GNU tar 1.35: 9 Tapes and Other Archive Media

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 formatted 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 discarded when they begin to produce data errors. EXABYTE tape cartridges should be discarded 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.

9.1 Device Selection and Switching

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 /usr/libexec/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 /usr/libexec/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. The exact path to this utility is determined when configuring the package. It is ‘prefix/libexec/rmt’, where prefix stands for your installation prefix. This location may also be overridden at runtime by using the ‘--rmt-command=command’ option (See section —rmt-command, for detailed description of this option. See section Remote Tape Server, for the description of rmt command).

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 preferable. 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>’.

9.2 Remote Tape Server

Suffix	Units	Byte Equivalent
b	Blocks	`size` x 512
B	Kilobytes	`size` x 1024
c	Bytes	`size`
G	Gigabytes	`size` x 1024^3
K	Kilobytes	`size` x 1024
k	Kilobytes	`size` x 1024
M	Megabytes	`size` x 1024^2
P	Petabytes	`size` x 1024^5
T	Terabytes	`size` x 1024^4
w	Words	`size` x 2

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 ‘prefix/libexec/rmt’ on any machine whose tape drive you want to use. tar calls 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. Its source code can be freely distributed. It is compiled and installed by default.

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’ option, or use the command ‘tar -C / …’.

Some versions of Unix (Ultrix 3.1 is known 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’, ‘--concatenate’, 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’, ‘--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.

9.3 Some Common Problems and their Solutions

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

9.4 Blocking

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 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 Basic Tar 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(26). 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’) larger than the actual blocking factor, and then use the ‘--read-full-records’ (‘-B’) option. (If you specify a blocking factor with ‘--blocking-factor’ and don’t use the ‘--read-full-records’ 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’ 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’ 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.

9.4.1 Format Variations

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’ 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.

9.4.2 The Blocking Factor of an Archive

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 (i.e., 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 (i.e., 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 useful 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.

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 (i.e., ‘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.

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 losing 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 accommodate 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.

9.5 Many Archives on One Tape

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 positioning 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

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.

9.5.1 Tape Positions and Tape Marks

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. You can do it manually via mt utility (see section The mt Utility). The restore script does that automatically (see section Using the Restore Script).

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**----------------

9.5.2 The mt Utility

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.

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:

If you don’t specify a tapename, mt uses the environment variable TAPE; if TAPE is not set, mt will use the default device specified in your ‘sys/mtio.h’ file (DEFTAPE variable). If this is not defined, the program will display a descriptive error message and exit with code 1.

mt returns a 0 exit status when the operation(s) were successful, 1 if the command was unrecognized, and 2 if an operation failed.

9.6 Using Multiple Tapes

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 file systems. Therefore, tar provides a special mode for creating multi-volume archives.

Multi-volume archive is a single tar archive, stored on several media volumes of fixed size. Although in this section we will often call ‘volume’ a tape, there is absolutely no requirement for multi-volume archives to be stored on tapes. Instead, they can use whatever media type the user finds convenient, they can even be located on files.

When creating a multi-volume archive, GNU tar continues to fill current volume until it runs out of space, then it switches to next volume (usually the operator is queried to replace the tape on this point), and continues working on the new volume. This operation continues until all requested files are dumped. If GNU tar detects end of media while dumping a file, such a file is archived in split form. Some very big files can even be split across several volumes.

Each volume is itself a valid GNU tar archive, so it can be read without any special options. Consequently any file member residing entirely on one volume can be extracted or otherwise operated upon without needing the other volume. Sure enough, to extract a split member you would need all volumes its parts reside on.

Multi-volume archives suffer from several limitations. In particular, they cannot be compressed.

GNU tar is able to create multi-volume archives of two formats (see section Controlling the Archive Format): ‘GNU’ and ‘POSIX’.

9.6.1 Archives Longer than One Tape or Disk

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’ option (see section How to Create Archives). A multi-volume archive can be manipulated like any other archive (provided the ‘--multi-volume’ option is specified), but is stored on more than one tape or file.

When you specify ‘--multi-volume’, 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.

The method tar uses to detect end of tape is not perfect, and fails on some operating systems or on some devices. If tar cannot detect the end of the tape itself, you can use ‘--tape-length’ option to inform it about the capacity of the tape:

When GNU tar comes to the end of a storage media, it asks you to change the volume. The built-in prompt for POSIX locale is(27):

Prepare volume #n for 'archive' and hit return:

where n is the ordinal number of the volume to be created and archive is archive file or device name.

