Re: [Qemu-devel] scsi-generic and max request size

[Alexander Graf]

On 22.12.2010, at 14:27, Christoph Hellwig wrote:

> On Wed, Dec 22, 2010 at 02:54:54PM +0100, Hannes Reinecke wrote:
>> Most modern HBAs are using separate codepaths for streaming/block I/O
>> anyway,
> 
> That's not true at all.  Every normal HBA justs passes normal SCSI
> commands to the SCSI targets.  It's just raid adapters that take special
> commands, and the megaraid one is extremly special as it actually
> emulates a few SCSI commands even in RAID mode, which almost no other
> HBA does.  Strictly speaking we should not allow scsi-generic with
> megaraid_sas, except for the separate passthrough channels that the real
> hardware has for things like tape drives.
> 
>> However, since Alex Graf is facing similar problems with the AHCI HBA of
>> his maybe we could retry again ...
> 
> AHCI is a ATA adapter, and should never be used with scsi-generic for
> disks.  Only for the ATAPI-attached cdroms/tapes/etc it could be used,
> although it's quite pointless.

It's not 100% pointless - ATAPI passthrough is a feature requested by users.
If we were to model ATAPI properly, it would end up using whatever SCSI layers 
we have below - which means ATAPI passthrough would be a mere matter of 
replacing the "scsi-cdrom" backend with a "scsi-passthrough" backend.

Now for the fun part. ATAPI can also do NCQ. So we actually end up having the 
exact same thing here as megasas. If we were to do ATAPI passthrough on CD-ROM 
with NCQ. NCQ goes through the normal read/write path of a block backend. 
Passthrough would do SG_IO.


Alex