Re: linux-aio usable?

Avi Kivity
2010-03-08T16:36:57Z

On 03/08/2010 06:28 PM, Anthony Liguori wrote:
>> I thought there was some autodetection involved, but perhaps I just
>> imagined it.
>
>
> There's no autodetection.
>
> linux-aio support in the kernel downgrades to synchronous IO if the
> underlying storage does not support linux-aio. There is no indication
> to userspace that this has happened.
>
> If this happens, besides having a slow guest, the guest VCPU will be
> starved during the I/O requests potentially resulting in things like
> soft lockups and time drift.
>
> Generally, speaking, linux-aio will work well under the following
> circumstances:
>
> - cache=off is specified
> - the underlying file system is XFS or you are using a block device
>
> We cannot detect this reliably though so it's really up to the user to
> decide whether to use it. We're working on improving the linux-aio
> kernel interface though to eliminate this detectability problem after
> which, we can enable it in a more automatic fashion.

Well, the common case of cache=none on a block device certainly can be
autodetected.

--
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