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Re: ata(4) and NCQ
on 26/04/2011 13:57 Manuel Bouyer said the following:
> On Wed, Apr 20, 2011 at 02:32:08PM +0300, Andriy Gapon wrote:
>> on 20/04/2011 03:38 Jonathan Stone said the following:
>>> Also that both FreeBSD and Linux took the plunge and handle ATA drives
>>> their SCSI mid-layer. I agree, that code is big; but maybe that
>>> alternative is
>>> worth examiining, too.
>> AFAIK, (Open)Solaris did the same.
>> And to clarify: you are actually talking about CAM layer. Yes, previously
>> layer was only used for SCSI.
> I'm still not sure what the rationale for this is. Managing the tags
> is simple and could easily be abstracted and shared by the 2 subsystems
> if we want to.
Right. And that's what CAM layer does (in addition to other things).
> Using the scsi code for ATA drives means that you either
Again, I don't understand this. We should first clearly define what SCSI code
is. In my understanding SCSI code has nothing to do with ATA disks (in FreeBSD
> have to teach the scsi sd driver to build ATA commands, and the whole
> scsipi to handle ATA commands in addition to SCSI and ATAPI; or
> or keep SCSI commands in the mid-layer and convert SCSI to ATA commands in
> the ATA layer (which I consider useless overhead).
FreeBSD did neither, but closer to the first option. There is a new peripheral
driver that uses ATA commands to talk to its disks. Most of the intermediate
layer is command-agnostic. Transport-specific places had to be taught about
ATA, of course.
> Actually I suspect the idea to have ATA drives show up as SCSI comes from
> windows. But this was done so on windows because of a limitation of the
> windows kernel: if you want to have a drive appears as ATA the controller
> has to follow the pciide 1 specification. if you have a smarter controller,
> the only way to have full control at the controller driver level is to
> register it as a SCSI driver.
Can't say about other OSes, but FreeBSD has chosen this approach because SATA
disks and AHCI controllers are much closer to SCSI concepts, than the primitive
Here is a very good overview written by a person who did all the work in
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