Subject: Re: /dev/[r]sd[5,6]* devices by default ?
To: David Laight <>
From: Brad Knowles <>
List: tech-kern
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, <>