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

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(a)designassembly.de&gt; > >>> > >>> 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?