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From: hitz@auspex.auspex.com (Dave Hitz)
Newsgroups: alt.sys.sun,comp.periphs.scsi
Subject: Re: Which Sun Disks to use?  SCSI vs IPI vs SMD?
Message-ID: <4461@auspex.auspex.com>
Date: 20 Nov 90 19:13:05 GMT
References: <1990Nov14.015823.27206@Neon.Stanford.EDU> <1990Nov14.051828.4221@cbnewsh.att.com>
Followup-To: alt.sys.sun
Distribution: na
Organization: Auspex Systems, Santa Clara
Lines: 48

In article <1990Nov14.051828.4221@cbnewsh.att.com> wcs@cbnewsh.att.com (Bill Stewart 201-949-0705 erebus.att.com!wcs) writes:
> We're planning to get a Sun server for NFS, and are trying to decide whether
> to get a cheap Sparcstation with SCSI, a Sparc2 with SCSI,
> or one of the big honking expensive servers with IPI or SMD.
> Are the SCSI systems fast enough?  Will UNIX 4.1.1 help performance
> substantially on the pre-Sparc2 machines?

For NFS servers, disk transfer-rate is less important than most people's
intuition first suggests.

The reason becomes clear when you look at how time is spent in an 8K
NFS read using a SCSI disk:

	Average seek				16   milliseconds
	Average Rotational latency		 7.5 milliseconds
	8K Data xfer				 4   milliseconds
	SCSI interrupt handling overhead	 2.5 milliseconds
	8K over the Ethernet			 8   milliseconds
						------------------
	Total					38.0 milliseconds

With an IPI, seek might drop to 15, and data xfer might drop to 3.
This gives an improvement of only about 2 milliseconds out of 38, or
about 5%.  In fact, percent improvement will be even less because I've
left out the time spent in processing the request in the CPU and in
moving the data around the system.

This argument works because NFS transfers are always 8K or less.  With
local access to very large files, transfer time eventually dominates
and IPI starts to make sense.  But for NFS, go with the cheap disks.

WARNING: All of this does not mean that a SPARCstation with SCSIs will
give you the same NFS performance as a SPARCserver with IPIs.  

What it does mean is that the SPARCserver isn't doing better because
of the type of disks it's using.  There are plenty of other bottlenecks
in a SPARCstation to slow things down.  Make your Sun salesman give
you benchmark results comparing the two systems you are considering.
A SPARCstation might well be able to handle the job for a while.

> We know about Auspex, but they start at $80K and we don't have a lot
> of client machines.

Sorry about that! :-)

-- 
Dave Hitz					work: 408-492-0900
UUCP: {uunet,mips,sun,bridge2}!auspex!hitz 	home: 408-739-7116