[libvirt] [PATCH 0/8 v5] Summary on block IO throttle

[Eric Blake]

On 11/23/2011 02:18 AM, Stefan Hajnoczi wrote:
>>>> Notes: Now all the planed features were implemented (#1#2 were implemented by
>>>> Zhi Yong Wu), the previous comments were all fixed up too. And the qemu part patches
>>>> have been accepted upstream and are expected to be part of the QEMU 1.1
>>>> release, git tree from Zhi Yong:
>>>>
>>>> http://repo.or.cz/w/qemu/kevin.git/shortlog/refs/heads/block
>>>
>>> I just realized that the block_set_io_throttle monitor command is not
>>> yet in released upstream qemu.  I will continue reviewing this series,
>>> but it's harder to justify including it into libvirt 0.9.8 unless we
>>> have a good feel that the design for qemu is stable and unlikely to
>> I strongly think that the syntax of this qemu monitor command will be
>> not changed later. :)
>> Moreover, it has not been changed since the patches for qemu block I/O
>> throttling is published.
> 
> It's good to be careful about adding libvirt features that are not yet
> in QEMU.  I got burnt by this with image streaming and have had to
> work extra hard to finally get the QEMU side merged with the libvirt
> side already set in stone.
> 
> However, Kevin has merged the QEMU I/O throttling patches into the
> block tree.  They are waiting for the 1.1 merge window because
> qemu.git has only been taking QEMU 1.0-rc patches for the past few
> weeks.  QEMU I/O throttling has been through review and accepted by
> the community:
> 
> http://patchwork.ozlabs.org/patch/124266/

How can libvirt reliably tell whether block throttling is supported?
Does that patch series alter the help output of 'qemu -h' to list
something like '-drive ...[,bps=...]'?  Is there a magic 'qemu -driver
xxx,?' call where we can see that the new bps attribute is settable?

I ask, because we need to alter libvirt to properly detect whether qemu
supports this new feature, so as to gracefully reject attempts to use
the XML if qemu does not support the feature.  See also:
https://www.redhat.com/archives/libvir-list/2012-January/msg00727.html

-- 
Eric Blake   eblake at redhat.com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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