f7343bd74d
pointed out by Rick Robinson. Found and fixed some grammar problems at the same time. Note: the reason for avoiding contractions is two-fold: 1) It makes the text easier to understand for people who speak English as a second language. 2) Expanding the contractions often reveals poorly worded passages.
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892 lines
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<!-- $Id: scsi.sgml,v 1.19 1996/10/04 22:54:14 wosch Exp $ -->
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<!-- The FreeBSD Documentation Project -->
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<!--
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<title>An introduction to SCSI and its use with FreeBSD</title>
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<author>(c) 1995-1996, Wilko Bulte, <tt/wilko@yedi.iaf.nl/
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<date>Sat Jul 6 20:57:39 MET DST 1996</date>
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Copyright 1995-1996, Wilko C. Bulte, Arnhem, The Netherlands
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<abstract>
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This document attempts to describe the background of SCSI, its
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(mis)use with FreeBSD and some common pitfalls.
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</abstract>
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-->
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<sect1><heading>What is SCSI?<label id="scsi"></heading>
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<p><em>Copyright © 1995, &a.wilko;.<newline>July 6, 1996.</em>
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SCSI is an acronym for Small Computer Systems Interface. It is an
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ANSI standard that has become one of the leading I/O buses in the
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computer industry. The foundation of the SCSI standard was laid by
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Shugart Associates (the same guys that gave the world the first
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mini floppy disks) when they introduced the SASI bus (Shugart Associates
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Standard Interface).
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After some time an industry effort was started to come to a more strict
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standard allowing devices from different vendors to work together.
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This effort was recognized in the ANSI SCSI-1 standard. The SCSI-1
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standard (approx 1985) is rapidly becoming obsolete. The current
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standard is SCSI-2 (see <ref id="scsi:further-reading" name="Further
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reading">), with SCSI-3 on the drawing boards.
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In addition to a physical interconnection standard, SCSI defines a
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logical (command set) standard to which disk devices must adhere.
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This standard is called the Common Command Set (CCS) and was
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developed more or less in parallel with ANSI SCSI-1. SCSI-2
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includes the (revised) CCS as part of the standard itself. The
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commands are dependent on the type of device at hand. It does not
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make much sense of course to define a Write command for a
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scanner.
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The SCSI bus is a parallel bus, which comes in a number of
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variants. The oldest and most used is an 8 bit wide bus, with
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single-ended signals, carried on 50 wires. (If you do not know what
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single-ended means, do not worry, that is what this document is all
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about.) Modern designs also use 16 bit wide buses, with
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differential signals. This allows transfer speeds of
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20Mbytes/second, on cables lengths of up to 25 meters. SCSI-2
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allows a maximum bus width of 32 bits, using an additional cable.
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Quickly emerging are Ultra SCSI (also called Fast-20) and Ultra2
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(also called Fast-40). Fast-20 is 20 mega-transfers per second
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(20 Mbytes/sec on a 8 bit bus), Fast-40 is 40 mega-transfers per
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second (40 Mbytes/sec on a 8 bit bus).
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Of course the SCSI bus not only has data lines, but also a number
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of control signals. A very elaborate protocol is part of the
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standard to allow multiple devices to share the bus in an efficient
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manner. In SCSI-2, the data is always checked using a separate
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parity line. In pre-SCSI-2 designs parity was optional.
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In SCSI-3 even faster bus types are introduced, along with a serial
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SCSI busses that reduces the cabling overhead and allows a higher
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maximum bus length. You might see names like SSA and Fiberchannel
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in this context. None of the serial buses are currently in widespread
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use (especially not in the typical FreeBSD environment). For
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this reason the serial bus types are not discussed any further.
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As you could have guessed from the description above, SCSI devices
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are intelligent. They have to be to adhere to the SCSI standard
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(which is over 2 inches thick BTW). So, for a hard disk drive for
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instance you do not specify a head/cylinder/sector to address a
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particular block, but simply the number of the block you want.
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Elaborate caching schemes, automatic bad block replacement etc
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are all made possible by this 'intelligent device' approach.
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On a SCSI bus, each possible pair of devices can communicate. Whether
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their function allows this is another matter, but the standard does
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not restrict it. To avoid signal contention, the 2 devices have to
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arbitrate for the bus before using it.
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The philosophy of SCSI is to have a standard that allows
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older-standard devices to work with newer-standard ones. So, an
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old SCSI-1 device should normally work on a SCSI-2 bus. I say
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Normally, because it is not absolutely sure that the implementation
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of an old device follows the (old) standard closely enough to be
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acceptable on a new bus. Modern devices are usually more
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well-behaved, because the standardization has become more strict
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and is better adhered to by the device manufacturers.
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Generally speaking, the chances of getting a working set of
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devices on a single bus is better when all the devices are SCSI-2
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or newer. This implies that you do not have to dump all your old
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stuff when you get that shiny 2Gb disk: I own a system on which a
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pre-SCSI-1 disk, a SCSI-2 QIC tape unit, a SCSI-1 helical scan
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tape unit and 2 SCSI-1 disks work together quite happily. From
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a performance standpoint you might want to separate your older
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and newer (=faster) devices however.
