@UNREVISED
A few special cases about tape handling warrant more detailed description. These special cases are discussed below.
Many complexities surround the use of tar on tape drives. Since
the creation and manipulation of archives located on magnetic tape was
the original purpose of tar, it contains many features making
such manipulation easier.
Archives are usually written on dismountable media--tape cartridges, mag tapes, or floppy disks.
The amount of data a tape or disk holds depends not only on its size, but also on how it is formatted. A 2400 foot long reel of mag tape holds 40 megabytes of data when formated at 1600 bits per inch. The physically smaller EXABYTE tape cartridge holds 2.3 gigabytes.
Magnetic media are re-usable--once the archive on a tape is no longer needed, the archive can be erased and the tape or disk used over. Media quality does deteriorate with use, however. Most tapes or disks should be disgarded when they begin to produce data errors. EXABYTE tape cartridges should be disgarded when they generate an error count (number of non-usable bits) of more than 10k.
Magnetic media are written and erased using magnetic fields, and should be protected from such fields to avoid damage to stored data. Sticking a floppy disk to a filing cabinet using a magnet is probably not a good idea.
@UNREVISED
This option is used to specify the file name of the archive tar
works on.
If the file name is `-', tar reads the archive from standard
input (when listing or extracting), or writes it to standard output
(when creating). If the `-' file name is given when updating an
archive, tar will read the original archive from its standard
input, and will write the entire new archive to its standard output.
If the file name contains a `:', it is interpreted as
`hostname:file name'. If the hostname contains an at
sign (@), it is treated as `user@hostname:file name'. In
either case, tar will invoke the command rsh (or
remsh) to start up an `/etc/rmt' on the remote machine. If
you give an alternate login name, it will be given to the rsh.
Naturally, the remote machine must have an executable `/etc/rmt'.
This program is free software from the University of California, and a
copy of the source code can be found with the sources for tar;
it's compiled and installed by default.
If this option is not given, but the environment variable TAPE is
set, its value is used; otherwise, old versions of tar used a default
archive name (which was picked when tar was compiled). The
default is normally set up to be the first tape drive or other
transportable I/O medium on the system.
Starting with version 1.11.5, GNU tar uses standard input and
standard output as the default device, and I will not try anymore
supporting automatic device detection at installation time. This was
failing really in too many cases, it was hopeless. This is now
completely left to the installer to override standard input and standard
output for default device, if this seems preferrable to him/her.
Further, I think most actual usages of tar are done with
pipes or disks, not really tapes, cartridges or diskettes.
Some users think that using standard input and output is running after trouble. This could lead to a nasty surprise on your screen if you forget to specify an output file name--especially if you are going through a network or terminal server capable of buffering large amounts of output. We had so many bug reports in that area of configuring default tapes automatically, and so many contradicting requests, that we finally consider the problem to be portably intractable. We could of course use something like `/dev/tape' as a default, but this is also running after various kind of trouble, going from hung processes to accidental destruction of real tapes. After having seen all this mess, using standard input and output as a default really sounds like the only clean choice left, and a very useful one too.
GNU tar reads and writes archive in records, I suspect this is the
main reason why block devices are preferred over character devices.
Most probably, block devices are more efficient too. The installer
could also check for `DEFTAPE' in `<sys/mtio.h>'.
rsh. This option exists
so that people who use something other than the standard rsh
(e.g., a Kerberized rsh) can access a remote device.
When this command is not used, the shell command found when
the tar program was installed is used instead. This is
the first found of `/usr/ucb/rsh', `/usr/bin/remsh',
`/usr/bin/rsh', `/usr/bsd/rsh' or `/usr/bin/nsh'.
The installer may have overriden this by defining the environment
variable RSH at installation time.
tar to write a multi-volume archive--one
that may be larger than will fit on the medium used to hold it.
See section Archives Longer than One Tape or Disk.
In order to access the tape drive on a remote machine, tar
uses the remote tape server written at the University of California at
Berkeley. The remote tape server must be installed as `/etc/rmt'
on any machine whose tape drive you want to use. tar calls
`/etc/rmt' by running an rsh or remsh to the remote
machine, optionally using a different login name if one is supplied.
A copy of the source for the remote tape server is provided. It is Copyright (C) 1983 by the Regents of the University of California, but can be freely distributed. Instructions for compiling and installing it are included in the `Makefile'.
Unless you use the --absolute-names (-P) option, GNU tar will
not allow you to create an archive that contains absolute file names
(a file name beginning with `/'.) If you try, tar will
automatically remove the leading `/' from the file names it
stores in the archive. It will also type a warning message telling
you what it is doing.
When reading an archive that was created with a different tar
program, GNU tar automatically extracts entries in the archive
which have absolute file names as if the file names were not absolute.
