Different types of backups

[John Ramsay_2]

Hi,
Could someone educate me a little about the characturistics about AT&T ,Berkely, best...types of backup schemes? What exactly am I doing by choosing 0m, or 0mn etc.
John

[A. Clay Stephenson · Accepted solution]

This is one of those it depends answers. You really have several things going on at once. First is density, generally 'm' -- shorthand for BEST is the one to use; next rewind (default) or 'n' no rewind. If you are only laying down one backup per tape then the rewind device is your boy; otherwise you use the norewind device node. Finally, and certainly the most confusing, is Berkeley or AT&T style. These choices have to do with how the tape is positioned for no-rewind devices after closing. In almost all cases (and unless you have to read a tape created with AT&T conventions) the Berkeley style is the way to go because it simply leaves the tape where it is.

Man 7 mt will really spell out the details for you.

[Bill Hassell]

These terms are not backup schemes but the behavior of the driver with regard to the device. AT&T and Berkeley behavior is best described in the mt(7) man page. The nomenclature (7) means that the specific manpage is in section 7 or the 7th man page 'brick' (because the printed man pages are brick-sized). The man command has a section option so if you type man mt you'll get mt(1) which is the mt command. Type the command: man 7 mt and you'll get section 7, the documentation section.

Simply put: the device file is contains a major number which is really the driver number, and the minor number contains driver options. The options for tapes include compression (or density) and rewind on close. Now if you want to position the tape to a specific location, the AT&T behavior (typical device files) is *not* easy to use. AT&T behavior is detailed in man 7 mt under Tape Behavioral Characteristics. To move the tape in an inuitive manner, always use a Berkeley device file (as described in the mt(1) man page). You'll need to create Berkeley device files as they don't exist by default (see mksf man page, -u option).

All that is background for a backup solution. All of the Unix legacy tools such as tar, cpio, pax, dump, vxdump, all perform no tape positioning is performed before or after the program runs. That means that the tape's position after a backup will depend on the device file used for the backup. That means that almost all backups should have mt positioning commands in a script to make sure the tape is where it is expected to be. In the good old days (tm), multiple backups would be done onto a single tape and the right device file was a requisite. However, today's critical requirements for data reliability relegates these practices to small data sets.

As a result, modern backup tools such as fbackup, Omniback and other commercial tools always rewind the tape prior to writing (regardless of the tape's device file features). By overriding the device file behavior, the backup will always be one-per-tape and will begin with a complete index. In contrast, there is no index for classic Unix tools other than reading the entire tape.