Re: [Xen-devel] Re: [Qemu-devel] [PATCH] qemu and qemu-xen: support empty write barriers in xen_disk

[Christoph Hellwig <hch@xxxxxx>]

On Wed, Nov 24, 2010 at 10:18:40AM -0800, Jeremy Fitzhardinge wrote:
> Linux wants is a useful thing to do and implement (especially since it
> amounts to standardising the ?BSD extension).  I'm not sure of their
> precise semantics (esp WRT ordering), but I think its already OK.

The nice bit is that a pure flush does not imply any odering at all.
Which is how the current qemu driver implements the barrier requests
anyway, so that needs some fixing.

> (BTW, in case it wasn't clear, we're seriously considering - but not yet
> committed to - using qemu as the primary PV block backend for Xen
> instead of submitting the existing blkback code for upstream.  We still
> need to do some proper testing and measuring to make sure it stacks up
> OK, and work out how it would fit together with the rest of the
> management stack.  But so far it looks promising.)

Good to know.  Besides the issue with barriers mentioned above there's
a few things that need addressing in xen_disk, if you (or Stefano or
Daniel) are interested:

 - remove the syncwrite tunable, as this is handled by the underlying
   posix I/O code if needed by using O_DSYNC which is a lot more
   efficient.
 - check whatever the issue with the use_aio codepath is and make it
   the default.  It should help the performance a lot.
 - Make sure to use bdrv_aio_flush for cache flushes in the aio
   codepath, currently it still uses plain synchronous flushes.

> 
>     J
---end quoted text---

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