Re: [PATCH 1/2] libata: switch to using block layer tagging support

[Jeff Garzik <jeff@garzik.org>]

James Bottomley wrote:
> On Wed, 2009-05-20 at 11:08 -0700, Gwendal Grignou wrote:
>> Form the ATA and SATA spec, NCQ is per device. It is possible to
>> assign the same tag on different port, the disks and PMP will not
>> care.
>> However, today, we assign tag on a port basis [see
>> __ata_qc_from_tag()], therefore only 32 commands can be inflight to
>> all devices behind a given port.
>>
>> Being able to a do a mapping qc <---> {pmp port, tag} instead of just
>> qc <---> {tag} will provide a performance boost when disks supporting
>> NCQ are connected behind a PMP. Maybe it can be done, by moving qcmd
>> from ata_port to ata_link.
>>
>> As Tejun said, the patch needs more work to be able to support same
>> tag used on 2 different links behind the same port.
>> Also, given we are changing the amount of commands we sent to the
>> controller, we would have to change can_queue from ATA_MAX_QUEUE to
>> ATA_MAX_QUEUE*n, where n is the number of ports supported by the
>> controller [max SATA_PMP_MAX_PORTS, but some controller, like SiI3132
>> only supports 5 devices, and other may only support n = 1]
>>
>> When done, the patch will require a great amount of testing, given we
>> will exercise the controllers in a brand new way. A white list might
>> be necessary.
>>
>> Jens, using SCSI tagging for ata commands is a great idea, but it is
>> no small feat...
> 
> So realistically, you want one block queue per PMP port rather than an
> artificial limits adjustment on the single queue per output phy ... this
> shouldn't be too hard: it is exactly the way the SAS transport class
> (and libsas) works today for expander connected SAS devices.

Well...   The limiting factor is "ATA command slots per SATA phy", which 
defines $N active ATA commands per port.  If a SATA NCQ device is 
attached, you may have $N active ATA commands queued.  If two SATA NCQ 
devices are attached to a PMP, which is attached to the SATA controller, 
the two devices share the limit of $N active ATA commands queued.  If 32 
devices are attached to a PMP, all 32 devices share the $N command queue 
limit.

But additionally, as Tejun demonstrated, you might have a mix of NCQ and 
non-NCQ devices attached to the PMP.

Thus, it is a case of nested limits:

- $N maximum active commands per port; typically N==32
- $M maximum active commands per device; typically N==1 or N==32

Regards,

	Jeff