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<sect2><heading>Components of SCSI</heading>
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<p>
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<!-- <sect3><heading>A <it>smart</it> interface</heading>
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<p> -->
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As said before, SCSI devices are smart. The idea is to put the
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knowledge about intimate hardware details onto the SCSI device
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itself. In this way, the host system does not have to worry
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about things like how many heads are hard disks has, or how many
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tracks there are on a specific tape device. If you are curious,
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the standard specifies commands with which you can query your
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devices on their hardware particulars. FreeBSD uses this
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capability during boot to check out what devices are connected
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and whether they need any special treatment.
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The advantage of intelligent devices is obvious: the device
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drivers on the host can be made in a much more generic fashion,
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there is no longer a need to change (and qualify!) drivers for
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every odd new device that is introduced.
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<!-- <sect3><heading>Do's and don't's on interconnections</heading>
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<p> -->
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For cabling and connectors there is a golden rule: get good
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stuff. With bus speeds going up all the time you will save
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yourself a lot of grief by using good material.
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So, gold plated connectors, shielded cabling, sturdy connector
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hoods with strain reliefs etc are the way to go. Second golden
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rule: do no use cables longer than necessary. I once spent 3 days
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hunting down a problem with a flaky machine only to discover that
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shortening the SCSI bus by 1 meter solved the problem. And the
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original bus length was well within the SCSI specification.
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<sect2><heading>SCSI bus types</heading>
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<p>
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From an electrical point of view, there are two incompatible bus
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types: single-ended and differential. This means that there are
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two different main groups of SCSI devices and controllers, which
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cannot be mixed on the same bus. It is possible however to use
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special converter hardware to transform a single-ended bus into a
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differential one (and vice versa). The differences between the
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bus types are explained in the next sections.
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In lots of SCSI related documentation there is a sort of jargon
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in use to abbreviate the different bus types. A small list:
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<itemize>
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<item>FWD: Fast Wide Differential
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<item>FND: Fast Narrow Differential
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<item>SE: Single Ended
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<item>FN: Fast Narrow
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<item>etc.
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</itemize>
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With a minor amount of imagination one can usually imagine what
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is meant.
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Wide is a bit ambiguous, it can indicate 16 or 32 bit buses. As
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far as I know, the 32 bit variant is not (yet) in use, so wide
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normally means 16 bit.
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Fast means that the timing on the bus is somewhat different, so
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that on a narrow (8 bit) bus 10 Mbytes/sec are possible instead
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of 5 Mbytes/sec for 'slow' SCSI. As discussed before, bus
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speeds of 20 and 40 megatransfers/second are also emerging
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(Fast-20 == Ultra SCSI and Fast-40 == Ultra2 SCSI).
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It should be noted that the data lines > 8 are only used for
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data transfers and device addressing. The transfers of commands
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and status messages etc are only performed on the lowest 8
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data lines. The standard allows narrow devices to operate on
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a wide bus. The usable bus width is negotiated
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between the devices. You have to watch your device addressing
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closely when mixing wide and narrow.
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<sect3><heading>Single ended buses</heading>
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<p>
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A single-ended SCSI bus uses signals that are either 5 Volts or
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0 Volts (indeed, TTL levels) and are relative to a COMMON
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ground reference. A singled ended 8 bit SCSI bus has
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approximately 25 ground lines, who are all tied to a single
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`rail' on all devices. A standard single ended bus has a
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maximum length of 6 meters. If the same bus is used with
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fast-SCSI devices, the maximum length allowed drops to 3
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meters. Fast-SCSI means that instead of 5Mbytes/sec the bus
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allows 10Mbytes/sec transfers.
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Fast-20 (Ultra SCSI) and Fast-40 allow for 20 and 40
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megatransfers/second respectively. So, F20 is 20 Mbytes/second
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on a 8 bit bus, 40 Mbytes/second on a 16 bit bus etc.
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For F20 the max bus length is 1.5 meters, for F40 it
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becomes 0.75 meters. Be aware that F20 is pushing
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the limits quite a bit, so you will quickly find out if your
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SCSI bus is electrically sound.
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Please note that this means that
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if some devices on your bus use 'fast' to communicate your
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bus must adhere to the length restrictions for fast buses!
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It is obvious that with the newer fast-SCSI devices the
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bus length can become a real bottleneck. This is why the
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differential SCSI bus was introduced in the SCSI-2 standard.
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For connector pinning and connector types please refer to the
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SCSI-2 standard (see <ref id="scsi:further-reading" name="Further
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reading">) itself, connectors etc are listed there in
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painstaking detail.
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Beware of devices using non-standard cabling. For instance
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Apple uses a 25pin D-type connecter (like the one on serial
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ports and parallel printers). Considering
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that the official SCSI bus needs 50 pins you can imagine
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the use of this connector needs some 'creative cabling'.
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The reduction of the number of ground wires they used
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is a bad idea, you better stick to 50 pins cabling
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in accordance with the SCSI standard. For Fast-20 and 40
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do not even think about buses like this.
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<sect3><heading>Differential buses</heading>
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<p>
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A differential SCSI bus has a maximum length of 25
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meters. Quite a difference from the 3 meters for a single-ended
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fast-SCSI bus. The idea behind differential signals is that
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each bus signal has its own return wire. So, each signal is
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carried on a (preferably twisted) pair of wires. The voltage
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difference between these two wires determines whether the
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signal is asserted or de-asserted. To a certain extent the
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voltage difference between ground and the signal wire pair is
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not relevant (do not try 10 kVolts though).
