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

Alan Cox wrote:
On Mer, 2006-03-22 at 18:09 -0500, Jeff Garzik wrote:
An IBM hypervisor on ppc64 communicates uses SCSI RPC messages. I think this would be quite nice for Xen, because SCSI (a) is a message-based model, and (b) implementing block using SCSI has a very high Just Works(tm) value which cannot be ignored. And perhaps (c) SCSI target code already exists, so implementing the server side doesn't require starting from scratch, but rather simply connecting the Legos.

A pure SCSI abstraction doesn't allow for shared head scheduling which
you will need to scale Xen sanely on typical PC boxes.

Not true at all. If you can do it with a block device, you can do it with a SCSI block device.

In fact, SCSI should make a few things easier, because the notion of host+bus topology is already present, and notion of messaging is already present, so you don't have to recreate that in a Xen block device infrastructure.

SCSI emulations
are also always full of bits people got wrong, often critical bits like
tagged queues and error sequences - things that break your journalled
file system.

This I'll grant you.


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