SMD Vs SCSI: Results of Survey

[maxlin!wbc at dartvax.dartmouth.edu]

Well, it seems that SCSI disks are the preferred choice for speed and
reliability.  I am planning to go with a Xylogics 753, which can controll
our existing Fujitsi Eagles as well as a new gigabyte drive (Fujitsu or
Hitachi)  Hopefully, overlapped seeks should gain something too.  A lot of
time is spent seeking on the swap disks.  Here are some reports on why smd
disks are better:

We have a3/160 with 3 3/50(60) clients, OS 3.4.  long ago we decided to go
to SMD despite the extra expense for the following reason: The scsi bus
seems to be able to talk to one device at a time.  IF the tape drive is
exicuting a seek or even a rewind the scsi disk is inaccessable until the
tape is done.  THis is sort of rediculous on a networked multiuser system.
We have only one SMD disk so I cannot confirm that the 753 can do multiple
seeks, but I have heard this is so.

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Berkeley-based systems like sun make intelligent use of the semantics of a
disk drive. This is the Fast File System in the documentation.  As an
example, there is an attempt to put all of the files in a directory on the
same cylinder group, allowing files to be accessed without moving the head
to a new cylinder.  Also, blocks are accumulated so that all of the blocks
in one cylinder are hopefully written at once, without moving the disk
heads.  The SCSC disks, however, hide all of the information from the
kernal, so no intelligent layout policy can be achieved.  The general run
of thumb is that  a local SCSI disk is the same performance as a remote
SMD disk. If that ration is 1:4, then a remote SCSI disk will perform 4
times slower than the remote SMD disk.

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I talked to several people when I was configuring a Sun 4/260 system that
we will order soon, and everyone said that there is a big difference in
speed with the SMD disks faster. I am planning to order CDC Sabre V's (1
GB). I had considered EDSI disks as being cheaper.  They also said that
the CDC Sabres are more reliable than disks like Maxtors.

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I'm not sure I follow your logic here.  If it is performance you want, a
large disk is the last alternative.  While it is true that they back more
bits per inch (circumferentially) and hence achieve higher transfer rates
per rotation, they also have more tracks, and a greater distance to seek.
Performance for me also includes reliability.  So, you are better off
getting several mid-sized disks and putting them on separate controllers.
You get higher reliability, better utilization (as you can be seeking on
one unit while reading on another), and about the same bandwidth.  Some
disks (typically the CDC units) have options for two controllers to talk
to them -- so you can have one CPU run "standby" in case the other ("hot")
CPU goes down.  Just be sure not to have both CPUs using the disk at the
same time or it will be corrupted.

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With 18 clients I would heartily recommend sticking to SMD drives.  They
(and their controllers) are *faster* for overall system throughput. The
753 is an excellent idea, being much better than the 451 in my experience.
Seek time is important, but its not the only factor.

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For anything performance intensive, go with SMDE over SCSI or ESDI.  The
transfer rates are higher, and seek times will be shorter.  Reliability is
likely to be better, too, which may justify the cost differential by
itself.  However, I am getting some ESDI disks that I will use for low
performace applications, like mail, news and on-line source code.  These
drives will be in one of those RT-135s that IBM is pushing right now, but
the same logic would apply to SCSI disks (which are usually ESDI disks
with a SCSI interface attached).  But, for NFS those big Fujitsus look
like the way to go to me.  I've got one of their 8" 690 MB drives now and
am very pleased with it.  An 892 MB on a 753 should be dynamite.

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Here at Washington University we have been very dissapointed with the
performance of Sun's SCSI disks.  Although we haven't tried other sources
for SCSI drives, I think your are going to be better off with an SMD.
While you're at it pick up a fast controller like the Ciprico Rimfire.

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	Wayne Cripps
	wbc at maxlin.DARTMOUTH.EDU		wbc at maxlin.dartvax.uucp
	Box 6188 Dartmouth College, Hanover N.H. 03755 (603) 646-3198