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It is beyond the scope of this document to explain why this
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differential idea is so much better. Just accept that
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electrically seen the use of differential signals gives a much
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better noise margin. You will normally find differential buses
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in use for inter-cabinet connections. Because of the lower cost
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single ended is mostly used for shorter buses like inside
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cabinets.
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There is nothing that stops you from using differential stuff
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with FreeBSD, as long as you use a controller that has device
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driver support in FreeBSD. As an example, Adaptec marketed the
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AHA1740 as a single ended board, whereas the AHA1744 was differential.
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The software interface to the host is identical for both.
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<sect3><heading>Terminators</heading>
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<p>
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Terminators in SCSI terminology are resistor networks that are
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used to get a correct impedance matching. Impedance matching
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is important to get clean signals on the bus, without
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reflections or ringing. If you once made a long distance
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telephone call on a bad line you probably know what reflections
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are. With 20Mbytes/sec traveling over your SCSI bus, you
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do not want signals echoing back.
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Terminators come in various incarnations, with more or less
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sophisticated designs. Of course, there are internal and
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external variants. Almost every SCSI device comes with a
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number of sockets in which a number of resistor networks can
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(must be!) installed. If you remove terminators from a device,
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carefully store them. You will need them when you ever decide to
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reconfigure your SCSI bus. There is enough variation in even
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these simple tiny things to make finding the exact replacement
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a frustrating business. There are also SCSI devices that have
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a single jumper to enable or disable a built-in terminator.
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There are special terminators you can stick onto a flat cable
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bus. Others look like external connectors, or a connector hood
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without a cable. So, lots of choice as you can see.
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There is much debate going on if and when you should switch
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from simple resistor (passive) terminators to active
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terminators. Active terminators contain slightly more elaborate
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circuit to give cleaner bus signals. The general consensus
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seems to be that the usefulness of active termination increases
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when you have long buses and/or fast devices. If you ever have
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problems with your SCSI buses you might consider trying an
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active terminator. Try to borrow one first, they reputedly are
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quite expensive.
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Please keep in mind that terminators for differential and
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single-ended buses are not identical. You should <bf>not
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mix</bf> the two variants.
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OK, and now where should you install your terminators? This is
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by far the most misunderstood part of SCSI. And it is by far
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the simplest. The rule is: <bf>every SCSI bus has 2 (two)
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terminators, one at each end of the bus.</bf> So, two and not
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one or three or whatever. Do yourself a favor and stick to
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this rule. It will save you endless grief, because wrong
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termination has the potential to introduce highly mysterious
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bugs.
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A common pitfall is to have an internal (flat)cable in a
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machine and also an external cable attached to the
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controller. It seems almost everybody forgets to remove the
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terminators from the controller. The terminator must now be on
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the last external device, and not on the controller! In
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general, every reconfiguration of a SCSI bus must pay attention
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to this.
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What I did myself is remove all terminators from my SCSI
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devices and controllers. I own a couple of external
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terminators, for both the Centronics-type external cabling and
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for the internal flat cable connectors. This makes
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reconfiguration much easier.
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On modern devices, sometimes integrated terminators are
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used. These things are special purpose integrated circuits that
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can be dis/en-abled with a control pin. It is not necessary to
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physically remove them from a device. You may find them on
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newer host adapters, sometimes they even are software
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configurable, using some sort of setup tool. Consult you
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documentation!
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<sect3><heading>Terminator power</heading>
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<p>
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The terminators discussed in the previous chapter need power to
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operate properly. On the SCSI bus, a line is dedicated to this
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purpose. So, simple huh?
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Not so. Each device can provide its own terminator power to
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the terminator sockets it has on-device. But if you have
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external terminators, or when the device supplying the
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terminator power to the SCSI bus line is switched off you are
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in trouble.
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The idea is that initiators (these are devices that initiate
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actions on the bus, a discussion follows) must supply
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terminator power. All SCSI devices are allowed (but not
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required) to supply terminator power.
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To allow for un-powered devices on a bus, the terminator
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power must be supplied to the bus via a diode. This prevents
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the backflow of current to un-powered devices.
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To prevent all kinds of nastiness, the terminator power is
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usually fused. As you can imagine, fuses might blow. This can,
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but does not have to, lead to a non functional bus. If multiple
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devices supply terminator power, a single blown fuse will not
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put you out of business. A single supplier with a blown fuse
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certainly will. Clever external terminators sometimes have a
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LED indication that shows whether terminator power is present.
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In newer designs auto-restoring fuses that 'reset'
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themselves after some time are sometimes used.
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<sect3><heading>Device addressing</heading>
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<p>
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Because the SCSI bus is, ehh, a bus there must be a way to
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distinguish or address the different devices connected to it.
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This is done by means of the SCSI or target ID. Each device has
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a unique target ID. You can select the ID to which a device
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must respond using a set of jumpers, or a dip switch, or
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something similar. Consult the documentation of your device for
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more information.
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Beware of multiple devices configured to use the same ID. Chaos
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normally reigns in this case. A pitfall is that one of the
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devices sharing the same ID sometimes even manages to answer
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to I/O requests!
