forrest@sybase.com (09/19/89)
After posting a series of questions about 3rd party disks on
Vaxstation 3100s I received a phone call from a DEC engineer who
worked on the VS3100 disk system. He told me several interesting
things:
1. It is true that VMS 5.2 (and lower) doesn't support the DISCONNECT
feature with 3rd party drives. (DEC drives DO do disconnect).
The reason for this is that when DEC was first developing the disk
system for the VS3100 they believed that not all disks would implement
DISCONNECT correctly. Rather than taking a chance, they just chose not
to do DISCONNECT on 3rd party drives. Because of potential legal concerns,
DEC has changed its mind on this and will support DISCONNECT on 3rd party
drives in a "future release" of VMS. By the way, the way DEC determines
if a drive is 3rd party is by strings returned by the drive.
As the boys down at Harvey Mudd discovered, you can patch the SCSI
driver to think that 3rd party drives are DEC drives by changing
various character string constants in the driver. (Don't ask me
exactly how to do this because I don't know). He also said that
the DISCONNECT function makes the biggest difference when you
have more than one drive.
2. Disks connected to a VS3100 use the asynchronous communication
method. This is because the local port hardware in the VS3100 can't
handle the higher speeds of the synchronous mode. The Vaxstation
3510 and 3520 can do synchronous data transfer.
3. DEC did not cripple the VS3100 disk system because VS3100 performance
would be too close to higher priced Vaxes. The decision to not support
DISCONECT was purely a technical decision. It is my guess that the
decision to not support synchronous mode was due to cost considerations.
What all this means is that, as of now, drives that look like 3rd
party drives to VMS will not do DISCONNECT, resulting in reduced
performance. There is a potential for problems with 3rd party drives
not doing DISCONNECT right. This sounds to me like the kind of problem
that used to exist with attaching 3rd party devices to the Unibus.
Most devices work but sometimes there's one that's in the twilight
zone. But, until the future release of VMS that does support DISCONNECT
comes out, there'll be no problems since you couldn't use DISCONNECT
with 3rd party drives unless you get a new (or patched) driver.
What surprises me is the lack of 3rd party drives for VS3100s. I
only saw two places advertising such drives in the latest issue of
Digital Review. On the other hand, since there are also only two
places I'm aware of that make 3rd party memory for VS3100s maybe
it's just too early.
Jon Forrest
forrest@sybase.com
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