Yeah - the more reading I'm doing - the more I'm finding out.
Alledgelly the Western Digial Raptor drives implement a version of
ATA-4 Tagged Queing which allows reordering of commands. Some
controllers support this. The 3ware docs say that the controller
support both reordering on the controller and to the drive. *shrug*
This of course is all supposed to go away with SATA II which as NCQ,
Native Command Queueing. Of course the 3ware controllers don't
support SATA II, but a few other do, and I'm sure 3ware will come out
with a controller that does.
Alex Turner
netEconomist
On 06 Apr 2005 23:00:54 -0400, Greg Stark <gsstark@mit.edu> wrote:
>
> Alex Turner <armtuk@gmail.com> writes:
>
> > SATA gives each drive it's own channel, but you have to share in SCSI.
> > A SATA controller typicaly can do 3Gb/sec (384MB/sec) per drive, but
> > SCSI can only do 320MB/sec across the entire array.
>
> SCSI controllers often have separate channels for each device too.
>
> In any case the issue with the IDE protocol is that fundamentally you can only
> have a single command pending. SCSI can have many commands pending. This is
> especially important for a database like postgres that may be busy committing
> one transaction while another is trying to read. Having several commands
> queued on the drive gives it a chance to execute any that are "on the way" to
> the committing transaction.
>
> However I'm under the impression that 3ware has largely solved this problem.
> Also, if you save a few dollars and can afford one additional drive that
> additional drive may improve your array speed enough to overcome that
> inefficiency.
>
> --
> greg
>
>