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For an 8 bit bus, a maximum of 8 targets is possible. The
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maximum is 8 because the selection is done bitwise using the 8
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data lines on the bus. For wide buses this increases to the
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number of data lines.
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The higher the SCSI target ID, the higher the priority the
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devices has. When it comes to arbitration between devices that
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want to use the bus at the same time, the device that has the
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highest SCSI ID will win. This also means that the SCSI
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host adapter usually uses target ID 7 (for narrow buses).
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For a further subdivision, the standard allows for Logical
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Units or LUNs for short. A single target ID may have multiple
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LUNs. For example, a tape device including a tape changer may
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have LUN 0 for the tape device itself, and LUN 1 for the
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tape changer. In this way, the host system can address each of
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the functional units of the tape changer as desired.
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<sect3><heading>Bus layout</heading>
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<p>
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SCSI buses are linear. So, not shaped like Y-junctions, star
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topologies, cobwebs or whatever else people might want to
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invent.
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You might notice that the terminator issue discussed earlier
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becomes rather hairy if your bus is not linear.
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The electrical characteristics, its noise margins and
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ultimately the reliability of it all are tightly related to
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linear bus rule.
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<bf>Stick to the linear bus rule!</bf>
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<sect2><heading>Using SCSI with FreeBSD</heading>
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<p>
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<sect3><heading>About translations, BIOSes and magic...</heading>
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<p>
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As stated before, you should first make sure that you have a
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electrically sound bus.
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When you want to use a SCSI disk on your PC as boot disk, you
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must aware of some quirks related to PC BIOSes. The PC BIOS in
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its first incarnation used a low level physical interface to the
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hard disk. So, you had to tell the BIOS (using a setup tool or a
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BIOS built-in setup) how your disk physically looked like. This
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involved stating number of heads, number of cylinders, number of
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sectors per track, obscure things like precompensation and
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reduced write current cylinder etc.
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One might be inclined to think that since SCSI disks are smart
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you can forget about this. Alas, the arcane setup issue is still
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present today. The system BIOS needs to know how to access your
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SCSI disk with the head/cyl/sector method in order to load the
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FreeBSD kernel during boot.
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The SCSI host adapter or SCSI controller you have put in your
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AT/EISA/PCI/whatever bus to connect your disk therefore has its
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own on-board BIOS. During system startup, the SCSI BIOS takes over
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the hard disk interface routines from the system BIOS. To fool the
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system BIOS, the system setup is normally set to No hard disk
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present. Obvious, isn't it?
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The SCSI BIOS itself presents to the system a so called
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<bf>translated</bf> drive. This means that a fake drive table is
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constructed that allows the PC to boot the drive. This
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translation is often (but not always) done using a pseudo drive
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with 64 heads and 32 sectors per track. By varying the number of
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cylinders, the SCSI BIOS adapts to the actual drive size. It is
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useful to note that 32 * 64 / 2 = the size of your drive in
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megabytes. The division by 2 is to get from disk blocks that are
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normally 512 bytes in size to Kbytes.
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Right. All is well now?! No, it is not. The system BIOS has
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another quirk you might run into. The number of cylinders of a
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bootable hard disk cannot be greater than 1024. Using the
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translation above, this is a show-stopper for disks greater than
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1 Gb. With disk capacities going up all the time this is causing
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problems.
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Fortunately, the solution is simple: just use another
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translation, e.g. with 128 heads instead of 32. In most cases new
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SCSI BIOS versions are available to upgrade older SCSI host
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adapters. Some newer adapters have an option, in the form of a
|
|
jumper or software setup selection, to switch the translation the
|
|
SCSI BIOS uses.
|
|
|
|
It is very important that <bf>all</bf> operating systems on the
|
|
disk use the <bf>same translation</bf> to get the right idea about
|
|
where to find the relevant partitions. So, when installing
|
|
FreeBSD you must answer any questions about heads/cylinders
|
|
etc using the translated values your host adapter uses.
|
|
|
|
Failing to observe the translation issue might lead to
|
|
un-bootable systems or operating systems overwriting each
|
|
others partitions. Using fdisk you should be able to see
|
|
all partitions.
|
|
|
|
You might have heard some talk of 'lying' devices?
|
|
Older FreeBSD kernels used to report the geometry
|
|
of SCSI disks when booting. An example from one of my systems:
|
|
|
|
<verb>
|
|
aha0 targ 0 lun 0: <MICROP 1588-15MB1057404HSP4>
|
|
sd0: 636MB (1303250 total sec), 1632 cyl, 15 head, 53 sec, bytes/sec 512
|
|
</verb>
|
|
Newer kernels usually do not report this information. e.g.
|
|
<verb>
|
|
(bt0:0:0): "SEAGATE ST41651 7574" type 0 fixed SCSI 2
|
|
sd0(bt0:0:0): Direct-Access 1350MB (2766300 512 byte sectors)
|
|
</verb>
|
|
|
|
Why has this changed?
