Imported Upstream version 2.4.4p3

[debian/amanda] / docs / TAPE.CHANGERS

AMANDA TAPE CHANGER SUPPORT

Originally by James da Silva <jds@cs.umd.edu>
Heavily modified by various contributors

1. INTRODUCTION

This document outlines the tape changer support design of Amanda 2.2
and beyond.  It defines a small interface for changer software to follow
so that Amanda can remain device-independent but still support the widest
range of tape software and hardware possible.

The interface is a bit simplistic and has only had complications added
when there is a body of field experience.


2. SPECIFYING A TAPE CHANGER IN AMANDA.CONF

All device-specifics are hidden by a glue program that the rest of Amanda
calls to interact with the tape changer.

The name of this changer program is given by the "tpchanger" variable
in the amanda.conf file.  Example entry:

        tpchanger "chg-multi"      # use multi-unit tape changer emulator

The tapedev parameter is ignored if a tpchanger program is specified,
unless the changer program itself reads tapedev from amanda.conf.
The chg-multi changer doesn't, as it reads all its configuration arguments
from its own configuration file, specified as changerfile.

If the tpchanger program does not begin with a '/', Amanda expects it
to reside in libexecdir, and possibly have the version suffix appended
depending on how Amanda was configured.

Two other amanda.conf parameters are available for changer use, however
their definition is left entirely up to the changer script itself.
They are changerfile and changerdev.  Typically changerfile will point
to the configuration file specific to the changer, or be a base name
of several related changer files.  The changerdev variable may point to
the /dev entry used to access the changer device.

See the documentation with the specific changer you're interested in to
see exaclty how these variables are used (if at all).


3. DEVICE-SPECIFIC TAPE CHANGER SCRIPT

The tape changer script/program is always from the directory with
amanda.conf.  It is never passed the configuration name it is running
on behalf of, but since amgetconf works without a name from the current
directory, that should be sufficient.

The script/program must support the following commands:

    <tpchanger> -slot <slot-specifier>

                If changer is loaded, unloads the current slot (if
                different than "slot-specifier") and puts that tape away,
                then loads the requested slot.  See the next section for
                the list of valid slot requests.

                Outputs to stdout the slot name and name of the device
                file to access the tape on success, or a slot name and
                error text.

                Returns 0 on success.

                Returns 1 on positioning error (eg at bottom of gravity
                stacker or slot empty).

                Returns 2 any other fatal error.
                The slot name may be invalid, but must be present.
                Error message goes to stdout in place of device name.

        Examples:
                % chg-multi -slot 0
                0 /dev/nrst8                    # exitcode returned is 0

                % chg-multi -slot 1
                1 slot 1 is empty               # exitcode returned is 1

                % chg-multi -slot bogus-slot
                <none> no slot `bogus-slot'     # exitcode returned is 2


    <tpchanger> -info 

                Outputs to stdout three or more fields:
                
                  * The current slot string (required)
                  
                  * The number of slots (required)

                  * Flag indicating whether the changer can go backwards
                    (0 if it can't, 1 if it can). (required)

                  * Flag indicating whether the changer is searchable
                    (optional).  Shows whether the changer supports
                    the -search and -label commands and is able to
                    load a tape given only the Amanda label string
                    (0 or omitted if it can't, 1 if it can). (optional)

        Examples:
                % chg-multi -info
                0 10 1                          # exitcode returned is 0

                % chg-zd-mtx -info
                0 10 1 1

    <tpchanger> -reset

                Resets the changer to known state and loads the first slot.

                Output and error handling are the same as

                  "<tpchanger> -slot first"

                In the case of a gravity stacker that must be reset
                by hand, this could be run (via "amtape <conf> reset")
                to inform the software the stacker is positioned back
                at the top.

        Examples:
                % chg-multi -reset
                0 /dev/nrst8                    # exitcode returned is 0

                % chg-multi -reset
                0 slot 0 is empty               # exitcode returned is 1

                % chg-multi -reset
                0 tape-changer not responding   # exitcode returned is 2

    <tpchanger> -eject

                Unloads the current slot (if loaded) and puts that tape away.

                Output and error handling are the same as the -slot command.

                Note that a tape may or may not be loaded when this command
                completes, depending on the hardware.

