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

From: Eric Blake
Subject: Re: [Qemu-block] [PATCH 0/5] block/qcow2-bitmap: Enable resize with persistent bitmaps
Date: Mon, 11 Mar 2019 11:18:30 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.5.1

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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