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

[Alberto Garcia]

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 This patch would definitely be superseded by a block reconfiguration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.

Berto