[conspire] SCSI disconnect (was: hrm...)

Rick Moen rick at linuxmafia.com
Tue Dec 5 18:18:26 PST 2006

[Hoping you don't mind my CC'ing conspire@, because of the cautionary
tale below.]

Quoting Paul Reiber (reiber at gmail.com):

> I'll continue down the route of pulling drives from the chain.
> Unfortunately the next obvious choice is the other 9GB which has the /
> partition on it...

Don't be too surprised if that does the trick.

Remember, all the gear we had was of unknown characteristics.  And
there's a particular way in which some crappo SCSI drives have been
known to be problematic:  Some of them don't support SCSI disconnect.

You can go into the Adaptec SCSI BIOS and turn off the Adaptec chip's
willingness to attempt disconnected operation for each specific SCSI ID,
individually.  Sometimes, you do that, and suddenly the drive works well
in conjunction with other drives.  But, of course, more slowly than you
would like, having reduced the chain to an IDE level of inefficiency.

If that's the case with one (or two) of those 9GB drives, then that will
make the second (and possibly third) drive I've ever encountered that's 
that miserably bad.  However, it's possible.  I've seen it:

Consider Tandem Computer, now long swallowed up and digested.  It used
to sell workstations with each one SCSI drive.  At a point a decade or
so ago, they came to IBM to buy a batch of 1.3 GB SCSI drives for such a
workstation, and IBM offered them a special deal on a big batch of them,
at very low distress prices because they didn't do disconnected

This wasn't a deal-breaker for Tandem, because the machine was spec'd to
have just the one hard drive, ergo inability to detach from the host
adapter and do independent operations ("SCSI disconnect") wasn't an
issue.  So, they bought some number of shipping palettes of the drives,
really cheaply.  They had a lot more than they needed, so they sold a
lot of them off to NCA Computer Products.  Where I bought one, had
enormous problems trying to use it on a multidrive chain, eventually
figured out that it worked if you disabled disconnect, and brought it
back to exchange it.

NCA Computer Products always had the best prices in the Valley, and
their reliance on grey market crappo merchandise was the main reason why.
They also always had a long line of people waiting to exchange defective
or otherwise malfunctioning gear.  I have no doubt that the IBM drive I
brought back went right back onto their shelf.

(I confirmed my suspicion about the drive by telephoning IBM, who
confirmed that it was a model lacking disconnect ability, and referring
me for any further support to the OEM'er, the then-already-defunct
Tandem Computer, whose S/N it bore.)

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