Re: [Qemu-devel] [PATCH v2 1/2] raw-posix: warn about BDRV_O_NATIVE_AIO if libaio is unavailable

[Kevin Wolf <kwolf@redhat.com>]

Am 23.07.2015 um 12:09 hat Christian Borntraeger geschrieben:
> Am 17.07.2015 um 16:23 schrieb Stefan Hajnoczi:
> > raw-posix.c silently ignores BDRV_O_NATIVE_AIO if libaio is unavailable.
> > It is confusing when aio=native performance is identical to aio=threads
> > because the binary was accidentally built without libaio.
> > 
> > Print a deprecation warning if -drive aio=native is used with a binary
> > that does not support libaio.  There are probably users using aio=native
> > who would be inconvenienced if QEMU suddenly refused to start their
> > guests.  In the future this will become an error.
> > 
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> 
> had that myself on a freshly installed system without libaio-devel.
> Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
> 
> 
> Another thing. Would it make sense to change the default to aio=native somewhen?
> From what I can tell this seems to outperform aio=threads in most cases.

aio=native requires cache.direct=on, which is not portable to all
platforms that qemu supports, and also doesn't work with all filesystems
on Linux (most notably tmpfs fails). Also recent benchmarks seem to
suggest that currently there is no clear winner between aio=native and
aio=threads, it depends too much on the host storage and the workload.

When we discussed the default cache mode a while back (with the options
cache=writeback and cache=none), it was considered more important to
have a default setting that works everywhere and performs good for quick
ad-hoc VMs and development/debugging work than one that performs best in
enterprise setups that should use a management tool anyway.

Kevin