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@xxxxxxxxxx> 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
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