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

Jeff Garzik
2006-03-24T12:38:56Z

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.

Jeff