(You should only type ‘y’ after you have changed the tape; otherwise tar will write over the volume it just finished.)

The volume number used by tar in its tape-changing prompt can be changed; if you give the ‘--volno-file=file-of-number’ option, then file-of-number should be an non-existing 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 Including a Label in the Archive, it only affects the number used in the prompt.)

If you want more elaborate behavior than this, you can write a special new volume script, that will be responsible for changing the volume, and instruct tar to use it instead of its normal prompting procedure:

The command can contain additional options, if such are needed. See section Running External Commands, for a detailed discussion of the way GNU tar runs external commands. It inherits tar’s shell environment. Additional data is passed to it via the following environment variables:

These variables can be used in the command itself, provided that they are properly quoted to prevent them from being expanded by the shell that invokes tar.

The volume script can instruct tar to use new archive name, by writing in to file descriptor $TAR_FD (see below for an example).

If the info script fails, tar exits; otherwise, it begins writing the next volume.

If you want tar to cycle through a series of files or tape drives, there are three approaches to choose from. First of all, you can give tar multiple ‘--file’ options. In this case 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). For example, suppose 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 -cM -f /dev/tape0 -f /dev/tape1 files

Finally, the most flexible approach is to use a volume script, that writes new archive name to the file descriptor $TAR_FD. For example, the following volume script will create a series of archive files, named ‘archive-vol’, where archive is the name of the archive being created (as given by ‘--file’ option) and vol is the ordinal number of the archive being created:

#! /bin/bash
# For this script it's advisable to use a shell, such as Bash,
# that supports a TAR_FD value greater than 9.

echo Preparing volume $TAR_VOLUME of $TAR_ARCHIVE.

name=`expr $TAR_ARCHIVE : '\(.*\)-.*'`
case $TAR_SUBCOMMAND in
-c)       ;;
-d|-x|-t) test -r ${name:-$TAR_ARCHIVE}-$TAR_VOLUME || exit 1
          ;;
*)        exit 1
esac

echo ${name:-$TAR_ARCHIVE}-$TAR_VOLUME >&$TAR_FD

The same script can be used while listing, comparing or extracting from the created archive. For example:

# Create a multi-volume archive:
$ tar -c -L1024 -f archive.tar -F new-volume .
# Extract from the created archive:
$ tar -x -f archive.tar -F new-volume .

Notice, that the first command had to use ‘-L’ option, since otherwise GNU tar will end up writing everything to file ‘archive.tar’.

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’, without ‘--multi-volume’ specified. To extract an archive member from one volume (assuming it is described that volume), use ‘--extract’, again without ‘--multi-volume’.

If an archive member is split across volumes (i.e., its entry begins on one volume of the media and ends on another), you need to specify ‘--multi-volume’ 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.

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’ (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’ again in conjunction with the ‘--append’, ‘--update’ or ‘--concatenate’ operation.

Notice that multi-volume support is a GNU extension and the archives created in this mode should be read only using GNU tar. If you absolutely have to process such archives using a third-party tar implementation, read Extracting Members Split Between Volumes.

9.6.2 Tape Files

To give the archive a name which will be recorded in it, use the ‘--label=volume-label’ (‘-V volume-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’. If you are creating a multi-volume archive with ‘--multi-volume’ (see section 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=volume-label’ option when reading an archive, it checks to make sure the label on the tape matches the one you gave. 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 The mt Utility.

--label="some-prefix `date +some-format`"

9.6.3 Concatenate Volumes into a Single Archive

Sometimes it is necessary to convert existing GNU tar multi-volume archive to a single tar archive. Simply concatenating all volumes into one will not work, since each volume carries an additional information at the beginning. GNU tar is shipped with the shell script tarcat designed for this purpose.

The script takes a list of files comprising a multi-volume archive and creates the resulting archive at the standard output. For example:

tarcat vol.1 vol.2 vol.3 | tar tf -

The script implements a simple heuristics to determine the format of the first volume file and to decide how to process the rest of the files. However, it makes no attempt to verify whether the files are given in order or even if they are valid tar archives. It uses dd and does not filter its standard error, so you will usually see lots of spurious messages.

