DSSI: What was it like?

From: Chuck McManis <cmcmanis_at_mcmanis.com>
Date: Sat Oct 14 11:35:19 2000

> DEC's proprietary DSSI interface: Can anyone give me some idea of
> why DEC
>came up with it, what benefits it offered over, say, SCSI, and whether it
>was entirely proprietary or related to something like IPI?
>
> For that matter, what the heck did DSSI abbreviate? My guess:
> Distributed
>Storage Subsystem Interface.

I'm working on a web site and I'm working on a driver for NetBSD :-) But
from what I've found DSSI was DEC's proposal for either SCSI-1 or SCSI-2.
Like SCSI it is an 8 bit parallel interface, unlike SCSI it is only a
"transport", which is to say it has primitives for passing chunks of data
(packets) from one device on the bus to another, but it does _not_ have any
"disk" operations. This is what allows it to be used as both an interface
to DSSI disks and as a cluster interconnect to other systems. On the DSSI
bus one can simply send a bunch of data to some target and it is up to the
target to understand what you are talking about.

This was one of the reasons I initially couldn't write a device driver for
it since the disk operations are MSCP and that wasn't defined in the
KA640-TM. I've since acquired a copy of the "secret" MSCP manual and the
source code to the Ultrix driver. The latter however is not as useful as I
would have hoped.

One of the more interesting aspects of this bus is that every "target" is
an independent entity (its more like an 8 bit parallel network than an I/O
bus). Thus disks have their own controller running their own OS etc and you
can actually "connect" to the disk and run software on it (notably the
formatter and parameter setup stuff). This also means that sharing a disk
amongst multiple systems on the same DSSI bus is not technically very
difficult, you just put another system on the bus and send the disk
commands and get in line :-)

The challenges of course are keeping track of who has files open and what
they are doing to them. This requires some cooperation on the part of the
disk drive, but it is provided for in the MSCP protocol ( you can "lock"
segments of the drive to your system.)

All in all I think it is superior to SCSI in many ways but because DEC held
on to it so tightly it was never adopted beyond DEC machines.

As for the abbreviation DSSI I've heard two different expansions used:
         "Digital Standard Storage Interconnect"
         "Digital Standard System Interconnect"
I've yet to find something that pins it down definitively.

--Chuck
Received on Sat Oct 14 2000 - 11:35:19 BST