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Revision 454ae734

ID454ae734f1d9f591345fa78376435a8e74bb4edd

Added by Stefano Stabellini over 10 years ago

xen_disk: support "direct-io-safe" backend option

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 <>
Signed-off-by: Ian Jackson <>

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