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Re: [Qemu-devel] [PATCH v5 00/11] virtio: virtio-blk data plane
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [PATCH v5 00/11] virtio: virtio-blk data plane |
Date: |
Fri, 7 Dec 2012 07:12:55 +0100 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
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 <address@hidden> 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