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 backend options that are part of the vbd xenstore interface. The existing "mode" and "device-type" as well as the new "cache". Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com> diff --git a/docs/misc/vbd-interface.txt b/docs/misc/vbd-interface.txt index 3952e73..0584e78 100644 --- a/docs/misc/vbd-interface.txt +++ b/docs/misc/vbd-interface.txt @@ -96,6 +96,29 @@ Guests SHOULD ignore numbers that they do not understand or recognise. They SHOULD check supplied numbers for validity. +Other backend options +--------------------- + +The interface introduces few xenstore nodes to specify other backend +options: + +* mode +Description: file or device opening flags +Mandatory: Yes +Supported values: "w" (read/write), "r" (read-only) + +* cache +Description: file or device cache mode +Mandatory: No +Supported values: "none" (no cache), "writeback" (writeback + caching), "writethrough" (writethrough caching) +* device-type +Description: type of device the backend corresponds to +Mandatory: No +Supported values: "cdrom", "disk" + + + Notes on Linux as a guest ------------------------- diff --git a/docs/misc/xl-disk-configuration.txt b/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 <script>, These scripts are normally called "block-<script>". +cache=<cache> +------------- + +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 +http://lists.xen.org/archives/html/xen-devel/2012-12/msg01154.html. +Otherwise using "none" is safe and gives you better performances. + + + =========================================== DEPRECATED PARAMETERS, PREFIXES AND SYNTAXES
On Wed, 2013-06-26 at 18:43 +0100, Stefano Stabellini wrote:> 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 backend options that are part of the vbd xenstore interface. > The existing "mode" and "device-type" as well as the new "cache".The exiting ones are already documented in xen/include/public/io/blkif.h It''s a bit of a shame that things are split over two files but I think putting the new one in blkif.h is better than duplicating things further.> > Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com> > > diff --git a/docs/misc/vbd-interface.txt b/docs/misc/vbd-interface.txt > index 3952e73..0584e78 100644 > --- a/docs/misc/vbd-interface.txt > +++ b/docs/misc/vbd-interface.txt > @@ -96,6 +96,29 @@ Guests SHOULD ignore numbers that they do not understand or > recognise. They SHOULD check supplied numbers for validity. > > > +Other backend options > +--------------------- > + > +The interface introduces few xenstore nodes to specify other backend > +options: > + > +* mode > +Description: file or device opening flags > +Mandatory: Yes > +Supported values: "w" (read/write), "r" (read-only) > + > +* cache > +Description: file or device cache mode > +Mandatory: No > +Supported values: "none" (no cache), "writeback" (writeback > + caching), "writethrough" (writethrough caching)I''m not sure the bracketed descriptions are adding anything.> +* device-type > +Description: type of device the backend corresponds to > +Mandatory: No > +Supported values: "cdrom", "disk" > + > + > + > Notes on Linux as a guest > ------------------------- > > diff --git a/docs/misc/xl-disk-configuration.txt b/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 <script>, > These scripts are normally called "block-<script>". > > > +cache=<cache> > +------------- > + > +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 > +http://lists.xen.org/archives/html/xen-devel/2012-12/msg01154.html. > +Otherwise using "none" is safe and gives you better performances. > + > + > + > > ===========================================> DEPRECATED PARAMETERS, PREFIXES AND SYNTAXES > > _______________________________________________ > Xen-devel mailing list > Xen-devel@lists.xen.org > http://lists.xen.org/xen-devel
On Thu, 27 Jun 2013, Ian Campbell wrote:> On Wed, 2013-06-26 at 18:43 +0100, Stefano Stabellini wrote: > > 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 backend options that are part of the vbd xenstore interface. > > The existing "mode" and "device-type" as well as the new "cache". > > The exiting ones are already documented in xen/include/public/io/blkif.h > > It''s a bit of a shame that things are split over two files but I think > putting the new one in blkif.h is better than duplicating things > further.Ah, that''s why I couldn''t find it under docs!> > Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com> > > > > diff --git a/docs/misc/vbd-interface.txt b/docs/misc/vbd-interface.txt > > index 3952e73..0584e78 100644 > > --- a/docs/misc/vbd-interface.txt > > +++ b/docs/misc/vbd-interface.txt > > @@ -96,6 +96,29 @@ Guests SHOULD ignore numbers that they do not understand or > > recognise. They SHOULD check supplied numbers for validity. > > > > > > +Other backend options > > +--------------------- > > + > > +The interface introduces few xenstore nodes to specify other backend > > +options: > > + > > +* mode > > +Description: file or device opening flags > > +Mandatory: Yes > > +Supported values: "w" (read/write), "r" (read-only) > > + > > +* cache > > +Description: file or device cache mode > > +Mandatory: No > > +Supported values: "none" (no cache), "writeback" (writeback > > + caching), "writethrough" (writethrough caching) > > I''m not sure the bracketed descriptions are adding anything.OK> > +* device-type > > +Description: type of device the backend corresponds to > > +Mandatory: No > > +Supported values: "cdrom", "disk" > > + > > + > > + > > Notes on Linux as a guest > > ------------------------- > > > > diff --git a/docs/misc/xl-disk-configuration.txt b/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 <script>, > > These scripts are normally called "block-<script>". > > > > > > +cache=<cache> > > +------------- > > + > > +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 > > +http://lists.xen.org/archives/html/xen-devel/2012-12/msg01154.html. > > +Otherwise using "none" is safe and gives you better performances. > > + > > + > > + > > > > ===========================================> > DEPRECATED PARAMETERS, PREFIXES AND SYNTAXES > > > > _______________________________________________ > > Xen-devel mailing list > > Xen-devel@lists.xen.org > > http://lists.xen.org/xen-devel > >
On Thu, 2013-06-27 at 12:25 +0100, Stefano Stabellini wrote:> On Thu, 27 Jun 2013, Ian Campbell wrote: > > On Wed, 2013-06-26 at 18:43 +0100, Stefano Stabellini wrote: > > > 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 backend options that are part of the vbd xenstore interface. > > > The existing "mode" and "device-type" as well as the new "cache". > > > > The exiting ones are already documented in xen/include/public/io/blkif.h > > > > It''s a bit of a shame that things are split over two files but I think > > putting the new one in blkif.h is better than duplicating things > > further. > > Ah, that''s why I couldn''t find it under docs!yeah ;-) If you are reediting this file anyway perhaps you could insert a reference (in both directions perhaps)> > > > > Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com> > > > > > > diff --git a/docs/misc/vbd-interface.txt b/docs/misc/vbd-interface.txt > > > index 3952e73..0584e78 100644 > > > --- a/docs/misc/vbd-interface.txt > > > +++ b/docs/misc/vbd-interface.txt > > > @@ -96,6 +96,29 @@ Guests SHOULD ignore numbers that they do not understand or > > > recognise. They SHOULD check supplied numbers for validity. > > > > > > > > > +Other backend options > > > +--------------------- > > > + > > > +The interface introduces few xenstore nodes to specify other backend > > > +options: > > > + > > > +* mode > > > +Description: file or device opening flags > > > +Mandatory: Yes > > > +Supported values: "w" (read/write), "r" (read-only) > > > + > > > +* cache > > > +Description: file or device cache mode > > > +Mandatory: No > > > +Supported values: "none" (no cache), "writeback" (writeback > > > + caching), "writethrough" (writethrough caching) > > > > I''m not sure the bracketed descriptions are adding anything. > > OK > > > > +* device-type > > > +Description: type of device the backend corresponds to > > > +Mandatory: No > > > +Supported values: "cdrom", "disk" > > > + > > > + > > > + > > > Notes on Linux as a guest > > > ------------------------- > > > > > > diff --git a/docs/misc/xl-disk-configuration.txt b/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 <script>, > > > These scripts are normally called "block-<script>". > > > > > > > > > +cache=<cache> > > > +------------- > > > + > > > +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 > > > +http://lists.xen.org/archives/html/xen-devel/2012-12/msg01154.html. > > > +Otherwise using "none" is safe and gives you better performances. > > > + > > > + > > > + > > > > > > ===========================================> > > DEPRECATED PARAMETERS, PREFIXES AND SYNTAXES > > > > > > _______________________________________________ > > > Xen-devel mailing list > > > Xen-devel@lists.xen.org > > > http://lists.xen.org/xen-devel > > > >
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