Re: [Qemu-devel] [PATCH] ATAPI pass through

[Christoph Hellwig]

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.