422 lines
17 KiB
Plaintext
422 lines
17 KiB
Plaintext
<!-- $Id$ -->
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<!-- The FreeBSD Documentation Project -->
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<!--
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<title>An introduction to ESDI hard disks and their use with FreeBSD</title>
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<author>(c) 1995, Wilko Bulte, <tt/wilko@yedi.iaf.nl/
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<date>Tue Sep 12 20:48:44 MET DST 1995</date>
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Copyright 1995, Wilko C. Bulte, Arnhem, The Netherlands
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<abstract>
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This document describes the use of ESDI disks in combination
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with the FreeBSD operating system. Contrary to popular
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belief, this is possible and people are using ESDI based
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systems successfully! This document tries to explain you
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how to do this.
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If you find something missing, plain wrong or have useful
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comments on how to improve
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the document please send mail to <tt/wilko@yedi.iaf.nl/
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</abstract>
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-->
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<sect1><heading>Using ESDI hard disks<label id="esdi"></heading>
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<p><em>Copyright © 1995, &a.wilko;.<newline>24 September 1995.</em>
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ESDI is an acronym that means Enhanced Small Device Interface.
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It is loosely based on the good old ST506/412 interface originally
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devised by Seagate Technology, the makers of the first affordable
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5.25" winchester disk.
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The acronym says Enhanced, and rightly so. In the first place
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the speed of the interface is higher, 10 or 15 Mbits/second
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instead of the 5 Mbits/second of ST412 interfaced drives.
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Secondly some higher level commands are added, making the ESDI
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interface somewhat 'smarter' to the operating system driver
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writers. It is by no means as smart as SCSI by the way. ESDI
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is standardized by ANSI.
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Capacities of the drives are boosted by putting more sectors
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on each track. Typical is 35 sectors per track, high capacity
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drives I have seen were up to 54 sectors/track.
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Although ESDI has been largely obsoleted by IDE and SCSI interfaces,
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the availability of free or cheap surplus drives makes them
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ideal for low (or now) budget systems.
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<sect2><heading>Concepts of ESDI</heading>
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<p>
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<sect3><heading>Physical connections</heading>
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<p>
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The ESDI interface uses two cables connected to each drive.
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One cable is a 34 pin flat cable edge connector that carries
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the command and status signals from the controller to the
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drive and vice-versa. The command cable is daisy chained
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between all the drives. So, it forms a bus onto which all
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drives are connected.
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The second cable is a 20 pin flat cable edge connector that
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carries the data to and from the drive. This cable is radially
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connected, so each drive has its own direct connection to the
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controller.
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To the best of my knowledge PC ESDI controllers are limited
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to using a maximum of 2 drives per controller. This is
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compatibility feature(?) left over from the WD1003 standard
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that reserves only a single bit for device addressing.
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<sect3><heading>Device addressing</heading>
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<p>
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On each command cable a maximum of 7 devices and 1 controller
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can be present. To enable the controller to uniquely
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identify which drive it addresses, each ESDI device is equipped
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with jumpers or switches to select the devices address.
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On PC type controllers the first drive is set to address 0,
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the second disk to address 1. <it>Always make sure</it> you
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set each disk to an unique address! So, on a PC with its
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two drives/controller maximum the first drive is drive 0, the
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second is drive 1.
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<sect3><heading>Termination</heading>
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<p>
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The daisy chained command cable (the 34 pin cable remember?)
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needs to be terminated at the last drive on the chain.
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For this purpose ESDI drives come with a termination resistor
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network that can be removed or disabled by a jumper when it
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is not used.
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So, one and <it>only</it> one drive, the one at
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the farthest end of the command
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cable has its terminator installed/enabled. The controller
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automatically terminates the other end of the cable.
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Please note that this implies that the controller must be
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at one end of the cable and <it>not</it> in the middle.
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<sect2><heading>Using ESDI disks with FreeBSD</heading>
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<p>
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Why is ESDI such a pain to get working in the first place?
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People who tried ESDI disks with FreeBSD are known to have
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developed a profound sense of frustration. A combination of
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factors works against you to produce effects that are
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hard to understand when you have never seen them before.