This is an important feature. A visitor here once gave a
tar tape to an operator to restore; the operator used Sun tar
instead of GNU tar, and the result was that it replaced large
portions of our `/bin' and friends with versions from the tape;
needless to say, we were unhappy about having to recover the file system
from backup tapes.
For example, if the archive contained a file `/usr/bin/computoy',
GNU tar would extract the file to `usr/bin/computoy',
relative to the current directory. If you want to extract the files in
an archive to the same absolute names that they had when the archive
was created, you should do a `cd /' before extracting the files
from the archive, or you should either use the --absolute-names (-P)
option, or use the command `tar -C / ...'.
Some versions of Unix (Ultrix 3.1 is know to have this problem), can claim that a short write near the end of a tape succeeded, when it actually failed. This will result in the -M option not working correctly. The best workaround at the moment is to use a significantly larger blocking factor than the default 20.
In order to update an archive, tar must be able to backspace the
archive in order to reread or rewrite a record that was just read (or
written). This is currently possible only on two kinds of files: normal
disk files (or any other file that can be backspaced with `lseek'),
and industry-standard 9-track magnetic tape (or any other kind of tape
that can be backspaced with the MTIOCTOP ioctl.
This means that the --append (-r), --update (-u), --concatenate (--catenate, -A), and --delete commands will not work on any other kind of file. Some media simply cannot be backspaced, which means these commands and options will never be able to work on them. These non-backspacing media include pipes and cartridge tape drives.
Some other media can be backspaced, and tar will work on them
once tar is modified to do so.
Archives created with the --multi-volume (-M), --label=archive-label (-V archive-label), and
--incremental (-G) options may not be readable by other version
of tar. In particular, restoring a file that was split over
a volume boundary will require some careful work with dd, if
it can be done at all. Other versions of tar may also create
an empty file whose name is that of the volume header. Some versions
of tar may create normal files instead of directories archived
with the --incremental (-G) option.
errors from system:
permission denied
no such file or directory
not owner
errors from tar:
directory checksum error
header format error
errors from media/system:
i/o error
device busy
@UNREVISED
Block and record terminology is rather confused, and it is also confusing to the expert reader. On the other hand, readers who are new to the field have a fresh mind, and they may safely skip the next two paragraphs, as the remainder of this manual uses those two terms in a quite consistent way.
John Gilmore, the writer of the public domain tar from which
GNU tar was originally derived, wrote (June 1995):
The nomenclature of tape drives comes from IBM, where I believe they were invented for the IBM 650 or so. On IBM mainframes, what is recorded on tape are tape blocks. The logical organization of data is into records. There are various ways of putting records into blocks, including
F(fixed sized records),V(variable sized records),FB(fixed blocked: fixed size records, n to a block),VB(variable size records, n to a block),VSB(variable spanned blocked: variable sized records that can occupy more than one block), etc. TheJCL`DD RECFORM=' parameter specified this to the operating system.The Unix man page on
tarwas totally confused about this. When I wrotePD TAR, I used the historically correct terminology (tarwrites data records, which are grouped into blocks). It appears that the bogus terminology made it into POSIX (no surprise here), and now Fran@,{c}ois has migrated that terminology back into the source code too.
The term physical block means the basic transfer chunk from or
to a device, after which reading or writing may stop without anything
being lost. In this manual, the term block usually refers to
a disk physical block, assuming that each disk block is 512
bytes in length. It is true that some disk devices have different
physical blocks, but tar ignore these differences in its own
format, which is meant to be portable, so a tar block is always
512 bytes in length, and block always mean a tar block.
The term logical block often represents the basic chunk of
allocation of many disk blocks as a single entity, which the operating
system treats somewhat atomically; this concept is only barely used
in GNU tar.
The term physical record is another way to speak of a physical
block, those two terms are somewhat interchangeable. In this manual,
the term record usually refers to a tape physical block,
assuming that the tar archive is kept on magnetic tape.
It is true that archives may be put on disk or used with pipes,
but nevertheless, tar tries to read and write the archive one
record at a time, whatever the medium in use. One record is made
up of an integral number of blocks, and this operation of putting many
disk blocks into a single tape block is called reblocking, or
more simply, blocking. The term logical record refers to
the logical organization of many characters into something meaningful
to the application. The term unit record describes a small set
of characters which are transmitted whole to or by the application,
and often refers to a line of text. Those two last terms are unrelated
to what we call a record in GNU tar.
When writing to tapes, tar writes the contents of the archive
in chunks known as records. To change the default blocking
factor, use the --blocking-factor=512-size (-b 512-size) option. Each record will
then be composed of 512-size blocks. (Each tar block is
512 bytes. See section The Standard Format.) Each file written to the archive uses
at least one full record. As a result, using a larger record size
can result in more wasted space for small files. On the other hand, a
larger record size can often be read and written much more efficiently.