        Examples:
                % chg-multi -eject
                0 /dev/nrst8                    # exitcode returned is 0

                % chg-multi -eject
                0 drive was not loaded          # exitcode returned is 1

The tape changer program MAY support the following commands:

    <tpchanger> -search <labelstr>

                Loads an Amanda tape by name (labelstr).
                
                Output and error handling are the same as the -slot command.

                Taper, amcheck and amtape will use this command if the
                changer reports it is searchable.

        Example:
                % chg-zd-mtx -search DailySet005
                5 /dev/nrst8                    # exitcode returned is 0

    <tpchanger> -label <labelstr>
                Associates the Amanda label <labelstr> with the barcode 
                of the currently loaded (in the tape drive) tape.

                Outputs to stdout the current slot and tape device.
                Amlabel will use this command if your changer is
                searchable to build up the barcode database.

        Example:
                % chg-zd-mtx -label DailySet006
                6 /dev/nrst8                    # exitcode returned is 0


For all the commands:

        An exit code of 0 implies that the operation was completely
        successful, and the output may be parsed by the Amanda code as
        described above.

        For non-zero exit codes, the first field is still the slot name,
        but the actual error messages are not fixed.  They are just
        displayed and/or logged as-is by the calling amanda program.

        An exit code of 1 implies the operation failed in a benign way,
        for example an empty slot or an attempt to go backwards in a
        gravity stacker.  The calling Amanda program will print the
        error message if appropriate and continue, perhaps requesting
        a different slot be loaded.

        Any other exit code is considered fatal and will cause Amanda
        to stop attempting to talk to the tape changer.


4.  SLOT NAMES AND THE "CURRENT" SLOT

Some tape changers, such as carousels and gravity stackers, have a hardware
notion of current position.  Others have no current position when no tape
is loaded: all tapes are in their slots and the changer arm is docked away
from the slots.

Nevertheless, Amanda requires tape-changer scripts to maintain the
notion of a "current" position.  This is for performance reasons: as
tapes tend to be loaded into the rack in order, and Amanda uses them in
order, the next tape to use can be found much quicker if the position
of the current one is remembered.  As an example, the chg-multi script
maintains the current position in a "chg-multi.state" file (or any other
file specified in a `statefile' line in the changer configuration file).

Amanda does not care how slots are available or how they are named.
They could be numbered 0 to N-1, numbered 1 to N, or even designated by
letter, A .. Z.  The only requirement is that the names do not contain
whitespace and that the names "current", "next", "prev", "first", "last"
and "advance" retain their meaning as follows:

        current The position of the last loaded tape, as described above

        next    The position after current, wrapping from the last slot to
                the first.

        prev    The position before current, wrapping from the first slot to
                the last.

        first   The first slot in the tape rack.

        last    The last slot in the tape rack.

        advance The same as "next" except the next tape may not be loaded if
                the changer supports advancing to the next slot without
                putting that tape in the drive.

The current position must be updated even if there is a positioning error
(such as "empty slot").  This allows amanda to step through the entire tape
rack by issuing successive "-slot next" positioning commands.


5. OPERATOR INTERFACE

The amtape program is the main operator interface to Amanda's tape
changer support.  The commands supported include:

        amtape <conf> slot <slot-specifier>
                Load the tape from the specified slot into the drive

        amtape <conf> eject
                Send an eject command to the tape-changer.  Effect is changer
                specific.

        amtape <conf> reset
                Send a reset command to the tape-changer.  Effect is changer
                specific.

        amtape <conf> show
                Go through the entire tape rack, showing the labels of all 
                amanda tapes encountered.

        amtape <conf> label <label>
                Find and load the tape with the specified label

        amtape <conf> taper
                Perform taper's scan algorithm (see below), loading the tape
                which would be picked for the next amdump run.

        amtape <conf> clean
                If a cleaning tape is defined by the changer, load it in
                the drive and put it away when done.
        
        amtape <conf> device
                Output the current tape device name.
        
        amtape <conf> current
                Display the contents of the current slot.
        
        amtape <conf> update
                Scan the entire tape rack and update the barcode database.

See the amtape(8) man page for more details.

In addition to amtape, amlabel has been modified to allow optionally
specifying a slot:

        amlabel <conf> <label> [slot <slot-specifier>]

Amcheck looks for the next tape in the rack the same way the taper does.
If multiple tapes are used in one night, amcheck attempts to find all the
needed tapes in turn if the tape-changer is random access.  On a one-way
gravity stacker, amcheck only finds the first tape, since finding the
subsequent ones would put the first one out of reach of that night's
amdump run.

