Re: linux-aio usable?

[Avi Kivity <avi@redhat.com>]

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.

-- 
error compiling committee.c: too many arguments to function