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

[Christoph Hellwig]

On Thu, Jul 28, 2011 at 09:47:05AM +0200, Kevin Wolf wrote:
> > 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?

There isn't any, at least for modern Linux.  O_DSYNC at this point is
equivalent to a range fdatasync for each write call, and given that we're
doing O_DIRECT the ranges flush doesn't matter.  If you do have a modern
host and an old guest it might end up beeing faster because the barrier
implementation in Linux used to suck so badly, but that's not inhrent
to the I/O model.  If you guest however doesn't support cache flushes
at all O_DIRECT | O_DSYNC is the only sane model to use for local filesystems
and block devices.
 

> 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.

More or less.  There's one corner case for all Linux I/O, and that is
only writes up to INT_MAX are supported, and larger writes (and reads)
get truncated to it.  It's pretty nasty, but Linux has been vocally
opposed to fixing this issue.