Tech Arsenal 1/Tech Arsenal (Arsenal Computer).ISO/tek-06/artcle.zip/DISKS-1.TXT

Text File | 1991-04-07 | 12KB | 210 lines

Hard Drives and NetWare:
A Brief Description and Usage Summary

By Mickey Applebaum
Wasatch Education Systems/NetWire Sysop

Since the early days of operation, drastic changes have been made
in the way computers and NetWare deal with the various types of
drives and controllers. This article briefly outlines drive types
and the restrictions that apply to each, and explains how to
prepare drives for use with NetWare. (The following information
is gathered from six years of working with Novell's NetWare
products and various third-party programs to allow different
drive types to work properly with the various versions of NetWare
from v4.57a onward.)

Because NetWare for the PC was first designed to operate on IBM
and compatible PC XT computers, NetWare relied on the hard disk
controller's BIOS to set up the drive properly. This method was a
rather simplistic way of accessing the hard drives and led to
problems in configuring nonstandard size drives.

At this time, two options were feasibly available to the general
PC marketplace: The first was Modified Frequency Modulation (MFM)
encoded controllers, and the second was Run Length Limited (RLL)
encoded controllers. For this discussion, the main difference
between the two is that an MFM controller formats a disk at 17
sectors per track, while an RLL controller does either 21, 26,
27, or 34 sectors per track.  With 286-based NetWare, Novell
engineers decided there was a better way to get the drives
physical configuration for number of heads, cylinders (also known
as tracks), sectors per track, and bytes per sector (This is also
referred to as the drive's geometry) quickly and reliably. This
method involves reading the computer's BIOS drive table directly,
after reading the drive type number from the CMOS area.

This method allowed NetWare to get the drive geometry information
consistently and reliably, and meant a standardization in the
drives supported. Unfortunately, this method also imposed
limitations--although these limitations would not be felt for
years to come.    All PC AT computers of the time were using the
AT standard disk controller design. NetWare had no problems
running on these drives since all the disks were formatted at 17
sectors per track and each sector contained 512 bytes of data. No
drive had more than 1,024 cylinders (a limitation of the original
AT BIOS), each track had no more than 63 sectors per track, and
there were no more than 16 heads. As a result, all hard drives
were limited to about 500MB, but that was considered more storage
space than anyone would need. The largest drive supported by the
BIOS drive type was only a 117MB drive (type 9 in the original AT
BIOS).   NetWare v2.0a worked fine in this framework for the
first couple of years. But as newer and larger drives became
available and as more drive manufacturers came out with hard
drives that didn't directly support the BIOS drive table, users
needed a product to use the increased capacity of these drives.

Probably the best known option of the time was the Golden Bow ROM
extension board. This board was a combination hardware/software
solution: The hardware consisted of a BIOS drive table extension
Read-Only Memory (ROM), and the software consisted of a patch for
the operating system files and utilities so they would recognize
the new ROM.

The Golden Bow ROM extension board allowed for the use of more
than 25 different drive types--most of which were specified by
drive manufacturer and model. The extension board also allowed
for the use of the then new RLL-encoded controllers for the AT
bus. The RLL controller formatted disks to a consistent 26
sectors per track, thus increasing disk capacity by 50 percent.
This option breathed life into NetWare v2.0a installations where
disk space was tight and being limited by the BIOS drive tables
on some computers was not acceptable.

NetWare v2.0a also introduced the Novell disk coprocessor board
(DCB), which was the first small computer system interface (SCSI)
host board available to the NetWare world. The DCB allowed the
use of the Adaptec 4000 and 4070 midstage controllers, which were
standalone hard disk controller cards that connected a SCSI host
either to an MFM disk (4000) or to a RLL (4070) certified ST-506.
(ST-506 is the standard hard disk designation for drives attached
to an AT-type controller.)

The DCB allowed for the use of drives such as the Maxtor 1240,
which formatted to 179MB on the Adaptec 4070, or the Maxtor
2190/Priam ID230 drives, which formatted to 240MB on the 4070. At
the time, these were considered the largest drives available for
the AT computers.   Soon after the DCB was introduced, several
embedded controller SCSI drive devices became available. The
major difference between these drives and the Maxtor 1240/2190
was that the controller was directly attached to the SCSI hard
drive. The embedded controller increased performance because the
data and controller signals did not have to be passed over any
cables  longer than half an inch. Probably the most famous drive
series of the time was the CDC Wren III series.

NetWare v2.0a, however, still had some major limitations. It
could only support drives on the AT controller that were
specifically defined in BIOS, or drives that were connected to a
DCB and defined in the NetWare DISKSET program. NetWare v2.0a had
a drive limitation of 255MB. Drives larger than 255MB could only
have a single NetWare partition of 255MB; the rest of the drive
would be wasted.

