Revision 454ae734
ID | 454ae734f1d9f591345fa78376435a8e74bb4edd |
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 <stefano.stabellini@eu.citrix.com>
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Files
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