Re: Why is NCQ enabled by default by libata? (2.6.20)

Mark Rustad
2007-03-27T18:18:12Z

On Mar 27, 2007, at 12:59 AM, Jeff Garzik wrote:

> Justin Piszcz wrote:
>> Without NCQ, performance is MUCH better on almost every operation,
>> with the exception of 2-3 items.
>
> Variables to take into account:
>
> * the drive (NCQ performance wildly varies)
> * the IO scheduler
> * the filesystem (if not measuring direct to blkdev)
> * application workload (or in your case, benchmark tool)
> * in particular, the threaded-ness of the apps
>
> For the overwhelming majority of combinations, NCQ should not /
> hurt/ performance.
>
> For the majority of combinations, NCQ helps (though it may not be
> often that you use more than 4-8 tags).
>
> In some cases, NCQ firmware may be broken. There is a Maxtor
> firmware id, and some Hitachi ids that people are leaning towards
> recommending be added to the libata 'horkage' list.

Some other variables that we have noticed: Some drive firmware goes
into "stupid" mode when write cache is turned off. Meaning that it
does not reorder any queued operations. Of course if you really care
about your data, you don't really want to turn write cache on.

Also the controller used can have unfortunate interactions. For
example the Adaptec SAS controller firmware will never issue more
than two queued commands to a SATA drive (even though the firmware
will happily accept more from the driver), so even if an attached
drive is capable of reordering queued commands, its performance is
seriously crippled by not getting more commands queued up. In
addition, some drive firmware seems to try to bunch up queued command
completions which interacts very badly with a controller that queues
up so few commands. In this case turning NCQ off performs better
because the drive knows it can't hold off completions to reduce
interrupt load on the host – a good idea gone totally wrong when used
with the Adaptec controller.

Today SATA NCQ seems to be an area where few combinations work well.
It seems so bad to me that a whitelist might be better than a
blacklist. That is probably overstating it, but NCQ performance is
certainly a big problem.

--
Mark Rustad, MRustad@gmail.com