The 255MB limitation was overcome with the release of NetWare
v2.1. The 17-sector limitation imposed on AT-type disk
controllers was also lifted, making the way for the new
technology of Enhanced Small Disk Interface (ESDI) drives. ESDI
is an enhanced mode of RLL operation that allows hard disks to be
formatted up to the theoretical limit of 63 sectors per track.


The controllers could also work efficiently with drives that have
more than 1,024 cylinders. Due to the limitations of the AT
controller from the computer's BIOS, however, most of these
controllers needed a driver to run correctly.

Sector Translation answered this need. Sector Translation is a
method whereby the controller, through the use of an intelligent
BIOS, converts the hard drive's physical geometry into a
compatible logical geometry by lowering the number of cylinders
and increasing the number of heads and sectors per track. This
process is done on the controller, and the controller takes care
of the logistics of making the computer think the drive is the
size defined in the CMOS/BIOS tables. Most drives use the BIOS
drive type 1 to operate.

NetWare v2.1 also introduced Value-Added Disk Drivers (VADDs).
The VADD specification allowed any disk controller manufacturer
to produce a driver  for its interfaces to operate in a NetWare
environment. (These drivers are similar to the drivers
manufacturers produced for DOS.) NetWare was now open to all the
available SCSI host adapters, which were starting to flood the PC
marketplace. Users had a viable option to the DCB for their SCSI
drive needs.

With the release of NetWare v2.1, the limitation of one partition
was lifted, and the ability to have multiple NetWare volumes on a
single drive going beyond 255MB made superservers a workable
option. A SCSI host adapter can control up to eight controllers,
and each controller can support up to two drive devices. Using
embedded controller drives limited the controller to only one
drive.

Soon the market was seeing the release of 300MB and 600MB hard
drives for both the SCSI and ESDI interfaces. At this point, some
of Novell's early decisions to stay within the constraints of the
AT BIOS drive definition caused problems. The most obvious
problem is referred to as Cylinder Wrap.


The Cylinder Wrap is a condition that occurs when a drive's
NetWare partition is set up with a DOS-compatible partitioning
program (such as the partition definition routines in NETGEN's
Custom option). Since the AT BIOS limit is 1,024 cylinders,
NetWare can't interpret the cylinder request if a drive has a
partition going beyond that limit. With a drive that has more
than 1,024 cylinders, NetWare subtracts 1,024 from the requested
cylinder and writes or reads from the remaining cylinder number.

Cylinder Wrap also occurs when Hot Fix is activated due to a
write fault. This possibility is worse than a write request
because only one block in a file may be damaged, but you must
scan all files to find that one block.   In pursuit of complete
compatibility, Novell has corrected this problem with special-
purpose files and the strict instruction to use the default mode
of NetWare installation to define the hard drive's partition
table. What this means is that you must run through the NetWare
Installation option from the NetGen menu twice.  The first time
selecting the Default Installation option, and going through the
steps up to the point that the partition table is written to the
disk.  And the second time into the NetWare Installation choice
selecting the Custom Installation option to define the volumes,
number of directory entries, and other system parameters. Also
when NetWare 386 was designed, the AT BIOS limitations were
completely removed.

The newest hard drives are the IDE drives developed by the Connor
Corporation. Integrated Drive Electronics (IDE, developed by
Connor Technologies) drives are similar to SCSI drives in that
the controller for the drive is embedded on the drive itself, but
IDE drives are also similar to ESDI drives in that they use the
computers BIOS to function and allow for limited sector
translation, and only two drives are supported in a single
machine. These drives are particularly appealing to NetWare users
because they have a fairly large capacity (200MB, with 500MB
drives on the horizon) at a fairly low cost (approximately
$1,000). Unfortunately, these drives have opened up a whole new
arena of disk errors.   Since these drives use a limited sector
translation capability (which converts physical capacity to match
a BIOS defined drive type of similar or lesser capacity), it is
impossible to use the full capacity of the drives on most
computers. This problem has two solutions: First, the CMOS user
definable drive type allows you to specify a drive's geometry and
force it into the computer's CMOS definitions directly from the
keyboard.  Unfortunately, since NetWare still relies on the
computer's BIOS drive tables for the AT Standard and ISADISK
drivers, the user definable drive types are unusable in a NetWare
environment.   The second solution is the disk preparation
software package Disk Manager -N from On-Track Systems. This
product prepares any AT Controller device for NetWare by putting
a stamp on the drive and by modifying the Operating Ssytem and
disk drivers for NetWare. This program not only allows for drive
types that do not fit directly into the BIOS drive table, but
also circumvents the 1,024 cylinder limitation.

Many drive solutions are available today. With the advent of
smaller physical drive sizes and the increase in magnetic media
reliability,  multi-gigabyte hard drives in a single 3.5" drive
frame (all completely addressable by a NetWare server) are not
too far off.