[libvirt] [PATCH] Support for qemu aio drive option

[Eric Blake]

On 05/17/2010 03:31 PM, Eric Blake wrote:
> [adding Matthias on cc]

Reviving an old thread, as it is something I plan on working in the near
future.

> 
> On 05/17/2010 03:07 PM, Chris Lalancette wrote:
>> On 05/12/2010 11:26 PM, Eric Blake wrote:
>>> From: Matthias Dahl <mdvirt at designassembly.de>
>>>
>>> qemu allows the user to choose what io storage api should be used, either the
>>> default (threads) or native (linux aio) which in the latter case can result in
>>> better performance.
>>>
>>
>> The implementation looks perfectly reasonable.  I'm just concerned that the
>> concept of what we are doing is too qemu specific, though.  Basically, I think
>> what we are trying to model here is the concept of an I/O backend implementation,
>> correct?  Should we maybe change this to be "<iobackend type='%s'/>", and then
>> have available enums like:
>>
>> aiothreads
>> aionative
>> ...
>>
>> That way, for other hypervisors that do something different (like VirtualBox,
>> which just has AIO on/off), we can have additional enums to describe their
>> behavior.  Even further, if a given hypervisor wanted to do something like
>> "Direct I/O" for the I/O backend (as an example), we could also use this element
>> to specify that.  What do you think?
> 
> Indeed, having a more-generic attribute "type", where only a subset of
> known enum values are appropriate per a given hypervisor, makes sense to
> me.

I'm thinking that the following XML changes make the most sense for this
(note that the choice of aio mode is per-disk), where the change is the
addition of the new <iobackend> element:

<domain ...>
  <devices>
    <disk type='block' device='disk'>
      <driver name='qemu' type='qcow2' cache='none'/>
      <source dev='...'/>
      <target dev='hda' bus='ide'/>
      <iobackend type='aiothreads'/>
    </disk>
  </devices>
</domain>

If <iobackend> is missing, it is the same as type='default'; type can be
'default' (whatever qemu prefers, which may differ over qemu versions),
'aiothreads' (use ,aio=threads in qemu -drive argument), or 'aionative'
(use ,aio=native in qemu -drive argument); where we can add other types
later if needed.

Does this deserve a new <iobackend> element, or should it merely be a
new optional attribute of the existing <driver> element, as in:

<driver name='qemu' type='qcow2' cache='none' iobackend='aiothreads'/>

-- 
Eric Blake   eblake at redhat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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