RAIT is an acronym for "Redundant Array of Inexpensive Tapes", where data is striped over several tape drives, with one drive writing an exclusive-or-sum of the others which can be used for error recovery. Any one of the data streams can be lost, and the data can still be recovered.

This means that a 3-drive RAIT set will write 2 "data" streams and one "parity" stream, and give you twice the capacity, twice the throughput, and the square of the failure rate (i.e. a 1/100 failure rate becomes 1/10,000, since a double-tape failure is required to lose data).

Similarly, a 5-drive RAIT set will give you 4 times the capacity, 4 times the throughput (with sufficient bus bandwidth), and the square of the failure rate.

This means you can back up partitions as large as four times your tape size with Amanda, with higher reliability and speed.

If you have several tape devices on your system [currently either 3 or 5 drive sets are supported] you tell Amanda to use them as a RAIT by listing them as a single tape device using /bin/csh curly-brace-and-comma notation, as in:

	tapedev = "rait:/dev/rmt/tps0d{4,5,6}n"
		

which means that /dev/rmt/tps0d4n, /dev/rmt/tps0d5n, and /dev/rmt/tps0d6n are to be treated as a RAIT set. You can now mount three tapes, and label them with amlabel, etc.

Also, you want to create a new tape-type entry, which lists an n-drive RAIT set, for this RAIT-set. So if you were using an entry like:

	define tapetype EXB-8500 {
	    comment "Exabyte EXB-8500 drive on decent machine"
	    length 4200 mbytes
	    filemark 48 kbytes
	    speed 474 kbytes
	}
		

You would want to make a new one like:

	define tapetype EXB-8500x3 {
	    comment "Exabyte EXB-8500 3 drive stripe on decent machine"
	    length 8400 mbytes
	    filemark 200 kbytes
	    speed 948 kbytes
	}
		

and change your tapetype entry to:

	tapetype EXB-8500x3
		

to tell Amanda about the multiple drive set.

To assist in disaster recovery (as well as changer scripts) the Amanda package now also includes amdd, which is a simple dd(1) replacement which supports (only) the "if=xxx", "of=xxx", "bs=nnn[kMb]" "skip=nnn" and "count=nnn" options, but which can read and write RAIT tapesets.

Using amdd and your usual Amanda unpack instructions will suffice for disaster recovery from RAIT tape-sets.