Re: ahci problems with sata disk.

[Jens Axboe <jens.axboe@oracle.com>]

On Tue, Jan 16 2007, Eric D. Mudama wrote:

[snip lots of stuff I agree completely with]

> If done properly, queueing should never hurt performance.  High queue
> depths will increase average latency of course, but shouldn't hurt
> overall performance.

It may never hurt performance, but there are common scenarios where you
are much better off not doing queuing even if you could. A good example
of that is a media serving service, where you end up reading a bunch of
files sequentially. It's faster to read chunks of each file sequentially
at depth 1 and move on, than queue a a request from each of them and
send them to the drive. On my laptop with an NCQ enabled drive, the
mentioned approach outperforms queuing by more than 100%.

> >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.
> 
> True.  And internal to the drive, normal elevator is "meh."  There are
> other algorithms for scheduling that perform better.

Well Linux doesn't default to using a normal elevator, so it's a moot
point.

-- 
Jens Axboe