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Re: [Xen-devel] [Qemu-devel] [PATCH v2] xen_disk: support "direct-io-safe" backend option

On 28/06/13 17:16, Stefano Stabellini wrote:
On Thu, 27 Jun 2013, Stefano Stabellini wrote:
Support backend option "direct-io-safe".  This is documented as
follows in the Xen backend specification:

  * direct-io-safe
  *      Values:         0/1 (boolean)
  *      Default Value:  0
  *      The underlying storage is not affected by the direct IO memory
  *      lifetime bug.  See:
  *        http://lists.xen.org/archives/html/xen-devel/2012-12/msg01154.html
  *      Therefore this option gives the backend permission to use
  *      O_DIRECT, notwithstanding that bug.
  *      That is, if this option is enabled, use of O_DIRECT is safe,
  *      in circumstances where we would normally have avoided it as a
  *      workaround for that bug.  This option is not relevant for all
  *      backends, and even not necessarily supported for those for
  *      which it is relevant.  A backend which knows that it is not
  *      affected by the bug can ignore this option.
  *      This option doesn't require a backend to use O_DIRECT, so it
  *      should not be used to try to control the caching behaviour.

Also, BDRV_O_NATIVE_AIO is ignored if BDRV_O_NOCACHE, so clarify the
default flags passed to the qemu block layer.

The original proposal for a "cache" backend option has been dropped
because it was believed too wide, especially considering that at the
moment the backend doesn't have a way to tell the toolstack that it is
capable of supporting it.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
Signed-off-by: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
George, should I go ahead and commit to the qemu-xen tree?

Yes, I think it's pretty important to have the override in there:

Acked-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>

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