daisy chaining disks-summary

From: Bruce Samuelson (texbell!utafll!bruce@cs.utexas.edu)
Date: Fri Apr 27 1990 - 15:08:34 CDT

Many thanks to the following people for their responses, some of whom gave
detailed, knowledgeable answers. Thanks especially to Mark McDonald of Apex
who patiently offered phone assistance.


Caveat: when a colleague and I daisy chained our drives, we were half brain
dead, and we're only users rather than sys adms even when we are alert. So
take the following with a grain of salt.

I can send a copy of the responses to anyone who needs more detail than is
included below.

Problem: How to daisy chain a second shoebox disk drive onto an existing
shoebox. The following assumes you have a 141 MB drive and Emulex disk
controller and are running SunOS 3.2. You can make the necessary

Part 1: Hardware modifications

1) Existing shoebox requires no modifications. Place it at the end of the
SCSI chain.

2) Open other shoebox. You'll have to pry it pretty hard to get it open once
you undo the two fasteners.

3) Remove tape drive and the tape controller connected underneath it by
unscrewing four nuts connecting it to the shoebox housing.

4) SCSI chains must be properly terminated. I think only the last device in
the chain should have terminating resistors. Therefore, remove two yellow
resistors from the disk controller, which is underneath the tape unit you
already removed. The resistors are labeled U22 and U35. Also remove a white
resistor which is on a card next to the disk drive. The resistor is between
two blue cable connectors right near the front of the disk drive, i.e., near
the open end of the case. It's the only white chip in this position. Now the
intermediate disk drive will not be terminated, as desired.

5) There should be a dip switch in the lower right corner of the disk
controller under the tape unit you removed. It is on the opposite side of the
controller card from the U35 resistor you removed. All the dip switches are
currently set to 0. Set switch number 1 to 1. This changes the address from
0 to 1. The original shoebox, which is at the end of the chain, will keep its
address of 0. I don't understand whether this address refers to just the
disk, or to both the disk and the tape.

6) Remove the cables from the tape drive so that it no longer participates in
the SCSI chain. The Sun documentation does not make provision for having a
second tape drive in a SCSI chain with two shoeboxes, and we haven't attempted
to keep the tape connected. However, one or two of the respondents to my
question believe this is possible. Here's some info david@symult.com received
from Sun on the topic:

"From what I understand you have two identical shoeboxes, disk drives both
jumpered as target 0, tape drives jumpered as target 4. What you have to do,
is open one of the shoeboxes, and jumper the second disk as target 1, and the
second tape drive as target 5. (I'm trying to daisy chain 3 shoeboxes, 2 of
them have a disk drive/tape drive, one of them has just a disk drive. Target
numbers for disk drives go from 0 to 3, tape drives from 4 to 7; for a total
of 8 SCSI devices max, as it should be)." [There are probably also software
changes involved which I won't go into here.]

7) Place the original shoebox at the end of the chain. Plug it into the SCSI
OUT port of the other shoebox, using the original cable. Plug the other
shoebox into the SCSI port of the 3/50, using its original cable.

Part 2: Software modifications (for SunOS 3.2)

1) Create a directory on the original disk, something like usr2, on which you
will mount a directory from the other disk.

2) Go to /dev and run MAKEDEV sd2.

3) Modify /etc/fstab to refer to sd2g, which you'll be mounting on /usr2.
You'll also need to comment out any mounts you had been doing from directories
on the defunct system.

4) Modify /etc/exports to export usr2.

5) If you've customized your kernel, make sure it has a device driver for sd2.
It should be in your GENERIC kernel.

6) You may have to modify /etc/rc, /etc/rc.local, /etc/rc.boot, depending on
your setup, to reflect the new way you're mounting files.

7) For users who had accounts on the disk drive you have moved, you may need
to change their entries in the passwd file to refer to /usr2 rather than
whatever directory they were using. You may be able to avoid this with
symbolic links. I haven't thought about this.

8) If you have some script for doing daily tape backups, this may need

Summary: what I have described is probably 80% right. Doing all this went by
in too much of a blur, and I was too tired, to understand what on earth I was
doing. I can send the detailed responses to anyone who requests them.

* Bruce Samuelson Internet: bruce@utafll.lonestar.org *
* Univ of Texas, Arlington Usenet: ...uunet!texbell!utafll!bruce *

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