lkml.org 
[lkml]   [2007]   [Mar]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: Why is NCQ enabled by default by libata? (2.6.20)
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.


> 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.


> 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.

Jeff


-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2007-03-27 20:41    [W:0.568 / U:0.888 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site