Re: How to improve db performance with $7K?

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