Subject: Re: /dev/[r]sd[5,6]* devices by default ?
To: David Laight <firstname.lastname@example.org>
From: Brad Knowles <email@example.com>
Date: 01/09/2002 22:29:55
At 8:00 PM +0000 2002/01/09, David Laight wrote:
> When we were sizing systems, it was generally not considered worthwhile
> (from a throughput point of view) putting more than 1 disk on a scsi
> channel. It was much more common to add extra controllers for the
> other disks. Certainly you wouldn't consider 7 disks on one 8-bit
> scsi channel for any form of server, the scsi data rate is far too slow.
When I was working at AOL, and we were architecting
low-latency/high-throughput systems, one of the rules-of-thumb that
we were given by the data/filesystems experts (including the key guy
from the team that built the world-renowned CLARiiON RAID array) was
that you should have no more than four SCSI disk devices per channel.
Indeed, we were told that this has pretty much always been the rule,
because as disk devices have gotten faster, so has SCSI.
Actually, this was the rule for systems that needed minimum
latency. If you could afford for some disk queries to be delayed
slightly, you could usually get five or six disk devices per channel
and get higher throughput. It all depends on the size of your
average disk queries, and whether you were in a transaction-oriented
environment or in a batch environment.
Brad Knowles, <firstname.lastname@example.org>