Further complicating the problem is that some tape drives ignore the blocking entirely. For these, a larger record size can still improve performance (because the software layers above the tape drive still honor the blocking), but not as dramatically as on tape drives that honor blocking.
When reading an archive, tar can usually figure out the record
size on itself. When this is the case, and a non-standard record size
was used when the archive was created, tar will print a message
about a non-standard blocking factor, and then operate normally. On
some tape devices, however, tar cannot figure out the record size
itself. On most of those, you can specify a blocking factor (with
--blocking-factor=512-size (-b 512-size)) larger than the actual blocking factor, and then use
the --read-full-records (-B) option. (If you specify a blocking factor
with --blocking-factor=512-size (-b 512-size) and don't use the --read-full-records (-B)
option, then tar will not attempt to figure out the recording size
itself.) On some devices, you must always specify the record size
exactly with --blocking-factor=512-size (-b 512-size) when reading, because tar cannot
figure it out. In any case, use --list (-t) before doing any
extractions to see whether tar is reading the archive correctly.
tar blocks are all fixed size (512 bytes), and its scheme for
putting them into records is to put a whole number of them (one or
more) into each record. tar records are all the same size;
at the end of the file there's a block containing all zeros, which
is how you tell that the remainder of the last record(s) are garbage.
In a standard tar file (no options), the block size is 512
and the record size is 10240, for a blocking factor of 20. What the
--blocking-factor=512-size (-b 512-size) option does is sets the blocking factor,
changing the record size while leaving the block size at 512 bytes.
20 was fine for ancient 800 or 1600 bpi reel-to-reel tape drives;
most tape drives these days prefer much bigger records in order to
stream and not waste tape. When writing tapes for myself, some tend
to use a factor of the order of 2048, say, giving a record size of
around one megabyte.
If you use a blocking factor larger than 20, older tar programs
might not be able to read the archive, so we recommend this as a limit
to use in practice. GNU tar, however, will support arbitrarily
large record sizes, limited only by the amount of virtual memory or the
physical characteristics of the tape device.
Format parameters specify how an archive is written on the archive media. The best choice of format parameters will vary depending on the type and number of files being archived, and on the media used to store the archive.
To specify format parameters when accessing or creating an archive,
you can use the options described in the following sections.
If you do not specify any format parameters, tar uses
default parameters. You cannot modify a compressed archive.
If you create an archive with the --blocking-factor=512-size (-b 512-size) option
specified (see section The Blocking Factor of an Archive), you must specify that
blocking-factor when operating on the archive. See section Controlling the Archive Format, for other
examples of format parameter considerations.
The data in an archive is grouped into blocks, which are 512 bytes. Blocks are read and written in whole number multiples called records. The number of blocks in a record (ie. the size of a record in units of 512 bytes) is called the blocking factor. The --blocking-factor=512-size (-b 512-size) option specifies the blocking factor of an archive. The default blocking factor is typically 20 (ie. 10240 bytes), but can be specified at installation. To find out the blocking factor of an existing archive, use `tar --list --file=archive-name'. This may not work on some devices.
Records are separated by gaps, which waste space on the archive media.
If you are archiving on magnetic tape, using a larger blocking factor
(and therefore larger records) provides faster throughput and allows you
to fit more data on a tape (because there are fewer gaps). If you are
archiving on cartridge, a very large blocking factor (say 126 or more)
greatly increases performance. A smaller blocking factor, on the other
hand, may be usefull when archiving small files, to avoid archiving lots
of nulls as tar fills out the archive to the end of the record.
In general, the ideal record size depends on the size of the
inter-record gaps on the tape you are using, and the average size of the
files you are archiving. See section How to Create Archives, for information on
writing archives.
@FIXME{Need example of using a cartridge with blocking factor=126 or more.}
Archives with blocking factors larger than 20 cannot be read
by very old versions of tar, or by some newer versions
of tar running on old machines with small address spaces.
With GNU tar, the blocking factor of an archive is limited
only by the maximum record size of the device containing the archive,
or by the amount of available virtual memory.
Also, on some systems, not using adequate blocking factors, as sometimes imposed by the device drivers, may yield unexpected diagnostics. For example, this has been reported:
Cannot write to /dev/dlt: Invalid argument
In such cases, it sometimes happen that the tar bundled by the
system is aware of block size idiosyncrasies, while GNU tar requires
an explicit specification for the block size, which it cannot guess.
This yields some people to consider GNU tar is misbehaving, because
by comparison, the bundle tar works OK. Adding -b
256, for example, might resolve the problem.
If you use a non-default blocking factor when you create an archive, you
must specify the same blocking factor when you modify that archive. Some
archive devices will also require you to specify the blocking factor when
reading that archive, however this is not typically the case. Usually, you
can use --list (-t) without specifying a blocking factor---tar
reports a non-default record size and then lists the archive members as
it would normally. To extract files from an archive with a non-standard
blocking factor (particularly if you're not sure what the blocking factor
is), you can usually use the --read-full-records (-B) option while
specifying a blocking factor larger then the blocking factor of the archive
(ie. `tar --extract --read-full-records --blocking-factor=300'.