Amrestore and amrecover do not yet include any tape changer support
directly, as amrestore knows nothing about the amanda.conf files or
server-side databases.  This is a deliberate decision to keep amrestore
independent, so it can be run from any host with a tape drive, even
if the original tape server host is down.  To use amrestore in a
tape-changer environment, use amtape to find the right tape, then run
amrestore giving the resulting tape device name.


6. HOW AMDUMP INTERACTS WITH THE TAPE CHANGER

Amanda does not require a particular tape label for a run.  Any label
that matches the labelstr regex and is determined to be "inactive"
according to the tapelist database, may be written to.  However, there
is a preferred 'next' tape, the one that is next in the cycle implied
by the current tapelist.

Amdump uses two algorithms, depending on whether the tape changer can go
backwards in the rack or not.  If multiple tapes are needed in a single
run, this algorithm is repeated in turn whenever a new tape is required.

Normal tape changers (those that can go backwards):

With a full-access tape changer, Amdump searches the entire rack for the
preferred tape label.  This tape will usually be found at the current
or next position, but might be located anywhere.  If the tape is found,
it is used.  If it is not found, the first tape encountered that matches
the labelstr and is not active is picked.

Gravity stackers (anything that can not go backwards):

To avoid going all the way to the bottom of the stacker only to find that
the preferred tape isn't present and nothing can be done, Amanda picks the
first tape (starting at the current position) that matches the labelstr
and is not active, regardless of whether it is the preferred tape.


7. BUILT-IN TAPE CHANGERS

================================
chg-multi (formerly chg-generic)
================================

This tape changer script supports several common configurations:

  - Using multiple tape drives in a single host to emulate a tape changer.
    This can also be used with a single physical drive to write several
    tapes in an amanda run.

  - Using a gravity stacker or a real changer configured to sequentially
    load the next tape when the current one is ejected.  Also supports
    a changer which cycles to the first tape after loading the last one.

  - Using a changer accessed through several "virtual" tape devices
    which determine which slot actually gets loaded in the tape drive.

The advantage of this changer script is that you do not need to get into
the complexity of dealing with a real changer interface.  All the action
goes through the tape device interface with standard "mt" commands,
which eases many portability issues.  Many common tape jukeboxes can be
configured in a sequential or cycle mode.

chg-multi ignores `tapedev' and `changerdev' because `changerfile' may
specify several tape devices to be used.  A sample configuration file
may be found in example/chg-multi.conf.

================================
chg-manual (formerly no-changer)
================================

This is a poor man's tape changer that requires the backup operator to
change tapes manually.  It expects `tapedev' in amanda.conf to point to
a valid tape device, and stores some status data in files whose names
start with the `changerfile'.  `changerdev' is ignored.

=============================
chg-mtx (formerly hp-changer)
=============================

An mtx-based tape changer script.  `changerdev' must specify the tape
device controlled by the mtx program, and `tapedev' must point to the
no-rewind tape device to be used.  More than likely, both `changerdev' and
`tapedev' will reference the same device file.  `changerfile' must specify
a prefix for status files maintained by this script.  It will maintain
files named `changerfile'/changer-clean and `changerfile'/changer-access.
You may have to edit the script to specify which slot contains a cleaning
tape (cleanslot).

The `mtx' program must support commands such as `-s', `-l' and `-u'.
If the one you've got requires `status', `load' and `unload', you should
use chg-zd-mtx instead (see below).

==========
chg-zd-mtx
==========

Based on chg-mtx, but modified in order to support the Zubkoff/Dandelion
version of mtx.  Eric DOUTRELEAU <Eric.Doutreleau@int-evry.fr>, who
contributed this script, reported that it works on a Solaris/sparc box
with a HP 1557A stacker.

In addition to the `changerfile'-clean and the `changerfile'-access files,
it maintains a `changerfile'-slot file that indicates the currently
loaded slot.

There are lots of comments at the start of the script describing how to
set it up.

========
chg-disk
========

Clone of the chg-zd-mtx, but modified to be applied on local directories
instead of tapes. This changer emulates a robotic that uses virtual
tapes instead of real ones, where the virtual tapes are real directories
on a hard disk.

