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x-blockdev-reopen & block-dirty-bitmaps

From: John Snow
Subject: x-blockdev-reopen & block-dirty-bitmaps
Date: Fri, 14 Feb 2020 13:54:36 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1

Hi, what work remains to make this a stable interface, is it known?

We're having a problem with bitmaps where in some cases libvirt wants to
commit an image back down to a base image but -- for various reasons --
the bitmap was left enabled in the backing image, so it would accrue new
writes during the commit.

Normally, when creating a snapshot using blockdev-snapshot, the backing
file becomes RO and all of the bitmaps become RO too.

The goal is to be able to disable (or enable) bitmaps from a backing
file before (or atomically just before) a commit operation to allow
libvirt greater control on snapshot commit.

Now, in my own testing, we can reopen a backing file just fine, delete
or disable a bitmap and be done with it -- but the interface isn't
stable, so libvirt will be reluctant to use such tricks.

Probably a loaded question, but:

- What's needed to make the interface stable?
- Are there known problem points?
- Any suggestions for workarounds in the meantime?

I'm wary of workarounds, because I'd rather do it the right way.

That said, here's an absolutely awful workaround I thought of that I
think everyone will hate:

- Allow bitmap commands like "enable" and "disable" to be "queued" when
applied to readonly bitmaps ...
- On reopen, apply queued bitmap commands.

Eh, this is bad because it's basically creating a command that we will
immediately deprecate as soon as x-blockdev-reopen is formalized.

All ears for better solutions, though.


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