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

[Chris Lalancette]

On 01/03/11 - 04:43:06PM, Eric Blake wrote:
> 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'/>

I slightly prefer making it a new attribute of <driver>.  The only reason I
can see to make it a new <iobackend> element is if we think it will get
significantly more complex in the future (necessitating new XML elements).  Do
we think that?

-- 
Chris Lalancette