Re: [Qemu-devel] [PATCH 1/2] linux aio: support flush operation

[Christoph Hellwig <hch@lst.de>]

On Wed, Jul 27, 2011 at 09:52:51PM +0200, Frediano Ziglio wrote:
> > 
> 
> Yes! Not at kernel level :-)

In that case we have a bad error handling problem somewhere in qemu.
the IOCB_CMD_FDSYNC aio opcode will always return EINVAL currently,
and we really should have cought that somewhere in qemu.

> Thanks. I'll try to port misaligned access to Linux AIO. Also I'll add some comments on code to avoid somebody do the same mistache I did.

It's direct I/O code in general that doesn't handle misaligned access.
Given that we should never get misaligned I/O from guests I just didn't
bother duplicating the read-modify-write code for that code path as well.

> Mainly however -k qemu-img and aio=native in blockdev options are silently ignored if nocache is not enabled.

Maybe we should indeed error out instead.  Care to prepare a patch for that?

> Also I notice that combining XFS, Linux AIO, O_DIRECT and O_DSYNC give impressive performance but currently there is no way to specify all that flags together cause nocache enable O_DIRECT while O_DSYNC is enabled with writethrough.

Indeed.  This has come up a few times, and actually is a mostly trivial
task.  Maybe we should give up waiting for -blockdev and separate cache
mode settings and allow a nocache-writethrough or similar mode now?  It's
going to be around 10 lines of code + documentation.