Subject: Re: SCSI-2 Adapters for SCO ODT 
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Organization: BT
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Date: Sat, 2 Jan 1993 18:23:48 +0000

In article <1993Jan02.052810.2589@clarinet.com> brad@clarinet.com writes:

>If your disks are sending at 1 mb/second, then you would have to have
>a lot of disks going at once before saturating a 5 mb/s SCSI-1 bus and
>finding yourself in need of SCSI-2.  (Particularly since disks spend
>most of their time seeking and waiting for sectors to spin around)

SCSI-2 devices need only support 5MB/sec transfer over the SCSI bus. 10MB/sec
is the optional FAST synch mode defined in SCSI-2. For instance the Adaptec
1542B is SCSI-2 but only 5MB/sec.

Caches tend to sort disk accesses and can greatly increase the average size
of contiguous transfers. Raw transfer rate is still just as important as
seek times.

>
>Don't get me wrong.  I would love to have devices that are spewing out
>data at 10 mb/second so that I can use SCSI-2 and EISA speeds.  But are
>these devices out there?   I wouldn't mind even pushing the limits of
>SCSI-1, but I'm not convinced we're doing that.    (Other devices, such
>as tapes, CD-roms etc. aren't anywhere near the usage levels of the
>disks.)

I've done some comparatve tests on an Adaptec 1542B and Ultrastor 14F. They
are similar controllers except that the 14F supports 10MB/sec FAST SCSI. With
ordinary SCSI-2 drives they are about the same speed, with FAST SCSI-2 drives
the 14F is significantly faster. Probably because propagation delays in getting
the data over the 2 busses is to some extent cumulative.


>
>If a disk stores 35 sectors/track, uses only one head at a time and spins
>at 3600 rpm -- this describes most disks

Not most current SCSI disks. Even our Quantum 425S which isn't a particularly
expensive drive uses zoning to give 78 sectors on the outer tracks and well
over 2MB/sec. Some are going 3-4MB/sec. 

>A surprisingly easy fix that's not done more is to
>read from multiple heads at the same time.  On a 16 head disk you can of
>course get 2 bytes per bit-time, 16 mb/second, though you become more
>vulnerable to errors.  Anybody doing this? 

I've though this too.

>
>Another method is to double to 7200 rpm, which has the advantage of reducing
>rotational latency to 8ms -- or increasing density.

Some drives are doing this. I don't know any models offhand but I've talk
about xithea Seagate barracuda which is or is soon to be released.

>
>One scheme larger systems use is to have several disk drives and store one
>bit per disk drive!  With 19 disk drives, you can store 16 bits and 3 bits
>of error correction.  This makes it possible to take any one disk drive
>offline -- and the system still works.   Indeed, when a drive goes bad on
>these systems, you replace it and slowly rebuild the ecc onto it.  You
>are vulnerable to errors while it is off and being rebuilt, and you must of
>course keep the drives in sync.
>
>Doubt anybody is doing this for SCO, but it's a way to get highly reliable
>disk operation and 16 megabyte/second throughput.

There are RAID/striping systems though which do something similar. One
controller which sounds interesting is the Ultrastor 124F EISA which supports up
to 5 independent FAST SCSI-2 channels and they've reported getting a sustained
20Mbytes/second on a RAID setup.

-----------------------------------------
Lawrence Kirby | fred@genesis.demon.co.uk
Wilts, England | 70734.126@compuserve.com
-----------------------------------------
Newsgroups: biz.sco.opendesktop