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Re: [PULL 15/18] qapi: introduce x-query-ramblock QMP command


From: Daniel P . Berrangé
Subject: Re: [PULL 15/18] qapi: introduce x-query-ramblock QMP command
Date: Thu, 9 Jun 2022 11:19:22 +0100
User-agent: Mutt/2.2.1 (2022-02-19)

On Thu, Jun 09, 2022 at 12:07:31PM +0200, Claudio Fontana wrote:
> Hello all,
> 
> it would be really good to be able to rely on this command or something 
> similar,
> to be able to know the approximate size of a migration before starting it.
> 
> in QEMU ram_bytes_total() returns what I would like to have,
> but there is currently no QMP way to get it without starting a migration,
> which when trying to optimize it/size it is just about too late.

Aside from the main VM RAM, what other RAM blocks are likely to have
a size large enough to be of consequence to the live migration
data copy, and whose size is not already known to the mgmt app from
the guest config choices it made ? VGA RAM could be a few 100MB I
guess, but the mgmt app knows about that. I've always assumed everything
else is just noise in comparison to the main RAM region.

Still I wonder how useful this is as its just a static figure, and the
problems with migration transfer are the bulking up of data when the
VM is repeatedly dirtying stuff at a high rate.

> Do you think x-query-ramblock could be promoted to non-experimental?

It would have to be re-written, as this current impl is just emitting
a huge printf formatted string. To be considered supportable, the data
would have to be formally modelled in QAPI instead.

IOW, it would be a case of introducing a new command that emits formal
data, convertintg 'info ramblock' to use that, and then deprecating this 
x-query-ramblock.

> Should another one be made available instead, like :
> query-ram-bytes-total ?

That would be simpler if you're just wanting it to give a single
figure.


With regards,
Daniel
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