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From: Rob_Steven_Kramarz@cup.portal.com
Newsgroups: comp.periphs.scsi
Subject: What makes a SCSI drive fast?
Message-ID: <41399@cup.portal.com>
Date: 18 Apr 91 17:27:09 GMT
Distribution: usa
Organization: The Portal System (TM)
Lines: 19

We have tested a great many SCSI models during the process of certifying
our disk array software for UNIX systems.  (My firm, 1776, Inc. develops
disk mirroring and disk array software running on standard SCSI drives.)
We have unfortunately found that there is no way to predict how fast
or slow a given drive will be from the specifications.  There are several
hidden factors that affect performance other than access time, including
the efficiency of the electronics in getting on and off the SCSI bus, and
how many retries are necessary due to SCSI errors.  As a result, we have
found expensive drives that are fast and cheap drives that are slow.  One
factor that is essential now is support for synchronous transfers which
can make a huge difference in speed with the appropriate host adapter.

In all honesty, I must say the device driver used makes a large difference
too.  We have found differences of an order of magnitude between drivers
with buffering technology (such as ours) and the simple drivers which are
usually shipped with the o.s.  Disk striping can double or triple
performance further.  

Rob Kramarz