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Re: bitmaps -- copying allocation status into dirty bitmaps

From: Max Reitz
Subject: Re: bitmaps -- copying allocation status into dirty bitmaps
Date: Mon, 4 Nov 2019 12:21:23 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1

On 01.11.19 16:42, John Snow wrote:
> Hi, in one of my infamously unreadable and long status emails, I
> mentioned possibly wanting to copy allocation data into bitmaps as a way
> to enable users to create (external) snapshots from outside of the
> libvirt/qemu context.
> (That is: to repair checkpoints in libvirt after a user extended the
> backing chain themselves, you want to restore bitmap information for
> that node. Conveniently, this information IS the allocation map, so we
> can do this.)
> It came up at KVM Forum that we probably do want this, because oVirt
> likes the idea of being able to manipulate these chains from outside of
> libvirt/qemu.
> Denis suggested that instead of a new command, we can create a special
> name -- maybe "#ALLOCATED" or something similar that can never be
> allocated as a user-defined bitmap name -- as a special source for the
> merge command.
> You'd issue a merge from "#ALLOCATED" to "myBitmap0" to copy the current
> allocation data into "myBitmap0", for instance.

Sounds fun, but is there actually any use for this if the only purpose
is to work as a source for merge?

I mean, it would be interesting if it worked exactly like a perma-RO
pseudo-bitmap that whenever you try to get data from it performs a
block-status call.  But as you say, that would probably be too slow, and
it would take a lot of code modifications, so I wonder if there is
actually any purpose for this.

> Some thoughts:
> - The only commands where this pseudo-bitmap makes sense is merge.
> enable/disable/remove/clear/add don't make sense here.
> - This pseudo bitmap might make sense for backup, but it's not needed;
> you can just merge into an empty/enabled bitmap and then use that.
> - Creating an allocation bitmap on-the-fly is probably not possible
> directly in the merge command, because the disk status calls might take
> too long...
> Hm, actually, I'm not sure how to solve that one. Merge would need to
> become a job (or an async QMP command?) or we'd need to keep an
> allocation bitmap object around and in-sync. I don't really want to do
> either, so maybe I'm missing an obvious/better solution.

All of what you wrote in this mail makes me think it would make much
more sense to just add a “block-dirty-bitmap-create-from” job with an
enum of targets.  (One of which would be “allocated-blocks”.)


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