Re: [RFC] support more qdisk types

[Wei Liu <wei.liu2@citrix.com>]

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.
> 

This is a good idea in general.

> 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. 

I think you mean "libxlu would need to be taught how to turn that blob
into an appropriate libxl_device_disk" -- libxl doesn't parse user
configuration, it deals with the libxl_device_disk directly to construct
arguments for QEMU.

Either it is done by extending target= or adding discrete options, the
heavy lifting is going to be in libxlu.  I don't think we want to parse
a string inside libxl.

> At the other extreme is extending xl-disk-configuration
> with discrete knobs for each possible config item and making the
> libxl_device_disk structure more hierarchical. E.g.
> 

I don't think the second half (hierarchical structure) is closely tied
to whether target= is extended or discrete knobs are used. And extending
the structure seems to be the right thing to do.

Wei.