|
|
|
|
This info is retrieved from the SCSI disk itself. Newer disks
|
|
often use a technique called zone bit recording. The idea is that
|
|
on the outer cylinders of the drive there is more space so more
|
|
sectors per track can be put on them. This results in disks that
|
|
have more tracks on outer cylinders than on the inner cylinders
|
|
and, last but not least, have more capacity. You can imagine that
|
|
the value reported by the drive when inquiring about the geometry
|
|
now becomes suspect at best, and nearly always misleading. When
|
|
asked for a geometry , it is nearly always better to supply the
|
|
geometry used by the BIOS, or <em>if the BIOS is never going to know
|
|
about this disk</em>, (e.g. it is not a booting disk) to supply a
|
|
fictitious geometry that is convenient.
|
|
|
|
<sect3><heading>SCSI subsystem design</heading>
|
|
<p>
|
|
FreeBSD uses a layered SCSI subsystem. For each different
|
|
controller card a device driver is written. This driver
|
|
knows all the intimate details about the hardware it
|
|
controls. The driver has a interface to the upper layers of the
|
|
SCSI subsystem through which it receives its commands and
|
|
reports back any status.
|
|
|
|
On top of the card drivers there are a number of more generic
|
|
drivers for a class of devices. More specific: a driver for
|
|
tape devices (abbreviation: st), magnetic disks (sd), cdroms (cd)
|
|
etc. In case you are wondering where you can find this stuff, it
|
|
all lives in <tt>/sys/scsi</tt>. See the man pages in section 4
|
|
for more details.
|
|
|
|
The multi level design allows a decoupling of low-level bit
|
|
banging and more high level stuff. Adding support for another
|
|
piece of hardware is a much more manageable problem.
|
|
|
|
<sect3><heading>Kernel configuration</heading>
|
|
<p>
|
|
Dependent on your hardware, the kernel configuration file must
|
|
contain one or more lines describing your host adapter(s).
|
|
This includes I/O addresses, interrupts etc.
|
|
Consult the man page for your
|
|
adapter driver to get more info. Apart from that, check out
|
|
/sys/i386/conf/LINT for an overview of a kernel config file.
|
|
LINT contains every possible option you can dream of. It
|
|
does <em>not</em> imply LINT will actually get you to a
|
|
working kernel at all.
|
|
|
|
Although it is probably stating the obvious: the kernel config
|
|
file should reflect your actual hardware setup. So, interrupts,
|
|
I/O addresses etc must match the kernel config file. During
|
|
system boot messages will be displayed to indicate whether
|
|
the configured hardware was actually found.
|
|
|
|
An example loosely based on the FreeBSD 2.0.5-Release kernel config
|
|
file LINT with some added comments (between []):
|
|
|
|
<verb>
|
|
|
|
# SCSI host adapters: `aha', `ahb', `aic', `bt', `nca'
|
|
#
|
|
# aha: Adaptec 154x
|
|
# ahb: Adaptec 174x
|
|
# ahc: Adaptec 274x/284x/294x
|
|
# aic: Adaptec 152x and sound cards using the Adaptec AIC-6360 (slow!)
|
|
# bt: Most Buslogic controllers
|
|
# nca: ProAudioSpectrum cards using the NCR 5380 or Trantor T130
|
|
# uha: UltraStore 14F and 34F
|
|
# sea: Seagate ST01/02 8 bit controller (slow!)
|
|
# wds: Western Digital WD7000 controller (no scatter/gather!).
|
|
#
|
|
|
|
[For an Adaptec AHA274x, 284x etc controller]
|
|
controller ahc0 at isa? bio irq ? vector ahcintr # port??? iomem?
|
|
|
|
[For an Adaptec AHA174x controller]
|
|
controller ahb0 at isa? bio irq ? vector ahbintr
|
|
|
|
[For an Ultrastor adapter]
|
|
controller uha0 at isa? port "IO_UHA0" bio irq ? drq 5 vector uhaintr
|
|
|
|
# Map SCSI buses to specific SCSI adapters
|
|
controller scbus0 at ahc0
|
|
controller scbus2 at ahb0
|
|
controller scbus1 at uha0
|
|
|
|
# The actual SCSI devices
|
|
disk sd0 at scbus0 target 0 unit 0 [SCSI disk 0 is at scbus 0, LUN 0]
|
|
disk sd1 at scbus0 target 1 [implicit LUN 0 if omitted]
|
|
disk sd2 at scbus1 target 3 [SCSI disk on the uha0]
|
|
disk sd3 at scbus2 target 4 [SCSI disk on the ahb0]
|
|
tape st1 at scbus0 target 6 [SCSI tape at target 6]
|
|
device cd0 at scbus? [the first ever CDROM found, no wiring]
|
|
|
|
</verb>
|
|
|
|
The example above tells the kernel to look for a ahc (Adaptec 274x)
|
|
controller, then for an Adaptec 174x board, and
|
|
so on. The lines following the controller specifications
|
|
tell the kernel to configure specific devices but
|
|
<em>only</em> attach them when they match the target ID and
|
|
LUN specified on the corresponding bus.
|
|
|
|
Wired down devices get 'first shot' at the unit numbers
|
|
so the first non 'wired down' device, is allocated the unit number
|
|
one greater than the highest 'wired down' unit number
|
|
for that kind of device.
