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 [EMAIL PROTECTED]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
