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

[Eric Blake]

On 7/30/20 7:02 AM, 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

And that is enough to make me think this series is -rc3 material. 
Although I'm not yet sure whether the solution is this series as 
written, or to patch libvirt to look elsewhere for bitmap information, 
or to patch qemu on incoming migration to not complain when reloading a 
RO bitmap, or something else.

> 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?

I'd really like to hear from Virtuozzo on the topic before committing to 
this series, but I will at least review it in the meantime.

> 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(-)

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org