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

[Jeff Garzik]

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


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