See section How to List Archives, for more information on the --list (-t)
operation. See section Options to Help Read Archives, for a more detailed explanation of that option.
Device blocking
tar, will do reads and writes
of the archive in records of block*512 bytes. This is true
even when the archive is compressed. Some devices requires that all
write operations be a multiple of a certain size, and so, tar
pads the archive out to the next record boundary.
The default blocking factor is set when tar is compiled, and is
typically 20. Blocking factors larger than 20 cannot be read by very
old versions of tar, or by some newer versions of tar
running on old machines with small address spaces.
With a magnetic tape, larger records give faster throughput and fit
more data on a tape (because there are fewer inter-record gaps).
If the archive is in a disk file or a pipe, you may want to specify
a smaller blocking factor, since a large one will result in a large
number of null bytes at the end of the archive.
When writing cartridge or other streaming tapes, a much larger
blocking factor (say 126 or more) will greatly increase performance.
However, you must specify the same blocking factor when reading or
updating the archive.
Apparently, Exabyte drives have a physical block size of 8K bytes.
If we choose our blocksize as a multiple of 8k bytes, then the problem
seems to dissapper. Id est, we are using block size of 112 right
now, and we haven't had the problem since we switched...
With GNU tar the blocking factor is limited only by the maximum
record size of the device containing the archive, or by the amount of
available virtual memory.
However, deblocking or reblocking is virtually avoided in a special
case which often occurs in practice, but which requires all the
following conditions to be simultaneously true:
tar
invocation.
tar, the `--compress-block'
option (or even older: `--block-compress') was necessary to
reblock compressed archives. It is now a dummy option just asking
not to be used, and otherwise ignored. If the output goes directly
to a local disk, and not through stdout, then the last write is
not extended to a full record size. Otherwise, reblocking occurs.
Here are a few other remarks on this topic:
gzip will complain about trailing garbage if asked to
uncompress a compressed archive on tape, there is an option to turn
the message off, but it breaks the regularity of simply having to use
`prog -d' for decompression. It would be nice if gzip was
silently ignoring any number of trailing zeros. I'll ask Jean-loup
Gailly, by sending a copy of this message to him.
compress does not show this problem, but as Jean-loup pointed
out to Michael, `compress -d' silently adds garbage after
the result of decompression, which tar ignores because it already
recognized its end-of-file indicator. So this bug may be safely
ignored.
tar might ignore the exit status returned, but I hate doing
that, as it weakens the protection tar offers users against
other possible problems at decompression time. If gzip was
silently skipping trailing zeros and also avoiding setting the
exit status in this innocuous case, that would solve this situation.
tar should become more solid at not stopping to read a pipe at
the first null block encountered. This inelegantly breaks the pipe.
tar should rather drain the pipe out before exiting itself.
tar to ignore blocks
of zeros in the archive. Normally a block of zeros indicates the
end of the archive, but when reading a damaged archive, or one which
was created by cat-ing several archives together, this option
allows tar to read the entire archive. This option is not on
by default because many versions of tar write garbage after
the zeroed blocks.
Note that this option causes tar to read to the end of the
archive file, which may sometimes avoid problems when multiple files
are stored on a single physical tape.
tar will not panic if an
attempt to read a record from the archive does not return a full record.
Instead, tar will keep reading until it has obtained a full
record.
This option is turned on by default when tar is reading
an archive from standard input, or from a remote machine. This is
because on BSD Unix systems, a read of a pipe will return however
much happens to be in the pipe, even if it is less than tar
requested. If this option was not used, tar would fail as
soon as it read an incomplete record from the pipe.
This option is also useful with the commands for updating an archive.
Tape blocking
@FIXME{Appropriate options should be moved here from elsewhere.}
When handling various tapes or cartridges, you have to take care of selecting a proper blocking, that is, the number of disk blocks you put together as a single tape block on the tape, without intervening tape gaps. A tape gap is a small landing area on the tape with no information on it, used for decelerating the tape to a full stop, and for later regaining the reading or writing speed. When the tape driver starts reading a record, the record has to be read whole without stopping, as a tape gap is needed to stop the tape motion without loosing information.
Using higher blocking (putting more disk blocks per tape block) will use
the tape more efficiently as there will be less tape gaps. But reading
such tapes may be more difficult for the system, as more memory will be
required to receive at once the whole record. Further, if there is a
reading error on a huge record, this is less likely that the system will
succeed in recovering the information. So, blocking should not be too
low, nor it should be too high. tar uses by default a blocking of
20 for historical reasons, and it does not really matter when reading or
writing to disk. Current tape technology would easily accomodate higher
blockings. Sun recommends a blocking of 126 for Exabytes and 96 for DATs.