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This has also led to the popular legend ESDI and FreeBSD
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is a plain NO-GO.
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The following sections try to list all the pitfalls and
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solutions.
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<sect3><heading>ESDI speed variants</heading>
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<p>
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As briefly mentioned before, ESDI comes in two speed flavors.
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The older drives and controllers use a 10 Mbits/second
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data transfer rate. Newer stuff uses 15 Mbits/second.
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It is not hard to imagine that 15 Mbits/second drive cause
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problems on controllers laid out for 10 Mbits/second.
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As always, consult your controller <it>and</it> drive
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documentation to see if things match.
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<sect3><heading>Stay on track</heading>
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<p>
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Mainstream ESDI drives use 34 to 36 sectors per track.
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Most (older) controllers cannot handle more than this
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number of sectors.
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Newer, higher capacity, drives use higher numbers of sectors
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per track. For instance, I own a 670 Mb drive that has
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54 sectors per track.
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In my case, the controller could not handle this number
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of sectors. It proved to work well except that it only
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used 35 sectors on each track. This meant losing a
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lot of disk space.
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Once again, check the documentation of your hardware for
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more info. Going out-of-spec like in the example might
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or might not work. Give it a try or get another more
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capable controller.
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<sect3><heading>Hard or soft sectoring</heading>
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<p>
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Most ESDI drives allow hard or soft sectoring to be
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selected using a jumper. Hard sectoring means that the
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drive will produce a sector pulse on the start of each
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new sector. The controller uses this pulse to tell when
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it should start to write or read.
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Hard sectoring allows a selection of sector size (normally
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256, 512 or 1024 bytes per formatted sector). FreeBSD uses
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512 byte sectors. The number of sectors per track also varies
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while still using the same number of bytes per formatted sector.
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The number of <em>unformatted</em> bytes per sector varies,
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dependent on your controller it needs more or less overhead
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bytes to work correctly. Pushing more sectors on a track
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of course gives you more usable space, but might give
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problems if your controller needs more bytes than the
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drive offers.
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In case of soft sectoring, the controller itself determines
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where to start/stop reading or writing. For ESDI
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hard sectoring is the default (at least on everything
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I came across). I never felt the urge to try soft sectoring.
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In general, experiment with sector settings before you install
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FreeBSD because you need to re-run the low-level format
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after each change.
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<sect3><heading>Low level formatting</heading>
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<p>
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ESDI drives need to be low level formatted before they
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are usable. A reformat is needed whenever you figgle
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with the number of sectors/track jumpers or the
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physical orientation of the drive (horizontal, vertical).
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So, first think, then format.
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The format time must not be underestimated, for big
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disks it can take hours.
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After a low level format, a surface scan is done to
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find and flag bad sectors. Most disks have a
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manufacturer bad block list listed on a piece of paper
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or adhesive sticker. In addition, on most disks the
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list is also written onto the disk.
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Please use the manufacturer's list. It is much easier
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to remap a defect now than after FreeBSD is installed.
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Stay away from low-level formatters that mark all
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sectors of a track as bad as soon as they find one
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bad sector. Not only does this waste space, it also
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and more importantly causes you grief with bad144
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(see the section on bad144).
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<sect3><heading>Translations</heading>
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<p>
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Translations, although not exclusively a ESDI-only problem,
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might give you real trouble.
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Translations come in multiple flavors. Most of them
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have in common that they attempt to work around the
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limitations posed upon disk geometries by the original
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IBM PC/AT design (thanks IBM!).
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First of all there is the (in)famous 1024 cylinder limit.
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For a system to be able to boot, the stuff (whatever
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operating system) must be in the first 1024 cylinders
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of a disk. Only 10 bits are available to encode the
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cylinder number. For the number of sectors the limit
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is 64 (0-63).
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When you combine the 1024 cylinder limit with the 16 head
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limit (also a design feature) you max out at fairly limited
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disk sizes.
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To work around this problem, the manufacturers of ESDI
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PC controllers added a BIOS prom extension on their boards.