The directory tree should be:
slot_root_dir -|
               |- info
               |- data -> slot1/
               |- slot1/
               |- slot2/
               |- ...
               |- slotn/

Where "slot_root_dir" is the tapedev "file:xxx" parameter and "n" the
tapecycle parameter.

Please refer to the HOWTO-FILE-DRIVER for details of usage.




=======================================================
chg-scsi-chio (formerly seagate-changer, then chg-chio)
=======================================================

A C program that relies on scsi tape-changer interfaces.  It may
either use the tape changer interface specified in chio.h (Gerd Knor's
SCSI media changer driver, a Linux kernel loadable module), or it may
use built-in tape changer interfaces available on HPUX, Solaris 2.5,
IRIX and possibly others, but only the chio and HPUX interfaces are
currently implemented .  `tapedev' specifies the tape device to be
used; `changer_dev' is the device used to talk to the kernel module
(for chio, usually /dev/ch0), and `changerfile' specifies a filename
in which the current slot number will be stored.

Now there is another way, to get the chg-scsi a little bit more
flexible.  If you use only one digit in the `tapedev' parameter, the
chg-scsi expects that changerfile points to a real configuration file,
not only a counting file.  In this configuration file you may specify
that the tapedrive needs an eject command and an optional waittime,
necessary after inserting the tape into the drive.  You are also able
to configure a range of slots which should be used by your
configuration.  The idea behind this is, that you don't want amanda to
cycle all the tapes if amanda searches exactly one tape.  If you have
a library which supports more than one drive you can also specify
which drive to use.  For each configuration (there should be at least
one) you have to specify a file, where amanda remembers which tape is
actually in the drive.  For future use there is also some stuff for
cleaning the drives.

In amanda.conf:

tapedev "x"       with x between 0 and 9, selects the configuration to use
changerfile "filename"            specifies the changer configuration file

In the changer-config-file the following could be set:
number_configs x        x between 0 and 9       number of configurations
                                                defined. This should be
                                                the first parameter in the
                                                config-file.
eject   x               x 0 or 1                1 means that the drives need
                                                an eject command, before the 
                                                robot can handle the tape.
sleep   x               x between 0 and MAX_INT specifies the seconds to wait
                                                before the drive could be used
                                                after inserting a tape. 5
                                                should be OK.
cleanmax x              x some positive int     How many cleanings does a 
                                                cleaning tape survive
changerdev  <device>                            The device for the robot

And then there come some configuration sections, separated by the word `config`
followed by the ordinal of that configuration (0 to 9). In each configuration
section you should specify:

drivenum x              x between 0 and the number of drives in the library
                                                This one specifies the drive to
                                                use with this configuration
dev     <device>                                The device for the tapedrive
startuse    x           x between 0 and maximum slotnumber of your library
                                                Starting here we may use the tapes
enduse   x              x between start and maximum slotnumber
                                                This is the last tape we may use
                                                in this configuration. If we reach
                                                this one the next will be start..
statfile <filename>                             Here we remember the last used
                                                slot for this configuration
cleancart       x       x between 0 and maximum slotnumber 
                                                In this slot we find the 
                                                cleaning tape
cleanfile <filename>                            Here we will remember how 
                                                often we used the cleaning tape
usagecount <filename>                           This points to a file which is
                                                deleted after cleaning the drive
                                                e.g. the usagetime of the drive
                                                                                         
Comments begin with an '#' until end of line.
Allowed separators are TAB and SPACE. 

========================================================================
chg-scsi (new interface, try to drive a robot with direct scsi commands)
========================================================================

The config and the sysntax is the same as for chg-scsi-chio. New
is the config type 

emubarcode      1

With this option and the option labelfile chg-scsi will try to create an inventory.
With this inventory it should be possible to use the search feature for loading tapes.

debuglevel x:y

This option will set the debug level and select for which part debug messages should be sent
to the debug file. In case of problems you should set it to 9:0

havebarcode 1

This will force the program to read the barcodes, and don not try to figure out if there is an
barcode reader available.

scsitapedev <devicename>

This device is used to control the tape, read status infos etc.

tapestatus <filename>

If this option is given on every eject/move the log pages of the tape
device will be dumped in this file. There are 2 log pages were you
can see how many read/write errors (corrected) are processed by the tape

labelfile <filename>

This file is used for the mapping from barcode labels to amanda volume labels.
It is used if the changer has a barcode reader. To initialize run amtape show,
this will add the mapping for the tapes in the magazine.

eject > 1

Use the mtio ioctl to eject the tape, use only if the standard (1) does
not work, and send the debug output (/tmp/amanda/chg-scsi.debug) to
th@ant.han.de

changerident <ident>

With this it is possible to specify which internal driver to use for
controlling/error handling of the robot

tapeident <ident>

Some as above but for the tape.


