Re: [RFC] support more qdisk types

[Jim Fehlig <jfehlig@suse.com>]

On 02/03/2016 02:56 AM, Ian Campbell wrote:
> On Tue, 2016-02-02 at 15:06 -0700, Jim Fehlig wrote:
>>> And extending
>>> the structure seems to be the right thing to do.
>> So what do you think of the approach in the RFC patch? It adds discrete knobs in
>> the disk config and extends the disk structure similarly. Before I can make
>> progress on this we need to agree on extending the config and libxl_device_disk
>> structure.
> My main concern is that this approach requires us to update libxl for each
> new possible backend type.

Yes, understood.

>
> The intention of the target= in the disk spec is that it consumes the rest
> of the line so it can be used to encode pretty much anything. Is it not
> possible (modulo bugs) to pass all the necessary information to qdisk in
> this form? I thought Dave S had made it possible to use qdisk in this way
> back in:
>
>     commit a8a1f236a2964506a22d1779648d8e1c8668cb1a
>     Author: David Scott <    dave.scott@eu.citrix.com    >
>     Date:   Tue Apr 23 10:59:26 2013 +0100
>
>         libxl: Only call stat() when adding a disk if we expect a device to exist.
>         
>         We consider calling stat() a helpful error check in the following
>         circumstances only:
>          1. the disk backend type must be PHYsical
>          2. the disk backend domain must be the same as the running libxl
>             code (ie LIBXL_TOOLSTACK_DOMID)
>          3. there must not be a hotplug script because this would imply that
>             the device won't be created until after the hotplug script has
>             run.
>         
>         With this fix, it is possible to use qemu's built-in block drivers
>         such as ceph/rbd, with a xl config disk spec like this:
>         
>         disk=[ 'backendtype=qdisk,format=raw,vdev=hda,access=rw,target=rbd:rbd/ubuntu1204.img' ]

I thought I tried disk config along those lines with no success. But I'll
certainly take a closer look at using target= to encode the config needed by
these qdisk block drivers. Thanks for the pointer.

Regards,
Jim