Re: QEMU/KVM: virtio-blk asynchronous IO

[Anthony Liguori <anthony@codemonkey.ws>]

Marcelo Tosatti wrote:
> Use asynchronous IO in the virtio block QEMU driver.
>
> virtio_blk_handle_output should not block for long periods, since it
> holds the mutex lock prohibiting other vcpu's from doing IO to QEMU
> devices. Without AIO write intensive benchmarks make guests hang for
> several seconds. Write performance also increases significantly.
>
> Also report errors properly.
>
> To take full advantage of parallel IO we need to allow for more than AIO
> thread per-fd, or use direct IO (-nocache) which uses kernel AIO.
>
> Separate patch allows virtio-block guest driver to queue more than one
> element in the virtio ring.
>
> Anthony, this patch abuses the virtqueue_push() interface by passing a
> VirtQueueElement with only "index" member valid, since we know this is
> all it uses. Doing so avoids allocation, zeroing and copy of an entire
> VirtQueueElement structure. What do you say?
>   

So I'd like to do this a little differently.  In a few minutes, I'll be 
sending a virtio patch series to QEMU.  In it, I introduce a standard 
way to deal with scatter/gather lists.  Assuming that goes well, I think 
we should add a proper bdrv_aio_readv/writev interface that takes an 
IOVector.  That simplifies the virtio-blk support quite a bit.  We also 
need to implement a reset handler when we introduce aio support that 
cancels and pending aio requests.

Regards,

Anthony Liguori


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