We were told that for some DLT drives, the blocking should be a multiple
of 4Kb, preferably 64Kb (-b 128) or 256 for decent performance.
Other manufacturers may use different recommendations for the same tapes.
This might also depends of the buffering techniques used inside modern
tape controllers. Some imposes a minimum blocking, or a maximum blocking.
Others request blocking to be some exponent of two.
So, there is no fixed rule for blocking. But blocking at read time should ideally be the same as blocking used at write time. At one place I know, with a wide variety of equipment, they found it best to use a blocking of 32 to guarantee that their tapes are fully interchangeable.
I was also told that, for recycled tapes, prior erasure (by the same drive unit that will be used to create the archives) sometimes lowers the error rates observed at rewriting time.
I might also use `--number-blocks' instead of `--block-number', so `--block' will then expand to `--blocking-factor' unambiguously.
@FIXME{Appropriate options should be moved here from elsewhere.}
Most tape devices have two entries in the `/dev' directory, or entries that come in pairs, which differ only in the minor number for this device. Let's take for example `/dev/tape', which often points to the only or usual tape device of a given system. There might be a corresponding `/dev/nrtape' or `/dev/ntape'. The simpler name is the rewinding version of the device, while the name having `nr' in it is the no rewinding version of the same device.
A rewinding tape device will bring back the tape to its beginning point
automatically when this device is opened or closed. Since tar
opens the archive file before using it and closes it afterwards, this
means that a simple:
$ tar cf /dev/tape directory
will reposition the tape to its beginning both prior and after saving directory contents to it, thus erasing prior tape contents and making it so that any subsequent write operation will destroy what has just been saved.
So, a rewinding device is normally meant to hold one and only one file.
If you want to put more than one tar archive on a given tape, you
will need to avoid using the rewinding version of the tape device. You
will also have to pay special attention to tape positioning. Errors in
positionning may overwrite the valuable data already on your tape. Many
people, burnt by past experiences, will only use rewinding devices and
limit themselves to one file per tape, precisely to avoid the risk of
such errors. Be fully aware that writing at the wrong position on a
tape loses all information past this point and most probably until the
end of the tape, and this destroyed information cannot be
recovered.
To save directory-1 as a first archive at the beginning of a tape, and leave that tape ready for a second archive, you should use:
$ mt -f /dev/nrtape rewind $ tar cf /dev/nrtape directory-1
Tape marks are special magnetic patterns written on the tape
media, which are later recognizable by the reading hardware. These
marks are used after each file, when there are many on a single tape.
An empty file (that is to say, two tape marks in a row) signal the
logical end of the tape, after which no file exist. Usually,
non-rewinding tape device drivers will react to the close request issued
by tar by first writing two tape marks after your archive, and by
backspacing over one of these. So, if you remove the tape at that time
from the tape drive, it is properly terminated. But if you write
another file at the current position, the second tape mark will be
erased by the new information, leaving only one tape mark between files.
So, you may now save directory-2 as a second archive after the first on the same tape by issuing the command:
$ tar cf /dev/nrtape directory-2
and so on for all the archives you want to put on the same tape.
Another usual case is that you do not write all the archives the same day, and you need to remove and store the tape between two archive sessions. In general, you must remember how many files are already saved on your tape. Suppose your tape already has 16 files on it, and that you are ready to write the 17th. You have to take care of skipping the first 16 tape marks before saving directory-17, say, by using these commands:
$ mt -f /dev/nrtape rewind $ mt -f /dev/nrtape fsf 16 $ tar cf /dev/nrtape directory-17
In all the previous examples, we put aside blocking considerations, but you should do the proper things for that as well. See section Blocking.
@UNREVISED
Just as archives can store more than one file from the file system, tapes can store more than one archive file. To keep track of where archive files (or any other type of file stored on tape) begin and end, tape archive devices write magnetic tape marks on the archive media. Tape drives write one tape mark between files, two at the end of all the file entries.
If you think of data as a series of records "rrrr"'s, and tape marks as "*"'s, a tape might look like the following:
rrrr*rrrrrr*rrrrr*rr*rrrrr**-------------------------
Tape devices read and write tapes using a read/write tape
head---a physical part of the device which can only access one
point on the tape at a time. When you use tar to read or
write archive data from a tape device, the device will begin reading
or writing from wherever on the tape the tape head happens to be,
regardless of which archive or what part of the archive the tape
head is on. Before writing an archive, you should make sure that no
data on the tape will be overwritten (unless it is no longer needed).
Before reading an archive, you should make sure the tape head is at
the beginning of the archive you want to read. (The restore
script will find the archive automatically. @FIXME{There is no such
restore script!}. @FIXME-xref{Scripted Restoration}). See section The mt Utility, for
an explanation of the tape moving utility.
