Re: [Xen-devel] introduce a cache options for PV disks

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

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@xxxxxxxxxxxxx>
> > 
> > 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@xxxxxxxxxxxxx
> > http://lists.xen.org/xen-devel
> 
> 

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