[Xen-devel] Re: [RFC PATCH 35/35] Add Xen virtual block device driver.

[Peter Chubb <peterc@xxxxxxxxxxxxxxxxxx>]

>>>>> "Jeff" == Jeff Garzik <jeff@xxxxxxxxxx> writes:

Jeff> Ian Pratt wrote:
>> 
>> We certainly should be pushing everyone toward using the 'xdX' etc
>> devices that are allocated to us. However, the installers of
>> certain older distros and other user space tools won't except
>> anything other than hdX/sdX, so its useful from a compatibility POV
>> even if it never goes into mainline, which I agree it probably
>> shouldn't.

Jeff> Yes, this is true.  Red Hat installer guys grumbled at me when I
Jeff> wrote the 'sx8' block driver: since it wasn't hda/sda, they had
Jeff> to write special code for it, as they apparently must do for any
Jeff> new block driver "class".  SuSE and other distros are probably
Jeff> similar, since each block driver provides its own special
Jeff> behaviors and feature exports.

Jeff> I should have spoken up a long time ago about this, but anyway:

Jeff> An IBM hypervisor on ppc64 communicates uses SCSI RPC messages.
Jeff> I think this would be quite nice for Xen, because SCSI (a) is a
Jeff> message-based model, and (b) implementing block using SCSI has a
Jeff> very high Just Works(tm) value which cannot be ignored.  And
Jeff> perhaps (c) SCSI target code already exists, so implementing the
Jeff> server side doesn't require starting from scratch, but rather
Jeff> simply connecting the Legos.

The IA64 virtualisation work (Xen and Linux-on-Linux) uses the SKI
simulator virtual scsi device --- which looks just like any other scsi
disk, but uses hypervisor calls to do read/write/open/close calls like
a user-mode process.  For performance, it needs to be extended a bit
to do asynchronous I/O and interrupt on completion.  As a halfway
house, the ski simscsi driver would be fairly easy to port, I think.

-- 
Dr Peter Chubb  http://www.gelato.unsw.edu.au  peterc AT gelato.unsw.edu.au
http://www.ertos.nicta.com.au           ERTOS within National ICT Australia

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