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

[Vladimir Sementsov-Ogievskiy]

30.07.2020 15:02, Max Reitz wrote:
> Hi,
>
> 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
> image.
>
> 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:
>
> https://bugzilla.redhat.com/show_bug.cgi?id=1858739#c3
>
> 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?

Sorry, I was on vocation for a week, so missed this. I see, it was merged, 
still, I'll look through it now.

Hmm.

1. Why we sync bitmaps on inactivation: it's obvious, it's the last point when 
it is possible to store them

2. Why we release bitmaps on inactivation: after inactivation, the bitmaps are 
not owned by Qemu. The image is not locked, someone other can change persistent 
bitmaps (target Qemu for example)..

> 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(-)


--
Best regards,
Vladimir