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

[Alan Cox <alan@lxorguk.ukuu.org.uk>]

On Gwe, 2006-03-24 at 07:38 -0500, Jeff Garzik wrote:
> > 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.

I don't believe this is true. The complexity of expressing sequences of
command ordering between virtual machines acting in a co-operative but
secure manner isn't as far as I can see expressable sanely in SCSI TCQ
> 
> 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.

Those are the easy bits. 

> > 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.

And every one you get wrong is a corruptor....

Alan