How to (really) disable NCQ in Linux

Thanks to @frostschutz, I could measure the write performance in Linux without NCQ feature. The kernel boot parameter libata.force=noncq disabled NCQ completely.

Regarding my Seagate 6TB write performance problem, there was no change in speed. Linux still reaches 180 MiB/s.

But then I had another idea:
The Linux driver does not use transfers of 32 MiB chunks. The kernel buffer is much smaller, especially if NCQ with 32 queues is enabled (32 queues * 32 MiB => 1 GiB AHCI buffer).

So I tested my SATA controller with 256 KiB transfers and voilà, it's possible to reach 185 MiB/s.

So I guess the Seagate ST6000AS0002 firmware is not capable of handling big ATA burst transfers. The ATA standard allows up to 65.536 logical blocks, which equals 32 MiB.

SMR - Shingled Magnetic Recording

Another possibility for the bad write performance could be the shingled magnetic recording technique, which is used by Seagate in these archive devices. Obviously, I triggered a rare effect with my FPGA implementation.

Setting the queue depth to 1 (/sys/block/sd*/device/queue_depth) disables NCQ, it is not necessary to use the kernel parameter libata.force=noncq (which can only be set at boot).

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commit 360f654e7cda850034f3f6252a7a7cff3fa77356

 Date:   Sat Sep 30 19:45:00 2006 +0900

 [PATCH] libata: turn off NCQ if queue depth is adjusted to 1
 
 Turn off NCQ if queue depth is adjusted to 1.