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 -- To unsubscribe from this list: send the line "unsubscribe linux-ide" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