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This BIOS extension handles disk I/O for booting (and for
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some operating systems <it>all</it> disk I/O) by using
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translation. For instance, a big drive might be presented
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to the system as having 32 heads and 64 sectors/track.
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The result is that the number of cylinders is reduced to
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something below 1024 and is therefore usable by the system
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without problems.
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It is noteworthy to know that FreeBSD does not use the
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BIOS after its kernel has started. More on this later.
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A second reason for translations is the fact that most
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older system BIOSes could only handle drives with 17 sectors
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per track (the old ST412 standard). Newer system BIOSes
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usually have a user-defined drive type (in most cases this is
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drive type 47).
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<em>Whatever you do to translations after reading this document,
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keep in mind that if you have multiple operating systems on the
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same disk, all must use the same translation</em>
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While on the subject of translations, I have seen one controller
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type (but there are probably more like this) offer the option
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to logically split a drive in multiple partitions as a BIOS
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option. I had select 1 drive == 1 partition because this
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controller wrote this info onto the disk. On power-up it
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read the info and presented itself to the system based on
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the info from the disk.
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<sect3><heading>Spare sectoring</heading>
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<p>
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Most ESDI controllers offer the possibility to remap bad sectors.
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During/after the low-level format of the disk bad sectors are
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marked as such, and a replacement sector is put in place
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(logically of course) of the bad one.
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In most cases the remapping is done by using N-1 sectors on
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each track for actual data storage, and sector N itself is
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the spare sector. N is the total number of sectors physically
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available on the track.
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The idea behind this is that the operating system sees
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a 'perfect' disk without bad sectors. In the case of
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FreeBSD this concept is not usable.
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The problem is that the translation from <it>bad</it> to <it>good</it>
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is performed by the BIOS of the ESDI controller. FreeBSD,
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being a true 32 bit operating system, does not use the BIOS
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after it has been booted. Instead, it has device drivers that
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talk directly to the hardware.
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<em>So: don't use spare sectoring, bad block remapping or
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whatever it may be called by the controller manufacturer when you
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want to use the disk for FreeBSD.</em>
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<sect3><heading>Bad block handling</heading>
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<p>
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The preceding section leaves us with a problem. The controller's
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bad block handling is not usable and still FreeBSD's filesystems
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assume perfect media without any flaws.
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To solve this problem, FreeBSD use the <it>bad144</it> tool.
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Bad144 (named after a Digital Equipment standard for bad block
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handling) scans a FreeBSD slice for bad blocks. Having found
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these bad blocks, it writes a table with the offending block
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numbers to the end of the FreeBSD slice.
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When the disk is in operation, the disk accesses are checked
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against the table read from the disk. Whenever a block number
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is requested that is in the bad144 list, a replacement block
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(also from the end of the FreeBSD slice) is used.
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In this way, the bad144 replacement scheme presents 'perfect'
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media to the FreeBSD filesystems.
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There are a number of potential pitfalls associated with
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the use of bad144.
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First of all, the slice cannot have more than 126 bad sectors.
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If your drive has a high number of bad sectors, you might need
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to divide it into multiple FreeBSD slices each containing less
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than 126 bad sectors. Stay away from low-level format programs
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that mark <em>every</em> sector of a track as bad when
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they find a flaw on the track. As you can imagine, the
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126 limit is quickly reached when the low-level format is done
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this way.
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Second, if the slice contains the root filesystem, the slice
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should be within the 1024 cylinder BIOS limit. During the
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boot process the bad144 list is read using the BIOS and this
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only succeeds when the list is within the 1024 cylinder limit.
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<em>Note</em> that the restriction is not that only the root
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<em>filesystem</em> must be within the 1024 cylinder limit, but
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rather the entire <em>slice</em> that contains the root filesystem.
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<sect3><heading>Kernel configuration</heading>
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<p>
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ESDI disks are handled by the same <it>wd</it>driver as
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IDE and ST412 MFM disks. The <it>wd</it> driver should work
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for all WD1003 compatible interfaces.
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Most hardware is jumperable for one of two different I/O
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address ranges and IRQ lines. This allows you to have
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two wd type controllers in one system.
