Re: [PATCH] speed up SATA

[Jeremy Higdon <jeremy@sgi.com>]

On Sun, Mar 28, 2004 at 06:36:23PM +0100, Jamie Lokier wrote:
> 
> This is what I mean: turn off write cacheing, and performance on PATA
> drops because of the serialisation and lost inter-command time.

Since you have to write the sectors in order (well, you don't have
to, but the drives all do this), you lose a rev between each write
when you don't queue commands or have write cacheing.

> With TCQ-on-write, you can turn off write cacheing and in theory
> performance doesn't have to drop, is that right?

Correct.  I have proven this to my satisfaction.


Regarding request sizes, the main benefit to very large (> 1MB)
request sizes to disk drives is elimination of the lost revolution
when write cacheing is disabled and you can't queue back-to-
back writes.

For hardware RAID, they frequently need 4MB or more to hit
peak performance.


I agree that we don't want a host driver to arbitrarily limit
I/O size, because it's difficult to predict what may be connected
to it.  It may be necessary due to deficiencies elsewhere, but
not desired.

jeremy