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Re: bitmaps -- copying allocation status into dirty bitmaps

From: Denis V. Lunev
Subject: Re: bitmaps -- copying allocation status into dirty bitmaps
Date: Fri, 1 Nov 2019 15:49:34 +0000

On 11/1/19 4:42 PM, 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.
original problem was a little bit incorrect. After some thoughts I found
that this is NOT enough. We should also add zeroed clusters to the
bitmap to merge! They do cover some data clusters from the original

Thus we should either provide "ALLOCATED" bitmap for other purposes,
and we should supply "CHANGED" which contains allocated AND
explicitly zeroed clusters.

> 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.
> Also, with regards to introspection, if we do create a special reserved
> name like #ALLOCATED, we need to make sure that this is available and
> obvious via the QAPI schema.
> --js

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