
     Chapter 8. AMANDA Tape Changer Support
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Chapter 8. AMANDA Tape Changer Support


James da Silva

Original text
AMANDA Core Team
<jds@amanda.org>

Stefan G. Weichinger

XML-conversion;Updates
AMANDA Core Team
<sgw@amanda.org>
Table of Contents


  Introduction

  Specifying_a_tape_changer_in_amanda.conf

  Device-specific_tapechanger_script

  Slot_names_and_the_"current"_slot

  Operator_interface

  How_amdump_interacts_with_the_tape_changer

  Builtin_tape-changers


        chg-multi_(formerly_chg-generic)

        chg-manual_(formerly_no-changer)

        chg-mtx_(formerly_hp-changer)

        chg-zd-mtx

        chg-scsi-chio_(formerly_seagate-changer,_then_chg-chio)

        chg-scsi_(new_interface,_try_to_drive_a_robot_with_direct_scsi
        commands)

        chg-chio

        chg-chs

        chg-rth

        chg-juke

        chg-rait

        chg-disk

        chg-iomega

        chg-null



Note

Refer to http://www.amanda.org/docs/tapechangers.html for the current version
of this document.

 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.

 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
file amanda.conf. 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).

 Device-specific tapechanger 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.

 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.

 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.

 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.

 Builtin 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-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 syntax 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 platforms:
- 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-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 How_to_use_the_AMANDA_file-driver for details of usage.

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 amanda.conf. - 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 using 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.
-------------------------------------------------------------------------------

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