found a lot of discussion about this topic on the mailing list years
  ago, including some patch submissions, but none of the conversations
  panned out.

  http://lists.gnu.org/archive/html/qemu-discuss/2011-10/msg00011.html
  http://copilotco.com/mail-archives/qemu.2008/msg00072.html
  http://web.archiveorange.com/archive/v/1XS1vcusGInZKG2e0ImX
  http://marc.info/?l=qemu-devel&m=117191084713590

  What would it take for this feature to be added, and can we use the
  patches submitted by Eddie Kohler to enable this feature?

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