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[PATCH 0/2] qcow2: Release read-only bitmaps when inactivated

From: Max Reitz
Subject: [PATCH 0/2] qcow2: Release read-only bitmaps when inactivated
Date: Thu, 30 Jul 2020 14:02:32 +0200


When beginning migration, the qcow2 driver syncs all persistent bitmaps
to disk and then releases them.  If the user decides to continue on the
source after migration, those bitmaps are re-loaded from the qcow2

However, we only do this for bitmaps that were actively synced, i.e. R/W
bitmaps.  RO bitmaps (those on backing images) are not written and thus
not released.  However, we still try to re-load them when continuing,
and that will then fail.

To fix this problem, I think we should just consider RO bitmaps to be in
sync from the beginning, so we can release them just like bitmaps that
we have actively written back to the image.  This is done by patch 1.

However, there’s a catch: Peter Krempa noted that it isn’t in libvirt’s
interest for the bitmaps to be released before migration at all, because
this makes them disappear from query-named-block-node’s dirty-bitmaps
list, but libvirt needs the bitmaps to be there:


If it’s really not feasible to keep the bitmaps around, then I suppose
what might work for libvirt is to query
image/format-specific/data/bitmaps in addition to dirty-bitmaps (every
bitmap that we released before migration must be a persistent bitmap).

What are your thoughts on this?

Max Reitz (2):
  qcow2: Release read-only bitmaps when inactivated
  iotests/169: Test source cont with backing bmap

 block/qcow2-bitmap.c       | 23 +++++++++++---
 tests/qemu-iotests/169     | 64 +++++++++++++++++++++++++++++++++++++-
 tests/qemu-iotests/169.out |  4 +--
 3 files changed, 84 insertions(+), 7 deletions(-)


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