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Date: Wed, 1 Jul 2009 13:29:31 +0200
From: Christoph Hellwig <hch@lst.de>
Subject: Re: [Qemu-devel] [PATCH] ATAPI pass through
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To: Paul Brook <paul@codesourcery.com>
Cc: Kevin Wolf <kwolf@redhat.com>, qemu-devel@nongnu.org, Bique Alexandre <alexandre.bique@citrix.com>, Alexander Graf <agraf@suse.de>

On Wed, Jul 01, 2009 at 10:36:01AM +0100, Paul Brook wrote:
> IIRC ATAPI and SCSI are not the same. They provide very similar functionality, 
> however the actual commands are different.

ATAPI has somewhat more limited command sets, mostly because it does not
support the traditional 6 byte commands and doesn't support the scsi
block commands (SBC) used for disks.  For CDROMs both traditional
parallel scsi and atap use the same command set (MMC in various
revisions)

> While it is possible to connect SATA drives to a SAS HBA, I believe this is 
> done by tunneling IDE/ATAPI commands, rather then using native SCSI commands.

There are no ATAPI hard drives, just plain ATA which is not a SCSI
command set at all.  For directly connected disks on a SAS HBA the HBA
speaks plain SATA to the disk (as the SAS and SATA link layers are the
same).  If you connect a SATA disk to a SAS expander the expander talks
SATA to the disk and the expander encapsulates it in STP which is just a
thin layer of SAS routing information around the ATA command block,
similar to the SSP protocol used to talk to SCSI devices which is a thin
layer around SCSI command blocks.

I would expect an ATAPI cdrom attached to a SAS expander to talk MMC
command blocks over ATAPI over STP, but I haven't actually seen this
setup in practice.