Re: [RFC] support more qdisk types

[Jim Fehlig <jfehlig@suse.com>]

On 01/27/2016 01:25 PM, Doug Goldstein wrote:
> On 1/27/16 12:32 PM, Konrad Rzeszutek Wilk wrote:
>> On Mon, Jan 25, 2016 at 05:25:02PM -0700, Jim Fehlig wrote:
>>> I would like to hear the community's opinion on supporting more qdisk types in
>>> xl/libxl, e.g. nbd, rbd, iSCSI, etc. I prefer supporting additional qdisk types
>>> in libxl over apps like xl or libvirt doing all the setup, producing a block
>>> device, and then passing that to libxl. Each libxl app would have to
>>> re-implement functionality already provided by qdisk. libxl already supports
>>> IDE, AHCI, SCSI, and Xen PV qdisks. My suggestion is to extend that to initially
>>> include nbd, rbd, and iSCSI. Sheepdog, ssh, etc. could be added in the future.
>> ssh?
>>> I considered several approaches to supporting additional qdisk types, based
>>> primarily on changes to the disk cfg and interface. At one extreme is to change
>>> nothing and use the existing 'target=' to encode all required config for the
>>> additional qdisk types. libxl would need to be taught how to turn the blob into
>>> an appropriate qdisk. At the other extreme is extending xl-disk-configuration
>> Either way - new backends would require changes in both libxl and libvirt right?
>> The libxl would need to understand the new 'target=' blob to parse it out?
>>
> libvirt would probably just do what its doing now. Since it can setup
> the connection and pass the file descriptor into libxl.

Hmm, I'm not sure I understand. AFAIK, there is no way to pass libxl a file
descriptor opened from an image file or block device.

>  Honestly I don't
> see the advantage here because libvirt does a better job from a security
> standpoint and unless the goal is to have everything and the kitchen
> sink in libxl/xl. There's already a number of ways to skin the cat (xl,
> libvirt, xapi, openstack), why another one?

I'm simply proposing to extend the types of devices supported by and already
supported backend - qdisk. libxl configures qdisk based on the contents of the
libxl_device_disk struct. This proposal extends the struct with info necessary
to support the additional qdisk types.

Regards,
Jim