RAIT Device

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.

RAIT can also be used to mirror a backup to two drives. A 2-drive RAIT duplicates the output stream and each output stream can have different drivers, like a real tape, a virtual tape (see VFS Device) or a mix of them.

Currently, only 2, 3 and 5 drive sets are supported.

Using a RAIT Device

If you have several tape devices on your system you tell Amanda to use them as a RAIT by listing them as a list of single tape devices, between curly braces, separated with a comma (the string needs curly braces, otherwise it is treated as a single device name):

tapedev "rait:{DEVICE1,DEVICE2,..}"
tapedev "rait:{tape:/dev/nst0, tape:/dev/nst1,tape:/dev/nst2}"

The curly-braces can be used only the differences in the strings like:

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

which means that tape:/dev/rmt/tps0d4n, tape:/dev/rmt/tps0d5n, and tape:/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 tapetype 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, with capacity and speed times N-1:

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.

How To

For instructions and suggestions for using the RAIT device, see How To:Set Up RAIT (Redundant Array of Independent Tapes).