Re: [Qemu-devel] [PATCH v5 00/11] virtio: virtio-blk data plane

[Stefan Hajnoczi <stefanha@redhat.com>]

On Thu, Dec 06, 2012 at 01:38:28PM +0200, Michael S. Tsirkin wrote:
> On Wed, Dec 05, 2012 at 09:46:59PM +0100, Stefan Hajnoczi wrote:
> > This series adds the -device virtio-blk-pci,x-data-plane=on property that
> > enables a high performance I/O codepath.  A dedicated thread is used to process
> > virtio-blk requests outside the global mutex and without going through the QEMU
> > block layer.
> > 
> > Khoa Huynh <khoa@us.ibm.com> reported an increase from 140,000 IOPS to 600,000
> > IOPS for a single VM using virtio-blk-data-plane in July:
> > 
> >   http://comments.gmane.org/gmane.comp.emulators.kvm.devel/94580
> > 
> > The virtio-blk-data-plane approach was originally presented at Linux Plumbers
> > Conference 2010.  The following slides contain a brief overview:
> > 
> >   http://linuxplumbersconf.org/2010/ocw/system/presentations/651/original/Optimizing_the_QEMU_Storage_Stack.pdf
> > 
> > The basic approach is:
> > 1. Each virtio-blk device has a thread dedicated to handling ioeventfd
> >    signalling when the guest kicks the virtqueue.
> > 2. Requests are processed without going through the QEMU block layer using
> >    Linux AIO directly.
> > 3. Completion interrupts are injected via irqfd from the dedicated thread.
> > 
> > To try it out:
> > 
> >   qemu -drive if=none,id=drive0,cache=none,aio=native,format=raw,file=...
> >        -device virtio-blk-pci,drive=drive0,scsi=off,x-data-plane=on
> > 
> > Limitations:
> >  * Only format=raw is supported
> >  * Live migration is not supported
> >  * Block jobs, hot unplug, and other operations fail with -EBUSY
> >  * I/O throttling limits are ignored
> >  * Only Linux hosts are supported due to Linux AIO usage
> > 
> > The code has reached a stage where I feel it is ready to merge.  Users have
> > been playing with it for some time and want the significant performance boost.
> > 
> > We are refactoring QEMU to get rid of the global mutex.  I believe that
> > virtio-blk-data-plane can eventually become the default mode of operation.
> > 
> > Instead of waiting for global mutex removal efforts to finish, I want to use
> > virtio-blk-data-plane as an example device for AioContext and threaded hw
> > dispatch refactoring.  This means:
> > 
> > 1. When the block layer can bind to an AioContext and execute I/O outside the
> >    global mutex, virtio-blk-data-plane can use this (and gain image format
> >    support).
> > 
> > 2. When hw dispatch no longer needs the global mutex we can use hw/virtio.c
> >    again and perhaps run a pool of iothreads instead of dedicated data plane
> >    threads.
> > 
> > But in the meantime, I have cleaned up the virtio-blk-data-plane code so that
> > it can be merged as an experimental feature.
> 
> I mostly looked at the virtio side of the patchset.
> I don't see any bugs here. I sent some improvement suggestions but
> we can do them in tree as well.

Thanks Michael.  I'll send follow-up patches to split the iov_discard()
function and to address config-wce.

Stefan