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When your hardware allows non-standard strappings, you
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can use these with FreeBSD as long as you enter the
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correct info into the kernel config file.
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An example from the kernel config file (they live in
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<tt>/sys/i386/conf</tt> BTW).
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<tscreen><verb>
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# First WD compatible controller
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controller wdc0 at isa? port "IO_WD1" bio irq 14 vector wdintr
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disk wd0 at wdc0 drive 0
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disk wd1 at wdc0 drive 1
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# Second WD compatible controller
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controller wdc1 at isa? port "IO_WD2" bio irq 15 vector wdintr
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disk wd2 at wdc1 drive 0
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disk wd3 at wdc1 drive 1
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</verb></tscreen>
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<!--
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<sect3><heading>Tuning your ESDI kernel setup</heading>
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<p>
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-->
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<sect2><heading>Particulars on ESDI hardware</heading>
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<p>
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<sect3><heading>Adaptec 2320 controllers</heading>
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<p>
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I successfully installed FreeBSD onto a ESDI disk controlled by a
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ACB-2320. No other operating system was present on the disk.
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To do so I low level formatted the disk using NEFMT.EXE
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(<it>ftp</it>able from <it>www.adaptec.com</it>) and answered NO
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to the question whether the disk should be formatted with a
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spare sector on each track. The BIOS on the ACD-2320 was
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disabled. I used the 'free configurable' option in the system
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BIOS to allow the BIOS to boot it.
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Before using NEFMT.EXE I tried to format the disk using the
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ACB-2320 BIOS builtin formatter. This proved to be a show stopper,
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because it did not give me an option to disable spare sectoring.
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With spare sectoring enabled the FreeBSD installation
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process broke down on the bad144 run.
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Please check carefully which ACB-232xy variant you have. The
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x is either 0 or 2, indicating a controller without or with
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a floppy controller on board.
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The y is more interesting. It can either be a blank,
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a "A-8" or a "D". A blank indicates a plain 10 Mbits/second
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controller. An "A-8" indicates a 15 Mbits/second controller
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capable of handling 52 sectors/track.
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A "D" means a 15 Mbits/second controller that can also
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handle drives with > 36 sectors/track (also 52 ?).
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All variations should be capable of using 1:1 interleaving. Use 1:1,
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FreeBSD is fast enough to handle it.
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<sect3><heading>Western Digital WD1007 controllers</heading>
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<p>
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I successfully installed FreeBSD onto a ESDI disk controlled by a
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WD1007 controller. To be precise, it was a WD1007-WA2. Other
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variations of the WD1007 do exist.
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To get it to work, I had to disable the sector translation and
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the WD1007's onboard BIOS. This implied I could not use
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the low-level formatter built into this BIOS. Instead, I grabbed
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WDFMT.EXE from www.wdc.com Running this formatted my drive
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just fine.
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<sect3><heading>Ultrastor U14F controllers</heading>
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<p>
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According to multiple reports from the net, Ultrastor ESDI
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boards work OK with FreeBSD. I lack any further info on
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particular settings.
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<!--
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<sect2><heading>Tracking down problems</heading>
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<p>
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-->
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<sect2><heading>Further reading<label id="esdi:further-reading"></>
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<p>
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If you intend to do some serious ESDI hacking, you might want to
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have the official standard at hand:
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The latest ANSI X3T10 committee document is:
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<itemize>
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<item>Enhanced Small Device Interface (ESDI) [X3.170-1990/X3.170a-1991]
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[X3T10/792D Rev 11]
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</itemize>
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On Usenet the newsgroup <htmlurl url="news:comp.periphs"
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name="comp.periphs"> is a noteworthy place to look
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for more info.
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The World Wide Web (WWW) also proves to be a very handy info source:
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For info on Adaptec ESDI controllers see <htmlurl
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url="http://www.adaptec.com/">.
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For info on Western Digital controllers see <htmlurl
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url="http://www.wdc.com/">.
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<sect2>Thanks to...
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<p>
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Andrew Gordon for sending me an Adaptec 2320 controller and ESDI disk
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for testing.
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