Tuesday Sep 11, 2007

NCQ sata_max_queue_depth tunable

Previously, i did some analysis on NCQ in OpenSolaris. It turned out that to get good multi-stream sequential read performance, you had to disable NCQ via the tunable 'sata_func_enable'. Disabling NCQ actually does two things: 1) sets the number of concurrent I/Os to 1 and 2) changes what you send down protocol wise.

Turns out, the first is all we really need to get good performance for the multi-stream sequential read case, and doing the second actually exposes a bug on certain firmware of certain disks. So i highly recommend the newly added 'sata_max_queue_depth' tunable instead of 'sata_func_enable'. As a reminder, put the following in /etc/system and reboot:

set sata:sata_max_queue_depth = 0x1

An admin command to allow you to do this on the fly without rebooting would be another step forward, but no official plans on that just yet.

Thanks Pawel!

Tuesday May 29, 2007

NCQ performance analysis

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