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

[Alan Cox <alan@xxxxxxxxxxxxxxxxxxx>]

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. 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.



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