(tar)Blocking

Many

Device

Media

Blocking
========

     *(This message will disappear, once this node revised.)*

   When writing to tapes, `tar' writes the contents of the archive in
chunks known as "blocks".  To change the default blocksize, use the
`--block-size=512-SIZE' (`-b 512-SIZE') option.  Each block will then
be composed of SIZE records.  (Each `tar' record is 512 bytes.

FIXME: xref Archive Format
   .)  Each file written to the archive uses at least one full block.
As a result, using a larger block size can result in more wasted space
for small files.  On the other hand, a larger block size can ofter be
read and written much more efficiently.

   Further complicating the problem is that some tape drives ignore the
blocking entirely.  For these, a larger block 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.

   Wher reading an archive, `tar' can usually figure out the block size
on itself.  When this is the case, and a non-standard block 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 block size itself.  On
most of those, you can specify a blocking factor (with
`--block-size=512-SIZE' (`-b 512-SIZE')) larger than the actual
blocking factor, and then use the `--read-full-blocks' (`-B') option.
(If you specify a blocking factor with `--block-size=512-SIZE' (`-b
512-SIZE') and don't use the `--read-full-blocks' (`-B') option, then
`tar' will not attempt to figure out the blocking size itself.)  On
some devices, you must always specify the block size exactly with
`--block-size=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.

   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
block sizes, limited only by the amount of virtual memory or the
physical characteristics of the tape device.

   If you are writing a compressed archive to tape with `--compress'
(`-Z') or `--gzip' (`-z') (
FIXME: pxref Input and Output
   ), `tar' will not block the archive correctly.  This doesn't matter
if you are writing the archive to a normal file or through a pipe, but
if you are writing it to a tape drive, then this causes problems.  Use
`--compress-blocks' or `--gzip-block' instead, to cause `tar' to
arrange to have blocking work correctly.

Format Variations

Blocking Factor