New command line option:
-status [all|types|robot|sense|ModeSenseRobot|ModeSenseTape|fd]

<all> will show the result form all options.
<types> will list the known driver types.
<robot> will show the status of all elements (tape/robot/slots..)
<sense> will show the result from a request sense
<ModeSenseRobot> will show the sense page from the robot
<ModeSenseTape> will show the sense page from the tape
<fd> will show the devices which are open, and some info about it.


At the moment changer with tape and robot on the same SCSI id (
but on different luns) will run on the following plattforms
HP_UX 10.20     
IRIX 6.x
Solaris
Linux
AIX
FreeBSD 3.0/4.0

Tape and robot on different IDs run native on
Linux
HP-UX 10.20
Irix 6.x
FreeBSD

Tape and robot on different IDs with special modules run on:

Solaris with sst kernel module, which is not any longer needed in solaris 2.8.
See in the contrib/sst directory
The configuration on solaris 2.8 with the sgen driver is done by creating
the file /kernel/drv/sgen.conf
This file should contain at the beginning the following
device-type-config-list="changer","sequential"

This will force the driver to attach only to the devices with type either
changer (the robot) and sequential (the tape).
Next you must tell the driver on which id it should check for devices
(tape on id 5, robot on id 6 in this example),
name="sgen" class="scsi" target=5 lun=0;
name="sgen" class="scsi" target=6 lun=0;

This will create the 2 device files
/dev/scsi/sequential/c0t5d0         (scsitapedev option in chg-scsi.conf)
/dev/scsi/changer/c0t6d0            (changer option in chg-scsi.conf)

So the complete sgen.conf looks like:
device-type-config-list="changer","sequential
name="sgen" class="scsi" target=5 lun=0;
name="sgen" class="scsi" target=6 lun=0;


For HP you have to create the special device files for the pass throu
interface. Check if the ctl driver is installed.
Example:
# lsdev -C ctl
    Character     Block       Driver          Class
      203          -1         sctl            ctl

Next check on which bus your drives are connected. (ioscan)

with the Character device num form the lsdev and the card instance from
ioscan create the special file.
Example:
mknod /dev/scsi/1 c 203 0x001000
                          ||||
                          ||| LUN of device
                          ||SCSI ID of the device
                          2 digit instance number from ioscan

On FreeBSD 4.0 the syntax for the device files has changed. Now you
have to tell chg-scsi the bus:target:lun combination. If you for
example on your scsi bus 0 target 3 an robot the syntax is
changerdev 0:3:0
To get this info you can use the camcontrol command, <camcontrl devlist>
will give you a list of known devices.
Don't specify dev and scsitapedev in your chg-scsi.conf !!, this will
not work.

On Linux you need either sg (generic scsi) as module or it must be compiled
into the kernel. If the sg driver doses not work try to use the ioctl interface.
For that you have to undef the LINUX_CHG define in changer-src/scsi-linux.c
Also you have to change the NORMAL_TIMEOUT in /usr/src/linux/drivers/scsi/scsi_ioctl.c
from (10 * HZ) to (5 * 60 * HZ).
On linux it does not run if you are using an aha1542 SCSI controller. The
driver can not handle the extended request sense.

On IRIX you find the SCSI pass through interfaces for every device in /dev/scsi

chg-scsi has been tested/run with the following devices
Exabyte 10h and eliant tape
HP-Surestore 1200e and C1553A tape
BreeceHill Q2.15 (EXB-120) and DLT7000 tape
Powerstor L200 and DLT7000 
ARCHIVE Python 28849-XXX 
TANDBERG TDS 1420
ADIC VLS DLT Library

It is now possible with a changer that has barcode reader to load tapes faster.
Also amdump will find tapes faster. Every time a tape is labeled the information
in the labelfile will be updated. To initialize the label mapping you can also
do an amtape config show. This will put the mapping for all tapes in the magazine
into the mapping file.

