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

[Kevin Wolf]

Am 27.07.2011 21:57, schrieb Christoph Hellwig:
> On Wed, Jul 27, 2011 at 09:52:51PM +0200, Frediano Ziglio wrote:
>> 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.

I understand that there may be reasons for using O_DIRECT | O_DSYNC, but
what is the explanation for O_DSYNC improving performance?

Christoph, on another note: Can we rely on Linux AIO never returning
short writes except on EOF? Currently we return -EINVAL in this case, so
I hope it's true or we wouldn't return the correct error code.

The reason why I'm asking is because I want to allow reads across EOF
for growable images and pad with zeros (the synchronous code does this
today in order to allow bdrv_pread/pwrite to work, and when we start
using coroutines in the block layer, these cases will hit the AIO paths).

Kevin