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

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'/>