Re: ahci problems with sata disk.

[Jeff Garzik <jeff@garzik.org>]

Mark Hahn wrote:
>>>> I though that NCQ was intended to increase performance ??
> 
> intended to increase _sales_ performance ;)

Yep.


> remember that you've always had command queueing (kernel elevator): the 
> main difference with NCQ (or SCSI tagged queueing) is when
> the disk can out-schedule the kernel.  afaikt, this means sqeezing
> in a rotationally intermediate request along the way.
> 
> that intermediate request must be fairly small and should be a read
> (for head-settling reasons).
> 
> I wonder how often this happens in the real world, given the relatively
> small queues the disk has to work with.

ISTR either Jens or Andrew ran some numbers, and found that there was 
little utility beyond 4 or 8 tags or so.


>> My hdparm test is a sequential read-ahead test, so it will
>> naturally perform worse on a Raptor when NCQ is on.
> 
> that's a surprisingly naive heuristic, especially since NCQ is concerned 
> with just a max of ~4MB of reads, only a smallish
> fraction of the available cache.

NCQ mainly helps with multiple threads doing reads.  Writes are largely 
asynchronous to the user already (except for fsync-style writes).  You 
want to be able to stuff the disk's internal elevator with as many read 
requests as possible, because reads are very often synchronous -- most 
apps (1) read a block, (2) do something, (3) goto step #1.  The kernel's 
elevator isn't much use in these cases.

	Jeff