dar_split(1) dar_split(1)
NAME
dar_split - dar helper to split an archive over several tapes
SYNOPSIS
dar_split { split_input | [-s] split_output } <filename>
dar_split -v
dar_split -h
DESCRIPTION
dar_split is to be used with dar to read or write a large archive from
or to several tapes.
COMMANDS
dar_split has two modes of operation: split_input and split_output
split_input in this mode dar_split copies <filename> to its
standard output. <filename> may be a special device
like /dev/tape or any other inode that has the
ability to access removeable volumes. When
dar_split reaches the end of <filename> it suspends
and waits for the user to press the return key in
order to continue. In the meanwhile the user can
rewind and change the tape. When dar_split is
awaken again, it reopens <filename> and sends its
content to its standard output as if it was the
continuation of what was read so far before reach-
ing the end of file.
split_output in this mode dar_split copies its standard input to
<filename>. However if <filename> is full,
dar_split suspends and waits for the user to press
the return key in order to continue. In the mean-
while, the user can rewind and change the tape.
When dar_split is awaken again, it reopens <file-
name> and continues to copy to <filename> what was
not yet written previously.
OPTIONS
-s is only available for split_output mode an leads dar_split to per-
form sync writes in place of normal writes. This has the drawback to
drastically reduce performances but may be necessary under certain cir-
cumstances where the operating system cache reports a write operation
as successful while the cache underlying medium is already full.
EXAMPLES OF USE
creating an archive over several tapes
dar -c - ...possibily other dar options... | dar_split
split_output /dev/tape
Here dar generates the archive to its standard output which is read by
dar_split and copied to /dev/tape. When the tape reaches its send,
dar_split asks for the user to hit return to continue.
reading an archive splitted over several tapes
dar_split split_input /dev/tape | dar -t - --sequential-read
...possibily other dar options...
Here dar_split reads /dev/tape and sends its content to dar which
*must* be used in --senquential-read mode in order to read an archive
from its standard input without the help of dar_slave. When the tape
reaches its end, dar_split asks for the user to hit return to continue.
Note that dar_split can also be used with tar for the same purpose as
with dar:
tar -cz ... | dar_split split_output /dev/tape
without -f option nor TAPE environment variable, tar sends the tar ar-
chive to stdout, which get fet to dar_split for slicing
dar_split split_input /dev/tape | tar -tz
without -f option nor TAPE environement variable, tar read the tar ar-
chive from stdin, however you must specify the correct option corre-
sponding to the compression used at archive creation time.
The advantage here is to be able to save a huge dar or tar archive on
several tapes that could be either partially filled or having different
sizes, without having to calculate in advance the size of slices to
specify. The drawback is that this implies sequential reading for dar
(for tar also, but tar cannot do else).
With or without filesystem
You can use dar_split with floppies, zip/jazz disks or usb key too, but
it has many disadvantages compared to the normal way of using this
media: doing that way avoids you relying on a filesystem, thus you can-
not have direct access to file contents, which dar knows how to take
advantage of, and you are instead stuck with sequential read access
which is long as it requires reading the whole archive (same way as tar
does).
When instead using dar alone in normal mode (non sequential reading
mode using filenames instead of standard input/output), you have the
opportunity to create redundancy data beside dar slices thanks to par2,
which is not possible without a filesystem. At restoration time, thanks
to dar archive's catalog, dar can directly seek to the file's data you
want to restore, which is very quick compared to reading/uncipher-
ing/uncompressing the whole archive...
Note that copying tape contents generated by dar_split to different
files which name would correspond to dar slices, does not make a valid
multi-sliced archive because the slice header is missing in slices
(except in the first). You can however concatenate all the pieces of
data generated by dar_split and have a valid single sliced dar archive
that you can use in direct access mode. However attention must be paied
not to add any extra data after data generated by dar through dar_split
(in particular if the last tape was not full), in that case you can
only read the archive with --sequential-read mode.
EXIT CODES
dar_split exists with the following codes:
0 upon normal execution
1 syntax error on command-line
2 could not open source or destination files
SIGNALS
Any signal sent to dar_split except SIG_PIPE will abort the program
immediately
SEE ALSO
dar(1), dar_xform(1), dar_manager(1), dar_slave(1), dar_cp(1)
KNOWN BUGS
http://sourceforge.net/p/dar/bugs/
AUTHOR
http://dar.linux.free.fr/
Denis Corbin
France
Europe
3rd Berkeley Distribution August 18th, 2018 dar_split(1)
dar 2.6.1 - Generated Sat Jan 26 15:22:01 CST 2019