If you want to add new archive file entries to a tape, you should advance the tape to the end of the existing file entries, backspace over the last tape mark, and write the new archive file. If you were to add two archives to the example above, the tape might look like the following:
rrrr*rrrrrr*rrrrr*rr*rrrrr*rrr*rrrr**----------------
mt Utility@UNREVISED
@FIXME{Is it true that this only works on non-block devices? should explain the difference, (fixed or variable).} See section The Blocking Factor of an Archive.
You can use the mt utility to advance or rewind a tape past a
specified number of archive files on the tape. This will allow you
to move to the beginning of an archive before extracting or reading
it, or to the end of all the archives before writing a new one.
@FIXME{Why isn't there an "advance 'til you find two tape marks
together"?}
The syntax of the mt command is:
mt [-f tapename] operation [number]
where tapename is the name of the tape device, number is the number of times an operation is performed (with a default of one), and operation is one of the following:
@FIXME{is there any use for record operations?}
@FIXME{Is there a better way to frob the spacing on the list?}
If you don't specify a tapename, mt uses the environment
variable TAPE; if TAPE does not exist, mt uses the device
`/dev/rmt12'.
mt returns a 0 exit status when the operation(s) were
successful, 1 if the command was unrecognized, and 2 if an operation
failed.
@FIXME{New node on how to find an archive?}
If you use --extract (--get, -x) with the --label=archive-label (-V archive-label) option specified,
tar will read an archive label (the tape head has to be positioned
on it) and print an error if the archive label doesn't match the
archive-name specified. archive-name can be any regular
expression. If the labels match, tar extracts the archive.
See section Including a Label in the Archive. @FIXME-xref{Matching Format Parameters}.
@FIXME{fix cross references} `tar --list --label' will cause
tar to print the label.
@FIXME{Program to list all the labels on a tape?}
@UNREVISED
Often you might want to write a large archive, one larger than will fit
on the actual tape you are using. In such a case, you can run multiple
tar commands, but this can be inconvenient, particularly if you
are using options like --exclude=pattern or dumping entire filesystems.
Therefore, tar supports multiple tapes automatically.
Use --multi-volume (-M) on the command line, and then tar will,
when it reaches the end of the tape, prompt for another tape, and
continue the archive. Each tape will have an independent archive, and
can be read without needing the other. (As an exception to this, the
file that tar was archiving when it ran out of tape will usually
be split between the two archives; in this case you need to extract from
the first archive, using --multi-volume (-M), and then put in the
second tape when prompted, so tar can restore both halves of the
file.)
GNU tar multi-volume archives do not use a truly portable format.
You need GNU tar at both end to process them properly.
When prompting for a new tape, tar accepts any of the following
responses:
tar to explain possible responses
tar to exit immediately.
tar to write the next volume on the file file name.
tar to run a subshell.
tar to begin writing the next volume.
(You should only type `y' after you have changed the tape;
otherwise tar will write over the volume it just finished.)
If you want more elaborate behavior than this, give tar the
--info-script=script-name (--new-volume-script=script-name, -F script-name) option. The file script-name is expected
to be a program (or shell script) to be run instead of the normal
prompting procedure. When the program finishes, tar will
immediately begin writing the next volume. The behavior of the
`n' response to the normal tape-change prompt is not available
if you use --info-script=script-name (--new-volume-script=script-name, -F script-name).
The method tar uses to detect end of tape is not perfect, and
fails on some operating systems or on some devices. You can use the
--tape-length=1024-size (-L 1024-size) option if tar can't detect the end of the
tape itself. This option selects --multi-volume (-M) automatically.
The size argument should then be the usable size of the tape.
But for many devices, and floppy disks in particular, this option is
never required for real, as far as we know.
The volume number used by tar in its tape-change prompt
can be changed; if you give the --volno-file=file-of-number option, then
file-of-number should be an unexisting file to be created, or else,
a file already containing a decimal number. That number will be used
as the volume number of the first volume written. When tar is
finished, it will rewrite the file with the now-current volume number.
(This does not change the volume number written on a tape label, as
per section Including a Label in the Archive, it only affects the number used in
the prompt.)
If you want tar to cycle through a series of tape drives, then
you can use the `n' response to the tape-change prompt. This is
error prone, however, and doesn't work at all with --info-script=script-name (--new-volume-script=script-name, -F script-name).
Therefore, if you give tar multiple --file=archive-name (-f archive-name) options, then
the specified files will be used, in sequence, as the successive volumes
of the archive. Only when the first one in the sequence needs to be
used again will tar prompt for a tape change (or run the info
script).
Multi-volume archives
With --multi-volume (-M), tar will not abort when it cannot
read or write any more data. Instead, it will ask you to prepare a new
volume. If the archive is on a magnetic tape, you should change tapes
now; if the archive is on a floppy disk, you should change disks, etc.
