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Re: [Qemu-devel] [PATCH v0 2/2] qmp: add block-set-copy-on-read command

From: Kevin Wolf
Subject: Re: [Qemu-devel] [PATCH v0 2/2] qmp: add block-set-copy-on-read command
Date: Thu, 14 Jun 2018 11:00:55 +0200
User-agent: Mutt/1.9.1 (2017-09-22)

Am 13.06.2018 um 18:41 hat Max Reitz geschrieben:
> On 2018-06-13 18:02, Eric Blake wrote:
> > On 06/13/2018 10:47 AM, Denis Plotnikov wrote:
> >> The command enables/disables copy-on-read mode for VM's disk while
> >> VM is running.
> >>
> >> This is needed when using external disk readers to shape access pattern
> >> to the disk backend.
> >>
> >> Signed-off-by: Denis Plotnikov <address@hidden>
> >> ---
> > 
> > Deferring thoughts on the actual design for later;
> But why? ;-)
> This patch would definitely be superseded by a block reconfiguration
> command (i.e. presumably one that makes reopen accessible over QMP).
> With such a command, you could insert or remove a copy-on-read filter
> node at any point in time.
> Since we definitely want block graph configuration, I don't think we
> want to add special commands now.

I agree, and it seems that we get more and more use cases for a block
reconfiguration commands. Only yesterday we talked about the "true"
blockdev way for libvirt to implement I/O throttling. The result was
that without reopen, we still need to use the old QMP command to set
BlockBackend-based I/O throttling instead of using a filter node.

So I'm eagerly awaiting Berto's promised patches for it.


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