Re: [PATCH] speed up SATA

[Jeremy Higdon <jeremy@sgi.com>]

On Fri, Apr 02, 2004 at 05:11:49PM +0100, Jamie Lokier wrote:
> Jeremy Higdon 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.
> 
> I don't see how the driver can write the sectors out of order, if
> there is no TCQ (we're talking PATA) and every write must be committed
> before it's acknowledged (write cache disabled).
>
> > > 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.
> 
> Are you refuting the following assertion by Eric D. Mudama's, based on
> your measurements?  In other words, are ATA's 32 TCQ slots enough to
> eliminate the performance advantage of write cacheing?

I must apologize.  I had thought the context was SCSI, but now I
see that it is linux-ide.  So please disregard comments about command
queueing.  If you have write cache disabled and no TCQ (latter is common,
former may or may not be), you want to write as big a chunk as you can.

I'm sorry about the confusion.

jeremy