Some info about tape backups
From: Craig Anderson
The following supplements the information on rmt devices in
InfoExplorer. It is based on my own personal experience with IBM tape
drives running on AIX 3.1. No warranty is expressed or implied.
CONFIGURING THROUGH SMIT:
BLOCK size (0=variable length) (ALL)
Sets the tape block size. When reading, the block size must be
set to the block size set when the tape was written. When
using some commands, tapes written with ANY block size can be
read if the block size is set to 0 (variable length) (see
"BLOCK SIZES" below).
Use DEVICE BUFFERS during writes (ALL)
Set to yes, the device will buffer data internally on writes.
This greatly improves performance, but under certain cases may
be undesirable since the data is not written to tape before
returning a good indication.
Use EXTENDED file marks (8mm only)
Extended file marks take up much more space than short (or
non-extended) file marks. But extended file marks can be
overwritten, allowing data not at the beginning of tape to be
overwritten (see "FILE MARKS" below).
RETENSION on tape change or reset (1/4" only)
If set to "no" then the tape will not be retentioned
automatically when the tape is inserted. Note that this will
take effect only after the device is used.
Tape devices support multiple tape files. Tape files are the
result of a backup/cpio/tar/dd type command, where the device is
opened, written to, and closed. Because tapes allow large
quantities of data to be written on a single tape, several backups
(that is, tape files), may be combined on one physical tape.
Between each tape file is a "tape file mark" or simply "file
mark". These file marks are used by the device driver to indicate
where one tape file ends and another begins.