9.7 Including a Label in the Archive

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(29) in conjunction with the ‘--create’ 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. See section Using Multiple Tapes, for information on creating multiple volume archives.

The volume label will be displayed by ‘--list’ along with the file contents. If verbose display is requested, it will also be explicitly marked as in the example below:

$ tar --verbose --list --file=iamanarchive
V--------- 0/0               0 1992-03-07 12:01 iamalabel--Volume Header--
-rw-r--r-- ringo/user       40 1990-05-21 13:30 iamafilename

However, ‘--list’ option will cause listing entire contents of the archive, which may be undesirable (for example, if the archive is stored on a tape). You can request checking only the volume label by specifying ‘--test-label’ option. This option reads only the first block of an archive, so it can be used with slow storage devices. For example:

$ tar --test-label --file=iamanarchive
iamalabel

If ‘--test-label’ is used with one or more command line arguments, tar compares the volume label with each argument. It exits with code 0 if a match is found, and with code 1 otherwise(30). No output is displayed, unless you also used the ‘--verbose’ option. For example:

$ tar --test-label --file=iamanarchive 'iamalabel'
⇒ 0
$ tar --test-label --file=iamanarchive 'alabel'
⇒ 1

When used with the ‘--verbose’ option, tar prints the actual volume label (if any), and a verbose diagnostics in case of a mismatch:

$ tar --test-label --verbose --file=iamanarchive 'iamalabel'
iamalabel
⇒ 0
$ tar --test-label --verbose --file=iamanarchive 'alabel'
iamalabel
tar: Archive label mismatch
⇒ 1

If you request any operation, other than ‘--create’, along with using ‘--label’ option, tar will first check if the archive label matches the one specified and will refuse to proceed if it does not. Use this as a safety precaution to avoid accidentally overwriting existing archives. For example, if you wish to add files to ‘archive’, presumably labeled with string ‘My volume’, you will get:

$ tar -rf archive --label 'My volume' .
tar: Archive not labeled to match 'My volume'

in case its label does not match. This will work even if ‘archive’ is not labeled at all.

Similarly, tar will refuse to list or extract the archive if its label doesn’t match the archive-label specified. 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(31). 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.

You can also use ‘--label’ to get a common information on all tapes of a series. 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 -cM -f /dev/tape -V "Daily backup for `date +%Y-%m-%d`"
$ tar --create --file=/dev/tape --multi-volume \
     --label="Daily backup for `date +%Y-%m-%d`"

9.8 Verifying Data as It is Stored

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’ operation. When this option is specified, tar checks archive members against their counterparts in the file system, and reports discrepancies on the standard error.

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 explicitly compare an already made archive with the file system by using the ‘--compare’ (‘--diff’, ‘-d’) option, instead of using the more automatic ‘--verify’ option. See section Comparing Archive Members with the File System.

Note that these two options have a slightly different intent. The ‘--compare’ option checks how identical are the logical contents of some archive with what is on your disks, while the ‘--verify’ option is really for checking if the physical contents agree and if the recording media itself is of dependable quality. So, for the ‘--verify’ operation, tar tries to defeat all in-memory cache pertaining to the archive, while it lets the speed optimization undisturbed for the ‘--compare’ option. If you nevertheless use ‘--compare’ 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’ 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.

The ‘--verify’ (‘-W’) option will not work in conjunction with the ‘--multi-volume’ (‘-M’) option or the ‘--append’ (‘-r’), ‘--update’ (‘-u’) and ‘--delete’ operations. See section The Five Advanced tar Operations, for more information on these operations.

Also, since tar normally strips leading ‘/’ from file names (see section Absolute File Names), a command like ‘tar --verify -cf /tmp/foo.tar /etc’ will work as desired only if the working directory is ‘/’, as tar uses the archive’s relative member names (e.g., ‘etc/motd’) when verifying the archive.

9.9 Write Protection

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 accidentally 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.

9.1 Device Selection and Switching		Device selection and switching
9.2 Remote Tape Server
9.3 Some Common Problems and their Solutions
9.4 Blocking
9.5 Many Archives on One Tape		Many archives on one tape
9.6 Using Multiple Tapes
9.7 Including a Label in the Archive
9.8 Verifying Data as It is Stored
9.9 Write Protection

9 Tapes and Other Archive Media