|
|
So, if you had a SCSI tape at target ID 2 it would be
|
|
configured as st2, as the tape at target ID 6 is wired down
|
|
to unit number 1. Note that <em>wired down devices need not
|
|
be found</em>
|
|
to get their unit number. The unit number for a wired down device
|
|
is reserved for that device, even if it is turned off at boot
|
|
time. This allows the device to be turned on and brought
|
|
on-line at a later time, without rebooting. Notice that a device's
|
|
unit number has <em>no</em> relationship with its target ID on
|
|
the SCSI bus.
|
|
|
|
Below is another example of a kernel config file as used by
|
|
FreeBSD version < 2.0.5. The difference with the first example is
|
|
that devices are not 'wired down'. 'Wired down' means
|
|
that you specify which SCSI target belongs to which device.
|
|
|
|
A kernel built to the config file below will attach
|
|
the first SCSI disk it finds to sd0, the second disk to sd1
|
|
etc. If you ever removed or added a disk, all other devices
|
|
of the same type (disk in this case) would 'move around'.
|
|
This implies you have to change <tt>/etc/fstab</tt> each time.
|
|
|
|
Although the old style still works, you
|
|
are <em>strongly</em> recommended to use this new feature.
|
|
It will save you a lot of grief whenever you shift your
|
|
hardware around on the SCSI buses. So, when you re-use
|
|
your old trusty config file after upgrading from a
|
|
pre-FreeBSD2.0.5.R system check this out.
|
|
|
|
<verb>
|
|
[driver for Adaptec 174x]
|
|
controller ahb0 at isa? bio irq 11 vector ahbintr
|
|
[for Adaptec 154x]
|
|
controller aha0 at isa? port "IO_AHA0" bio irq 11 drq 5 vector ahaintr
|
|
[for Seagate ST01/02]
|
|
controller sea0 at isa? bio irq 5 iomem 0xc8000 iosiz 0x2000 vector seaintr
|
|
controller scbus0
|
|
|
|
device sd0 [support for 4 SCSI harddisks, sd0 up sd3]
|
|
|
|
device st0 [support for 2 SCSI tapes]
|
|
|
|
[for the cdrom]
|
|
device cd0 #Only need one of these, the code dynamically grows
|
|
</verb>
|
|
|
|
|
|
Both examples support SCSI disks. If during boot more
|
|
devices of a specific type (e.g. sd disks) are found than are
|
|
configured in the booting kernel, the system will simply allocate
|
|
more devices, incrementing the unit number starting at the last
|
|
number 'wired down'. If there are no 'wired down' devices
|
|
then counting starts at unit 0.
|
|
|
|
Use <tt>man 4 scsi</tt> to check for the latest info on the SCSI
|
|
subsystem. For more detailed info on host adapter drivers use eg
|
|
<tt>man 4 aha</tt> for info on the Adaptec 154x driver.
|
|
|
|
<sect3><heading>Tuning your SCSI kernel setup</heading>
|
|
<p>
|
|
Experience has shown that some devices are slow to respond to INQUIRY
|
|
commands after a SCSI bus reset (which happens at boot time).
|
|
An INQUIRY command is sent by the kernel on boot to see what
|
|
kind of device (disk, tape, CDROM etc) is connected to a
|
|
specific target ID. This process is called device probing by the way.
|
|
|
|
To work around the 'slow response' problem, FreeBSD allows a
|
|
tunable delay time
|
|
before the SCSI devices are probed following a SCSI bus reset.
|
|
You can set this delay time in your kernel configuration file
|
|
using a line like:
|
|
|
|
<verb>
|
|
options SCSI_DELAY=15 #Be pessimistic about Joe SCSI device
|
|
</verb>
|
|
This line sets the delay time to 15 seconds. On my own system I had to
|
|
use 3 seconds minimum to get my trusty old CDROM drive to be recognized.
|
|
Start with a high value (say 30 seconds or so) when you have problems
|
|
with device recognition. If this helps, tune it back until it just stays
|
|
working.
|
|
|
|
<sect3><heading>Rogue SCSI devices</heading>
|
|
<p>
|
|
Although the SCSI standard tries to be complete and concise, it is
|
|
a complex standard and implementing things correctly is no easy task.
|
|
Some vendors do a better job then others.
|
|
|
|
This is exactly where the 'rogue' devices come into view. Rogues are
|
|
devices that are recognized by the FreeBSD kernel as behaving slightly
|
|
(...) non-standard. Rogue devices are reported by the kernel when
|
|
booting. An example for two of my cartridge tape units:
|
|
|
|
<verb>
|
|
Feb 25 21:03:34 yedi /kernel: ahb0 targ 5 lun 0: <TANDBERG TDC 3600 -06:>
|
|
Feb 25 21:03:34 yedi /kernel: st0: Tandberg tdc3600 is a known rogue
|
|
|
|
Mar 29 21:16:37 yedi /kernel: aha0 targ 5 lun 0: <ARCHIVE VIPER 150 21247-005>
|
|
Mar 29 21:16:37 yedi /kernel: st1: Archive Viper 150 is a known rogue
|
|
</verb>
|
|
|
|
For instance, there are devices that respond to
|
|
all LUNs on a certain target ID, even if they are actually only one
|
|
device. It is easy to see that the kernel might be fooled into
|
|
believing that there are 8 LUNs at that particular target ID. The
|
|
confusion this causes is left as an exercise to the reader.
