Re: [Qemu-devel] [PATCH 0/5] block/qcow2-bitmap: Enable resize with persistent bitmaps

[Eric Blake <eblake@redhat.com>]

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On 3/5/19 5:43 PM, John Snow wrote:
> This series aims to enable block resizes when persistent bitmaps are
> in use. The basic approach here is to recognize that we now load all
> persistent bitmaps in memory, and so we can rely on in-memory resizes
> and then flush the changed metadata back to disk.
> 
> One part that is potentially now quite strange is that bitmap resizes
> may happen twice: once during the qcow2 resize event only if persistent
> bitmaps are found, and then again as part of the generic resize callback
> event whether or not we have any persistent bitmaps.
> 
> The second round is required if we are not using qcow2 or we have only
> transient bitmaps. The first round is required as we wish to flush the
> bitmaps back to disk atomically with the qcow2 resize to avoid violating
> our invariants for the bitmap metadata which is checked in many places.
> 
> This is harmless; hbitmap_truncate will recognize the second round as
> a no-op.

FWIW - I have not yet reviewed this series closely, but I think it would
be wise to get this initial cut in before softfreeze (we can make
further tweaks to fix bugs in assumptions during rc1 and rc2, but it's a
lot harder to add the series at all if it misses softfreeze).

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


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