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

From: Alberto Garcia
Subject: Re: [Qemu-devel] [PATCH v0 2/2] qmp: add block-set-copy-on-read command
Date: Thu, 14 Jun 2018 11:03:22 +0200
User-agent: Notmuch/0.18.2 (http://notmuchmail.org) Emacs/24.4.1 (i586-pc-linux-gnu)

On Thu 14 Jun 2018 11:00:55 AM CEST, Kevin Wolf <address@hidden> wrote:
> 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 This patch would definitely be superseded by a block reconfiguration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.

They're coming, it's just a bit tricker than I initially though. I'll
probably send the first series as an RFC if I don't manage to fix all
problems, so we can discuss them a bit.


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