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[Xen-devel] [PATCH v2] introduce a cache options for PV disks

Document a per-disk cache option in the xl config file to allow users to
select the cache mode that the backend should use to open the disk file
or device.

Document a "cache" xenstore backend option, part of the vbd xenstore

Add a reference to xen/include/public/io/blkif.h in

Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>

diff --git a/docs/misc/vbd-interface.txt b/docs/misc/vbd-interface.txt
index 3952e73..f873db0 100644
--- a/docs/misc/vbd-interface.txt
+++ b/docs/misc/vbd-interface.txt
@@ -125,3 +125,9 @@ because they directly map the bottom 8 bits of the xenstore 
 directly to the Linux guest's device number and throw away the rest;
 they can crash due to minor number clashes.  With these guests, the
 workaround is not to supply problematic combinations of devices.
+Other frontend and backend options
+See xen/include/public/io/blkif.h for the full list of options.
diff --git a/docs/misc/xl-disk-configuration.txt 
index 5bd456d..51d376f 100644
--- a/docs/misc/xl-disk-configuration.txt
+++ b/docs/misc/xl-disk-configuration.txt
@@ -178,6 +178,25 @@ information to be interpreted by the executable program 
 These scripts are normally called "block-<script>".
+Description:           Specifies what type of disk cache to use in the backend
+Supported values:      writeback, writethrough, none
+Mandatory:             No
+Default value:         depends on the backend type
+This does not affect the guest's view of the device. 
+Not all backends implement all cache modes: "phy" only supports "none",
+"qdisk" supports all of them and defaults to "writeback".
+It's important to note that if you are storing the VM disk on a network
+filesystem or a network block device (NFS or ISCSI) it might not be safe
+to use "none", see
+Otherwise using "none" is safe and gives you better performances.
diff --git a/xen/include/public/io/blkif.h b/xen/include/public/io/blkif.h
index b9b9d98..f4a271c 100644
--- a/xen/include/public/io/blkif.h
+++ b/xen/include/public/io/blkif.h
@@ -97,6 +97,16 @@
  *      The type of the backing device/object.
+ *
+ * cache
+ *      Values:         "none", "writeback", "writethrough"
+ *      Optional
+ *
+ *      The cache mode that the backend should use to open the backing
+ *      device/object. The backend is not required to support all the
+ *      possible cache modes. If an unsupported cache is requested, the
+ *      backend is free to ignore it.
+ *
  *--------------------------------- Features ---------------------------------
  * feature-barrier

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