Re: [Xen-devel] [PATCH 3/3] docs: add more info about target= in disk config

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Jim Fehlig writes ("[PATCH 3/3] docs: add more info about target= in disk 
config"):
> target= in disk config can be used to convey arbitrary
> configuration information to backends. Add a bit more info
> to xl-disk-configuration.txt to clarify this, including some
> simple nbd and rbd qdisk configurations.
> ---
>  docs/misc/xl-disk-configuration.txt | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/docs/misc/xl-disk-configuration.txt 
> b/docs/misc/xl-disk-configuration.txt
> index 29f6ddb..0918fb8 100644
> --- a/docs/misc/xl-disk-configuration.txt
> +++ b/docs/misc/xl-disk-configuration.txt
> @@ -75,7 +75,15 @@ Special syntax:
>     the target was already specified as a positional parameter.  This
>     is the only way to specify a target string containing metacharacters
>     such as commas and (in some cases) colons, which would otherwise be
> -   misinterpreted.
> +   misinterpreted. Meta-information in a target string can be used to
> +   specify configuration information for a qdisk block backend. For
> +   example the nbd and rbd qdisk block backends can be configured with
> +
> +     target=nbd:192.168.0.1:5555
> +     target=rbd:pool/image:mon_host=192.186.0.1\\:6789
> +
> +   Note the use of double backslash ('\\') for metacharacters that need
> +   escaped.

I'm not entirely comfortable with documenting this as supported.
The difficulties I see are:


In the usual configuration, libxl decides for itself what (libxl)
backend to use.  Different versions of libxl might make different
choices, so a configuration that works with one version of libxl might
not work with another.  That's fine for an undocumented feature but
not so good if it's actually advertised.  At the very least the docs
need to say that to rely on this you must specify backend=qdisk.


And this is a layering violation, or rather a violation of the
expected semantics of the target string.

I think it would be much better to support nbd and rbd explicitly in
libxl.  Maybe we should have a "protocol=" parameter, so you could
write something like this:
   disk=["vdev=xvda, protocol=nbd, target=192.168.0.1:5555"]

This would allow libxl to make better choices about backends, even if
right now all it does is force the use of the qemu backend and pass
the target string to qemu.


Finally, if this is actually true, it is a bug.  The specification
(docs/misc/xl-disk-configuration.txt) says:

  Description:           Block device or image file path.  When this is
                         used as a path, /dev will be prepended
                         if the path doesn't start with a '/'.

See also what is said under `script='.


Ian.

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