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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

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@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.

> +* 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
> 
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> http://lists.xen.org/xen-devel



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