From: Ed Cooley <edc@openage.COM>
Subject: SCSI as secondary drive
Resent-From: mmdf@xenitec.on.ca
Submit-To: scogen@xenitec.on.ca
Organization: [resent by] The SCOGEN gateway and Propagation Society
Date: Thu, 5 Nov 1992 14:25:48 GMT
Precedence: bulk


> I've seen this asked here, and am asking myself, for opinions and results.

> A client has an SCO UNIX box with an IDE drive as the primary disk.  They
> now want to expand and add a SCSI disk as a secondary drive.  I've seen 
> several people here asking about this and was hoping they would enlighten
> me as to their success/failure and pitfalls of doing so.  Knowing what you
> did would also help to reduce the time it takes me to install (not having
> to blaze a new trail to the same place and all).  

> any comments more than welcome!!

> brian

We have noticed that a lot of more current IDE drives make 
excellent primary drives on UNIX systems.  Specifically 
higher-capacity drives tend to be more recent in design and have 
data transfer rates between 12-16 Mbps, and access times as low 
as 12 ms.  Often times these drives will have some onboard cache 
as well, such as the Quantums.  

Using the SCSI drive as a secondary drive is easy to implement.  
In most cases simply be sure to disable WD interface.  In the 
case of the Adaptec 1540B, just disable the BIOS.  With EISA 
adapters like the 1740 or the DPT 2012B, run EISA Config and 
disable the WD mode.  IDE drives, like other ST506 interfaces, 
typically can have two drives daisy-chained together if the 
jumpers are set properly.  The primary IDE drive however, must 
also use IRQ 14 and the primary disk address.  Therefore, the IDE 
drive must be the boot disk.

Since SCSI host adapters are often multi-threaded in design, and 
make use of bus-mastering as well, they can accept multiple 
requests upon SCSI targets simultaneously.  This makes them well 
suited for multi-tasking or network operating systems.  In 
theory, you could see good performance gains by moving heavily 
used programs, data, and user directories off on to the SCSI 
drive.  This will be totally dependent upon the load placed on 
the system, and the drive and adapter combination you choose 
though.  

Selecting an ISA adapter with a SCSI hard disk that only operates
in asynchronous mode will likely reduce your performance more
than help it.  So my suggestions are to buy the best
performance SCSI adapter you or your customer can justify.  For 
an ISA system, the Adaptec 1540B is a good choice since it has a 
high level of compatibility, and supports 5 MB/s synchronous 
burst modes.  The DPT 2011 is a great ISA choice if you are into 
caching adapters, which can substantially increase performance on 
write operations (the EAFS or AFS takes care of read-ahead 
caching).  For EISA adapters, the Adaptec 1740 and DPT 2012 
support 10 MB/s synchronous burst modes.  Of course there are 
others, these came off the top of my head.

Perhaps most importantly though, is the drive you pick.  If 
performance is important, be aware that most SCSI drives do NOT 
support the 10 MB/s synchronous operation.  This is especially 
true with popular inexpensive drives like the Fujitsus (known 
around here as skillets because you can cook eggs on them).  I 
woul however actively suggest Hewlett-Packard's OEM disk drives 
sold through distributors to computer dealers.  These drives come 
with 5 year warranties, and support 10 MB/s synchronous 
operation.  It will make a difference in performance.

For installation, you'll find SCO tends to set up their drivers 
at the manufacturers' default IRQ, addresses, etc.  To install 
the disk itself, run "mkdev hd."  Choose the appropriate adapter,
and identify what the drive target ID (TID or SCSI address) is, 
as well as the logical unit number (LUN).  SCSI target ID 
defaults differ between manufacturers.  The host adapter itself 
is almost always set for 7, SCO expects tape drives to be at 2, 
and CD-ROM drives to be at 5.  That leaves you with 0, 1, 3, 4, 
and 6.  If this is on a bootable SCSI adapter (no IDE drive), 
then SCO expects the first two SCSI drives to be set at 0 and 1 
respectively.  After completing "mkdev hd" you'll need to relink 
and reboot in between to modify the kernel for the new drive.  
This modifies /etc/conf/cf.d/mscsi as well.  Once you've 
rebooted, run "mkdev hd" again.  This will kick you into divvy 
and make filesystems.  After which you should create the 
subdirectory(s) which will be the mount-point(s) for the new 
filesystem(s).  Then, run "mkdev fs" to specify the associated 
device names, mount points, and create the lost+found entries.  
Reboot and go into multi-user mode to access the new SCSI drive.

Hope this helps some.

As with any purchase, you will need to make the call between 
price and performance.

--------------------------------------------------------------------------------
Ed Cooley           12209 Bayswater Road    "An independent service and
Technical Manager   Gaithersburg, MD 20878   support provider for SCO
Open Age, Inc.      Main: 301/948-6422       products."
edc@openage.com     Fax:  301/948-9644