Uses for a couple boxes full of Compaq 2.1gig Fast-Wide SCSI2?

[Rich Braun]

Mark Spencer wrote:
> I recently got approximately twenty [Compaq 2.1Gb SCSI) drives ...
> I'm wondering what use they would be on a PC? I'm not very familiar with the
> SCSI architecture, but I would assume these drives would outperform an IDE
> setup by a significant margin, especially on a system under heavy load.
> 
> Is SCSI2 limited to eight devices?
> 
> What would recommended SCSI2 fast-wide ISA or PCI adapters be?  Do the
> Adaptec's work well with linux?

I suppose I'm sort of a resident expert on those...  I was in the firmware
group which designed them, at least if they're designated RZ28L.  Right
before DEC got out of the disk drive business, our biggest customer was
Compaq, which used them in its servers.  (Compaq today owns only part of the
old DEC; Intel, Quantum, and Cisco own some of the other parts.)

I ran a Unix/VMS lab for the SCSI-2 firmware development group, and I wrote
a tiny bit of the firmware in those drives (a flash loader and a bug-fix
here and there).  The RZ28L was developed in the same generation as the
Seagate Barracuda, at the time 1" 2Gb drives were being introduced.  This
is just before magnetorisistive (MR) disk-head fabrication became economical.
IBM, Seagate, and DEC drives were considered leading the technology race.
MR has since pushed capacities to 20Gb, and back then we expected 100Gb
to be around the upper limit.

OK, here's a little about SCSI-2 and Linux.

SCSI-2 is one of the most successful multi-vendor specifications; it tightened
up the previous SCSI standard and enabled vastly superior cross-vendor
portability, without sacrificing performance or capacity the way so many
other standards have done.  (Consider the 1.44Mb diskette, or NTSC video,
or the non-enhanced IDE, or VHS videotape, 8mm audio tape, etc., etc.)

It provides an economical "narrow", a high-performance "wide", and a
niche-market "ultra-wide" set of bus-width options (8, 16, and 32 bits).
It provides standard and "fast" bus speeds (5 and 10 MHz).  And it provides
bus parity along with two options for signalling, so-called 'single-ended'
versus 'differential' (the latter is considered more reliable for longer
cable runs, but requires almost twice as many connector pins).  It also
defines two or three different types of cable connector options.

As you can see, manufacturers can make about ten or so different types of
mutually-incompatible SCSI-2 devices.  But the nice thing about the spec
is that once you've picked the performance/price level of your system, you
can pretty much pick and choose components across multiple vendors

The drives you have are wide/fast.  That means 16 bits of data is
transmitted ten millions times per second, giving the shared bus up to
20 megabytes/sec of throughput capacity.  On wide SCSI-2 bus, an extra
bit is devoted to the device ID, so you can have 16 devices on the bus
instead of the 8 supported by narrow SCSI-2.  The host adapter (often
called the 'controller', which isn't really precise since drive
control logic is embedded in each drive) occupies one ID, usually the
highest value of 15, so you can plug 15 drives into a single host
adapter.  The SCSI ID's of the drives are numbered 0 to 14; by convention,
on the PC architecture the boot device is 0 (on a Sparc box, the boot
device is usually 3).

Back when I was working on this, Adaptec was consolidating its hold on
SCSI-2 bus technology through some fairly strong-arm marketing
techniques.  Part of the reason so many vendors' devices are compatible is
that they tend to use Adaptec silicon in the drives, host adapters, and
test equipment used at manufacturing and development sites.

Some of the first Linux device-driver support was for the Adaptec line of
adapters; the 1542 was the biggest seller in the AT-bus era.  Nowadays a
lot of computers have an Adaptec controller built into the motherboard.  For
a fairly low price, you can get a wide SCSI version (2940W) and plug it into
any PCI Pentium machine and have Linux boot up and install without any
difficulty.  Linux tends to run circles around any other platform in
squeezing performance out of these drives.

With that many drives, if you have a few extra bucks you might want to
get a hardware RAID controller.  That way if any of the drives fails,
you won't lose any data, and you can simply plug in a spare.  Look in
the current Linux docs to find out which RAID devices are supported (I
haven't tried this yet).

I ran a big Usenet server on these RZ-series drives for a couple of years,
and wound up replacing them with Seagate Barracuda drives as they failed.  The
7200rpm speed seemed to make them run hot, and Seagate had cooler-running
bearing technology than we did at DEC.  The Seagates are still running in
that Linux-based news server years later.  However, in any lesser application,
I had no failures of the RZ28's (Usenet's a mean and nasty thing to do to
a hard drive!).

cheers,
-rich

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