Each volume of a multi-volume archive is an independent tar
archive, complete in itself. For example, you can list or extract any
volume alone; just don't specify --multi-volume (-M). However, if one
file in the archive is split across volumes, the only way to extract
it successfully is with a multi-volume extract command `--extract
--multi-volume' (`-xM') starting on or before the volume where
the file begins.
For example, let's presume someone has two tape drives on a system
named `/dev/tape0' and `/dev/tape1'. For having GNU
tar to switch to the second drive when it needs to write the
second tape, and then back to the first tape, etc., just do either of:
$ tar --create --multi-volume --file=/dev/tape0 --file=/dev/tape1 files $ tar cMff /dev/tape0 /dev/tape1 files
To create an archive that is larger than will fit on a single unit of the media, use the --multi-volume (-M) option in conjunction with the --create (-c) option (see section How to Create Archives). A multi-volume archive can be manipulated like any other archive (provided the --multi-volume (-M) option is specified), but is stored on more than one tape or disk.
When you specify --multi-volume (-M), tar does not report an
error when it comes to the end of an archive volume (when reading), or
the end of the media (when writing). Instead, it prompts you to load
a new storage volume. If the archive is on a magnetic tape, you
should change tapes when you see the prompt; if the archive is on a
floppy disk, you should change disks; etc.
You can read each individual volume of a multi-volume archive as if it were an archive by itself. For example, to list the contents of one volume, use --list (-t), without --multi-volume (-M) specified. To extract an archive member from one volume (assuming it is described that volume), use --extract (--get, -x), again without --multi-volume (-M).
If an archive member is split across volumes (ie. its entry begins on
one volume of the media and ends on another), you need to specify
--multi-volume (-M) to extract it successfully. In this case, you
should load the volume where the archive member starts, and use
`tar --extract --multi-volume'---tar will prompt for later
volumes as it needs them. See section Extracting an Entire Archive, for more
information about extracting archives.
--info-script=script-name (--new-volume-script=script-name, -F script-name) is like --multi-volume (-M), except that
tar does not prompt you directly to change media volumes when
a volume is full--instead, tar runs commands you have stored
in script-name. For example, this option can be used to eject
cassettes, or to broadcast messages such as `Someone please come
change my tape' when performing unattended backups. When script-name
is done, tar will assume that the media has been changed.
Multi-volume archives can be modified like any other archive. To add files to a multi-volume archive, you need to only mount the last volume of the archive media (and new volumes, if needed). For all other operations, you need to use the entire archive.
If a multi-volume archive was labeled using --label=archive-label (-V archive-label)
(see section Including a Label in the Archive) when it was created, tar will not
automatically label volumes which are added later. To label subsequent
volumes, specify --label=archive-label (-V archive-label) again in conjunction with the
--append (-r), --update (-u) or --concatenate (--catenate, -A) operation.
@FIXME{There should be a sample program here, including an exit before end. Is the exit status even checked in tar? :-(}
Beware that there is no real standard about the proper way, for a
tar archive, to span volume boundaries. If you have a multi-volume
created by some vendor's tar, there is almost no chance you could
read all the volumes with GNU tar. The converse is also true:
you may not expect multi-volume archives created by GNU tar to
be fully recovered by vendor's tar. Since there is little chance
that, in mixed system configurations, some vendor's tar will work on
another vendor's machine, and there is a great chance that GNU tar
will work on most of them, your best bet is to install GNU tar
on all machines between which you know exchange of files is possible.
@UNREVISED
To give the archive a name which will be recorded in it, use the --label=archive-label (-V archive-label) option. This will write a special block identifying volume-label as the name of the archive to the front of the archive which will be displayed when the archive is listed with --list (-t). If you are creating a multi-volume archive with --multi-volume (-M) (@FIXME-pxref{Using Multiple Tapes}), then the volume label will have `Volume nnn' appended to the name you give, where nnn is the number of the volume of the archive. (If you use the --label=archive-label (-V archive-label) option when reading an archive, it checks to make sure the label on the tape matches the one you give. See section Including a Label in the Archive.
When tar writes an archive to tape, it creates a single
tape file. If multiple archives are written to the same tape, one
after the other, they each get written as separate tape files. When
extracting, it is necessary to position the tape at the right place
before running tar. To do this, use the mt command.
For more information on the mt command and on the organization
of tapes into a sequence of tape files, see section The mt Utility.
People seem to often do:
--label="some-prefix `date +some-format`"
or such, for pushing a common date in all volumes or an archive set.
This option causes tar to write out a volume header at
the beginning of the archive. If --multi-volume (-M) is used, each
volume of the archive will have a volume header of `name
Volume n', where n is 1 for the first volume, 2 for the
next, and so on.
