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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

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@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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