Re: linux-aio usable?

[Anthony Liguori <anthony@codemonkey.ws>]

On 03/08/2010 08:26 AM, Avi Kivity wrote:
> On 03/08/2010 04:25 PM, Dustin Kirkland wrote:
>> On Mon, Mar 8, 2010 at 3:48 AM, Avi Kivity<avi@redhat.com>  wrote:
>>> On 03/08/2010 11:48 AM, Bernhard Schmidt wrote:
>>>> On Mon, Mar 08, 2010 at 11:10:29AM +0200, Avi Kivity wrote:
>>>>>> Are there any potential pitfalls?
>>>>> It won't work well unless running on a block device (partition or 
>>>>> LVM).
>>>> What does "work well" mean in this context? Potential dataloss?
>>> No, it becomes synchronous (=extra slow).
>> But for this to happen, the user would have had to consciously enter
>> into the situation by creating/using a non block device,
>> non-pre-allocated backing disk AND specify the aio=native option,
>> correct?
>>
>
> 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.

Regards,

Anthony Liguori