|
|
|
|
The SCSI subsystem of FreeBSD recognizes devices with bad habits by
|
|
looking at the INQUIRY response they send when probed. Because the
|
|
INQUIRY response also includes the version number of the device
|
|
firmware, it is even possible that for different firmware versions
|
|
different workarounds are used. See e.g. /sys/scsi/st.c and
|
|
/sys/scsi/scsiconf.c for more info on how this is done.
|
|
|
|
This scheme works fine, but keep in mind that it of course only
|
|
works for devices that are KNOWN to be weird. If you are the first
|
|
to connect your bogus Mumbletech SCSI CDROM you might be the one
|
|
that has to define which workaround is needed.
|
|
|
|
After you got your Mumbletech working, please send the required
|
|
workaround to the FreeBSD development team for inclusion in the
|
|
next release of FreeBSD. Other Mumbletech owners will be grateful
|
|
to you.
|
|
|
|
<sect3><heading>Multiple LUN devices</heading>
|
|
<p>
|
|
In some cases you come across devices that use multiple
|
|
logical units (LUNs) on a single SCSI ID. In most cases
|
|
FreeBSD only probes devices for LUN 0. An example are
|
|
so called bridge boards that connect 2 non-SCSI harddisks
|
|
to a SCSI bus (e.g. an Emulex MD21 found in old Sun systems).
|
|
|
|
This means that any devices with LUNs != 0 are not normally
|
|
found during device probe on system boot. To work around this
|
|
problem you must add an appropriate entry in /sys/scsi/scsiconf.c
|
|
and rebuild your kernel.
|
|
|
|
Look for a struct that is initialised like below:
|
|
<verb>
|
|
{
|
|
T_DIRECT, T_FIXED, "MAXTOR", "XT-4170S", "B5A",
|
|
"mx1", SC_ONE_LU
|
|
}
|
|
</verb>
|
|
|
|
For you Mumbletech BRIDGE2000 that has more than one LUN,
|
|
acts as a SCSI disk
|
|
and has firmware revision 123 you would add something like:
|
|
|
|
<verb>
|
|
{
|
|
T_DIRECT, T_FIXED, "MUMBLETECH", "BRIDGE2000", "123",
|
|
"sd", SC_MORE_LUS
|
|
}
|
|
</verb>
|
|
|
|
The kernel on boot scans the inquiry data it receives against
|
|
the table and acts accordingly. See the source for more info.
|
|
|
|
<sect3><heading>Tagged command queueing</heading>
|
|
<p>
|
|
Modern SCSI devices, particularly magnetic disks, support
|
|
what is called tagged command queuing (TCQ).
|
|
|
|
In a nutshell, TCQ allows the device to have multiple I/O
|
|
requests outstanding at the same time. Because the device
|
|
is intelligent, it can optimise its operations (like
|
|
head positioning) based on its own request queue. On
|
|
SCSI devices like RAID (Redundant Array of Independent
|
|
Disks) arrays the TCQ function is indispensable to take
|
|
advantage of the device's inherent parallelism.
|
|
|
|
Each I/O request is uniquely identified by a 'tag' (hence
|
|
the name tagged command queuing) and this tag is used by
|
|
FreeBSD to see which I/O in the device drivers queue is
|
|
reported as complete by the device.
|
|
|
|
It should be noted however that TCQ requires device driver
|
|
support and that some devices implemented it 'not quite
|
|
right' in their firmware. This problem bit me once, and
|
|
it leads to highly mysterious problems. In such cases,
|
|
try to disable TCQ.
|
|
|
|
<sect3><heading>Busmaster host adapters</heading>
|
|
<p>
|
|
Most, but not all, SCSI host adapters are bus mastering controllers.
|
|
This means that they can do I/O on their own without putting load onto
|
|
the host CPU for data movement.
|
|
|
|
This is of course an advantage for a multitasking operating system like
|
|
FreeBSD. It must be noted however that there might be some rough edges.
|
|
|
|
For instance an Adaptec 1542 controller can be set to use different
|
|
transfer speeds on the host bus (ISA or AT in this case). The controller
|
|
is settable to different rates because not all motherboards can handle
|
|
the higher speeds. Problems like hangups, bad data etc might be the
|
|
result of using a higher data transfer rate then your motherboard
|
|
can stomach.
|
|
|
|
The solution is of course obvious: switch to a lower data transfer rate
|
|
and try if that works better.
|
|
|
|
In the case of a Adaptec 1542, there is an option that can be put
|
|
into the kernel config file to allow dynamic determination of the
|
|
right, read: fastest feasible, transfer rate. This option is
|
|
disabled by default:
|
|
|
|
<verb>
|
|
options "TUNE_1542" #dynamic tune of bus DMA speed
|
|
</verb>
|
|
|
|
Check the man pages for the host adapter that you use. Or better
|
|
still, use the ultimate documentation (read: driver source).
|
|
|
|
<sect2><heading>Tracking down problems</heading>
|
|
<p>
|
|
The following list is an attempt to give a guideline for the most
|
|
common SCSI problems and their solutions. It is by no means
|
|
complete.
|
|
|
|
<itemize>
|
|
<item>
|
|
Check for loose connectors and cables.