@FIXME{Should the arg to --label be a quoted string?? No.}
To avoid problems caused by misplaced paper labels on the archive media, you can include a label entry--an archive member which contains the name of the archive--in the archive itself. Use the --label=archive-label (-V archive-label) option in conjunction with the --create (-c) operation to include a label entry in the archive as it is being created.
If you create an archive using both --label=archive-label (-V archive-label) and --multi-volume (-M), each volume of the archive will have an archive label of the form `archive-label Volume n', where n is 1 for the first volume, 2 for the next, and so on. @FIXME-xref{Multi-Volume Archives}, for information on creating multiple volume archives.
If you list or extract an archive using --label=archive-label (-V archive-label), tar will
print an error if the archive label doesn't match the archive-label
specified, and will then not list nor extract the archive. In those cases,
archive-label argument is interpreted as a globbing-style pattern
which must match the actual magnetic volume label. See section Excluding Some Files, for
a precise description of how match is attempted(7).
If the switch --multi-volume (-M) is being used, the volume label
matcher will also suffix archive-label by ` Volume [1-9]*'
if the initial match fails, before giving up. Since the volume numbering
is automatically added in labels at creation time, it sounded logical to
equally help the user taking care of it when the archive is being read.
The --label=archive-label (-V archive-label) was once called `--volume', but is not available under that name anymore.
To find out an archive's label entry (or to find out if an archive has
a label at all), use `tar --list --verbose'. tar will print the
label first, and then print archive member information, as in the
example below:
$ tar --verbose --list --file=iamanarchive V--------- 0 0 0 1992-03-07 12:01 iamalabel--Volume Header-- -rw-rw-rw- ringo user 40 1990-05-21 13:30 iamafilename
To get a common information on all tapes of a series, use the --label=archive-label (-V archive-label) option. For having this information different in each series created through a single script used on a regular basis, just manage to get some date string as part of the label. For example:
$ tar cfMV /dev/tape "Daily backup for `date +%Y-%m-%d`"
$ tar --create --file=/dev/tape --multi-volume \
--volume="Daily backup for `date +%Y-%m-%d`"
Also note that each label has its own date and time, which corresponds
to when GNU tar initially attempted to write it, often soon
after the operator launches tar or types the carriage return
telling that the next tape is ready. Comparing date labels does give
an idea of tape throughput only if the delays for rewinding tapes
and the operator switching them were negligible, which is ususally
not the case.
@FIXME{was --volume}
This option causes tar to verify the archive after writing it.
Each volume is checked after it is written, and any discrepancies
are recorded on the standard error output.
Verification requires that the archive be on a back-space-able medium. This means pipes, some cartridge tape drives, and some other devices cannot be verified.
You can insure the accuracy of an archive by comparing files in the
system with archive members. tar can compare an archive to the
file system as the archive is being written, to verify a write
operation, or can compare a previously written archive, to insure that
it is up to date.
To check for discrepancies in an archive immediately after it is
written, use the --verify (-W) option in conjunction with
the --create (-c) operation. When this option is
specified, tar checks archive members against their counterparts
in the file system, and reports discrepancies on the standard error. In
multi-volume archives, each volume is verified after it is written,
before the next volume is written.
To verify an archive, you must be able to read it from before the end of the last written entry. This option is useful for detecting data errors on some tapes. Archives written to pipes, some cartridge tape drives, and some other devices cannot be verified.
One can explicitely compare an already made archive with the file system by using the --compare (--diff, -d) option, instead of using the more automatic --verify (-W) option. See section Comparing Archive Members with the File System.
Note that these two options have a slightly different intent. The
--compare (--diff, -d) option how identical are the logical contents of some
archive with what is on your disks, while the --verify (-W) option is
really for checking if the physical contents agree and if the recording
media itself is of dependable quality. So, for the --verify (-W)
operation, tar tries to defeat all in-memory cache pertaining to
the archive, while it lets the speed optimization undisturbed for the
--compare (--diff, -d) option. If you nevertheless use --compare (--diff, -d) for
media verification, you may have to defeat the in-memory cache yourself,
maybe by opening and reclosing the door latch of your recording unit,
forcing some doubt in your operating system about the fact this is really
the same volume as the one just written or read.
The --verify (-W) option would not be necessary if drivers were indeed able to detect dependably all write failures. This sometimes require many magnetic heads, some able to read after the writes occurred. One would not say that drivers unable to detect all cases are necessarily flawed, as long as programming is concerned.
Almost all tapes and diskettes, and in a few rare cases, even disks can be write protected, to protect data on them from being changed. Once an archive is written, you should write protect the media to prevent the archive from being accidently overwritten or deleted. (This will protect the archive from being changed with a tape or floppy drive--it will not protect it from magnet fields or other physical hazards).
The write protection device itself is usually an integral part of the physical media, and can be a two position (write enabled/write disabled) switch, a notch which can be popped out or covered, a ring which can be removed from the center of a tape reel, or some other changeable feature.
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