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

Mark Rustad
2007-03-27T22:12:10Z

On Mar 27, 2007, at 1:38 PM, Jeff Garzik wrote:

> Mark Rustad wrote:
>> reorder any queued operations. Of course if you really care about
>> your data, you don't really want to turn write cache on.
>
> That's a gross exaggeration. FLUSH CACHE and FUA both ensure data
> integrity as well.
>
> Turning write cache off has always been a performance-killing
> action on ATA.

Perhaps. Folks I work with would disagree with that, but I am not
enough of a storage expert to judge. My statement mirrors the
judgement of folks I work with that know more than I do.

>> 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.
>
> All of that can be fixed with an Adaptec firmware upgrade, so not
> our problem here, and not a reason to disable NCQ in libata core.

It theoretically could be, but we are using the latest Adaptec
firmware. Until there exists firmware that fixes it, it remains an
issue. We worked with Adaptec to isolate this issue, but no
resolution has been forthcoming from them. I agree that this does not
mean that NCQ should be disabled in libata core, but some combination
of controller/drive/firmware blacklist may need to be managed, as
distasteful as that is.

>> 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.
>
> Real world testing disagrees with you. NCQ has been enabled for a
> while now. We would have screaming hordes of users if the majority
> of configurations were problematic.

I didn't say that it is a majority or that it doesn't work, it just
often doesn't perform. If it didn't work there would be lots of
howling for sure. I'm also not saying that it is a libata problem. It
seems mostly to be controller and drive firmware issues - and the odd
fan issue (if you saw the thread: [BUG 2.6.21-rc3-git9] SATA NCQ
failure with Samsum HD401LJ).

I guess I am mainly lamenting the current state of SATA/NCQ devices
and sharing what little I have picked up about it - which is that I
want SAS disks in my next system!

--
Mark Rustad, MRustad@gmail.com