Path: utzoo!attcan!uunet!xstor!iverson
From: iverson@xstor.UUCP (Tim Iverson)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: SCSI Board Info Request
Message-ID: <213@xstor.UUCP>
Date: 17 Oct 90 19:02:15 GMT
References: <9254@orca.wv.tek.com> <1990Oct07.173315.6532@nstar.uucp> <209@xstor.UUCP> <1990Oct13.231715.1548@nstar.uucp> <1990Oct16.223855.4793@sj.ate.slb.com>
Reply-To: iverson@xstor.UUCP (Tim Iverson)
Distribution: na
Organization: Storage Dimensions, Inc.
Lines: 38

In article <1990Oct16.223855.4793@sj.ate.slb.com> poffen@sj.ate.slb.com (Russ Poffenberger) writes:

>You will find that the AHA1542 will outperform on transfer rate, but because of
>the intelligent "sector mapping" it does, [...]

The AHA1542 does not do any "intelligent" sector mapping.  Assuming that
you're referring to the fact that it reports an artificial geometry in order
to satisfy utilities that require one, there is no overhead to speak of
converting the 64head/32sector geometry into a SCSI LBA.

	e.g.  lba = (cyl << 11) | (hd << 5) | (sec - 1)

Not only is overhead on this calculation negligible, but almost all SCSI HAs
must perform the exact same calculation, so when comparing performance, this
factor is a wash - both do it, and both take about the same time to do so.

>[...] there tends to be a little overhead
>when it comes to average access and track to track seek times.

This difference between published times and coretest times *is* due to the
geometry translation, but not due to calculation overhead.  Because of the
translation, there is usually more than one real cylinder for each
artificial cylinder, thus coretest thinks it's seeking 1 cylinder when it
may be seeking farther.  This does not affect comparisons of coretest
results obtained for the same or similar HAs.

The per command overhead on the AHA 154x host adapters is due mostly to
command queueing.  The 154x HA is designed to perform well in multi-threaded
situations.  As a consequence, it does not perform as well as possible in
single-threaded situations.  Putting a 1542 in a DOS only machine is like
buying a Ferrari for your grandmother - it can go 200+, but she'll never
drive it faster than 35.

>Russ Poffenberger               DOMAIN: poffen@sj.ate.slb.com
>Schlumberger Technologies       UUCP:   {uunet,decwrl,amdahl}!sjsca4!poffen

- Tim Iverson
  uunet!xstor!iverson