For all problems please contact th@ant.han.de. Please include in your
mail the debug file. (/tmp/amanda/chg-scsi.debug)


===========================================================================
chg-chio (new perl script that replaces the original chg-chio written in C)
===========================================================================

This script is based on the FreeBSD version of chio, a program to
communicate with the jukebox.  This script has for the moment only
been test with FreeBSD and is likely not to work on any other system.
Let me know if this is the case and send me the output of the chio
program for your version of chio.

It does not restrict the number of tapes, except that if there is only
one tape in the juke, it is supposed to be in max_slot and not in slot 1.

[This is the first version of the changer script and I would
appreciate all comments on it, at nick.hibma@jrc.it.  It has been
tested only with FreeBSD 2.2.5 and the accompanying chio program.]

==============================
chg-chs (formerly chs-changer)
==============================

A tape changer script very similar to chg-multi, that uses the `chs'
program to change tapes.  As in chg-multi, `tapedev' is ignored.
`changerfile' names its configuration file, similar to chg-multi.conf.
`changerdev' will be passed to CHS in a -f command-line switch, unless
it is set to an empty string or "/dev/null" (watch out for default
values!)

==============================
chg-rth (formerly rth-changer)
==============================

A perl5 script that controls an HPc1553 tape drive via a Peripheral
Vision Inc. SCSI control subsystem that interprets commands sent on
the SCSI bus.  It expects `tapedev' to specify the tape device to be
used.  `changerfile' and `changerdev' are ignored.

==============================
chg-juke
==============================

A shell script that uses the Fermilab "juke" software (see 
http://www.fnal.gov/fermitools/, the "juke" link) to control
tape chagners.  It supports mounting multiple tapes for RAIT 
tapedrive sets, both multiple jukeboxes, or one jukebox with
multiple tape drives, or combinations.  'juke' must be configured
to know tape drives by the same name Amanda calls them.

It uses 'changerfile' to track Amanda's current tape state, 
'tapedev' must be the tape drive (or RAIT set) name, and 'changerdev' 
is the juke software's name for the changer, or a csh-glob that
expands to several jukebox names (i.e. "changer{a,b,c}")

==============================
chg-rait
==============================

A shell script that runs other changers in tandem, and returns
a rait:{dev1,dev2,...} tape device based on the results of 
each other changer.  So if you wanted to have 2 stackers
striped with no parity, and you have chg-mtx support for your
stackers, you would use the follwing changerfile:
 
   nchangers=3
   tpchanger_1="chg-mtx"
   changerdev_1="/dev/mtx1"
   changerfile_1="/some/file1"
   tapedev_1="/some/dev"
   tpchanger_2="chg-mtx"
   changerdev_2="/dev/mtx2"
   changerfile_2="/some/file2"
   tapedev_2="/some/dev"
   tpchanger_3="chg-null"
   changerdev_3="/dev/null"
   changerfile_3="/some/file3"
   tapedev_3="/some/dev"

The third uses the null changer.  The tapedev_n entries are only needed
if the changerfile in question uses them.


================================
chg-iomega
================================

This changer script is designed for IOMEGA or JAZZ disks of various sizes
as well as any other removable disk media.

This is a PURELY MANUAL changer. It requests insertion of disk media via
messages on /dev/tty. So it cannot be used via crontab.

Make sure you comply with any of the following.
- Add statements 
        tpchanger "chg-iomega"
        tapedev "file:<mount_point_of_removable_disk>"
        (e.g. tapedev "file:/mnt/iomega" )
        tapetype IOMEGA      

        
        define tapetype IOMEGA {
            comment "IOMega 250 MB floppys"
            length 250 mbytes
            filemark 100 kbytes
            speed 1 mbytes
        }
  to your /etc/amanda/<backup_set>/amanda.conf file
- Add entry to /etc/fstab to specify mount point of removable disk
  and make this disk mountable by any user.
- Format all disks, add a "data" sub directory and label all disks
  by a call to amlabel.
- Be aware that as of version 2.4.4p1, amanda can't handle backups that are
  larger than the size of the removable disk media. So make sure
  /etc/amanda/<backup_set>/disklist specifies chunks smaller than the 
  disk size.


==============================
chg-null
==============================

A trivial changer which loads/unloads on a null: device.  Useful with
chg-rait to throw away a parity stripe by puttin on a null jukebox, 
or for testing.