Re: [PATCH] speed up SATA

From: Jeff Garzik
Date: Sat Mar 27 2004 - 19:50:52 EST

Nick Piggin wrote:
Aside, you should make 2 or 4 tags the default though: that
still gives you the pipelining without sacrificing latency
and usibility.

I would rather have a sane block layer default, that works without my intervention :)

Ditto the max_sectors issue, the driver for the hardware w/ a 256-depth queue should attempt to queue up to the hardware maximum. The block layer can be smart and limit things further from there...

IIRC the AIC7xxx driver sets the queue depth to 200+ on one of my drives. That may be 2.4, though...

The only area (I think) where large queues outperform the IO
scheduler are lots of parallel, scattered reads. I think this
is because the drive can immediately return cached data even
though it looks like a large seek to the IO scheduler.
(This is from testing on my single, old SCSI disk though.)

Most ATA drives have their own internal disk schedulers, and all of the major vendors seem to have done something useful with TCQ. Newer drives definitely take advantage of the additional knowledge gained via knowledge of the entire queue rather than just a single request.

The parallel-reads case you mention is definitely a winner with TCQ, but writes are also, for ATA-specific reasons (among others): since ATA has been one-command-at-a-time for so long, write caching became necessary for decent performance. So ATA disks needed a decent internal IO scheduler, just maintain a decent level of performance.

TCQ-on-write for ATA disks is yummy because you don't really know what the heck the ATA disk is writing at the present time. By the time the Linux disk scheduler gets around to deciding it has a nicely merged and scheduled set of requests, it may be totally wrong for the disk's IO scheduler. TCQ gives the disk a lot more power when the disk integrates writes into its internal IO scheduling.


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