|
|
<item>
|
|
Check and double check the location and number of your terminators.
|
|
<item>
|
|
Check if your bus has at least one supplier of terminator power
|
|
(especially with external terminators.
|
|
<item>
|
|
Check if no double target IDs are used.
|
|
<item>
|
|
Check if all devices to be used are powered up.
|
|
<item>
|
|
Make a minimal bus config with as little devices as possible.
|
|
<item>
|
|
If possible, configure your host adapter to use slow bus speeds.
|
|
<item>
|
|
Disable tagged command queuing to make things as simple as
|
|
possible (for a NCR hostadapter based system see man
|
|
ncrcontrol)
|
|
<item>
|
|
If you can compile a kernel, make one with the SCSIDEBUG option,
|
|
and try accessing the device with debugging turned on for
|
|
that device. If your device does not even probe at startup,
|
|
you may have to define the address of the device that
|
|
is failing, and the desired debug level in
|
|
<tt>/sys/scsi/scsidebug.h</tt>.
|
|
If it probes but just does not work, you can use the
|
|
<tt>scsi(8)</tt> command to dynamically set a debug level to
|
|
it in a running kernel (if SCSIDEBUG is defined).
|
|
This will give you COPIOUS debugging output with which to confuse
|
|
the gurus. see <tt>man 4 scsi</tt> for more exact information.
|
|
Also look at <tt>man 8 scsi</tt>.
|
|
</itemize>
|
|
|
|
<sect2><heading>Further reading<label id="scsi:further-reading"></heading>
|
|
<p>
|
|
If you intend to do some serious SCSI hacking, you might want to
|
|
have the official standard at hand:
|
|
|
|
Approved American National Standards can be purchased from ANSI at
|
|
11 West 42nd Street, 13th Floor, New York, NY 10036, Sales Dept:
|
|
(212) 642-4900. You can also buy many ANSI standards and most
|
|
committee draft documents from Global Engineering Documents, 15
|
|
Inverness Way East, Englewood, CO 80112-5704, Phone: (800)
|
|
854-7179, Outside USA and Canada: (303) 792-2181, FAX: (303) 792-
|
|
2192.
|
|
|
|
Many X3T10 draft documents are available electronically on the SCSI
|
|
BBS (719-574-0424) and on the ncrinfo.ncr.com anonymous ftp site.
|
|
|
|
Latest X3T10 committee documents are:
|
|
<itemize>
|
|
<item>AT Attachment (ATA or IDE) [X3.221-1994] (<em>Approved</em>)
|
|
<item>ATA Extensions (ATA-2) [X3T10/948D Rev 2i]
|
|
<item>Enhanced Small Device Interface (ESDI) [X3.170-1990/X3.170a-1991] (<em>Approved</em>)
|
|
<item>Small Computer System Interface - 2 (SCSI-2) [X3.131-1994] (<em>Approved</em>)
|
|
<item>SCSI-2 Common Access Method Transport and SCSI Interface Module (CAM)
|
|
[X3T10/792D Rev 11]
|
|
</itemize>
|
|
Other publications that might provide you with additional information are:
|
|
<itemize>
|
|
<item>"SCSI: Understanding the Small Computer System Interface", written by NCR
|
|
Corporation. Available from: Prentice Hall, Englewood Cliffs, NJ, 07632
|
|
Phone: (201) 767-5937 ISBN 0-13-796855-8
|
|
|
|
<item>"Basics of SCSI", a SCSI tutorial written by Ancot Corporation
|
|
Contact Ancot for availability information at:
|
|
Phone: (415) 322-5322 Fax: (415) 322-0455
|
|
|
|
<item>"SCSI Interconnection Guide Book", an AMP publication (dated 4/93, Catalog
|
|
65237) that lists the various SCSI connectors and suggests cabling schemes.
|
|
Available from AMP at (800) 522-6752 or (717) 564-0100
|
|
|
|
<item>"Fast Track to SCSI", A Product Guide written by Fujitsu.
|
|
Available from: Prentice Hall, Englewood Cliffs, NJ, 07632
|
|
Phone: (201) 767-5937 ISBN 0-13-307000-X
|
|
|
|
<item>"The SCSI Bench Reference", "The SCSI Encyclopedia", and the "SCSI Tutor",
|
|
ENDL Publications, 14426 Black Walnut Court, Saratoga CA, 95070
|
|
Phone: (408) 867-6642
|
|
|
|
<item>"Zadian SCSI Navigator" (quick ref. book) and "Discover the Power of SCSI"
|
|
(First book along with a one-hour video and tutorial book), Zadian Software,
|
|
Suite 214, 1210 S. Bascom Ave., San Jose, CA 92128, (408) 293-0800
|
|
</itemize>
|
|
|
|
On Usenet the newsgroups <htmlurl
|
|
url="news:comp.periphs.scsi" name="comp.periphs.scsi">
|
|
and <htmlurl url="news:comp.periphs" name="comp.periphs">
|
|
are noteworthy places to look for more info. You can also
|
|
find the SCSI-Faq there, which is posted periodically.
|
|
|
|
Most major SCSI device and host adapter suppliers operate ftp sites
|
|
and/or BBS systems. They may be valuable sources of information
|
|
about the devices you own.
|