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| 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.
| | See {{man|7|amanda-devices}}. |
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| 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).
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| 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.
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| This means you can back up partitions as large as four times your tape size with Amanda, with higher reliability and speed.
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| 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.
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| Currently, only 2, 3 and 5 drive sets are supported.
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| = Using a RAIT Device =
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| 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):
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| tapedev "rait:{DEVICE1,DEVICE2,..}"
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| tapedev "rait:{tape:/dev/nst0, tape:/dev/nst1,tape:/dev/nst2}"
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| The curly-braces can be used only the differences in the strings like:
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| tapedev "rait: tape:/dev/rmt/tps0d{4,5,6}n"
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| which means that <tt>tape:/dev/rmt/tps0d4n</tt>, <tt>tape:/dev/rmt/tps0d5n</tt>, and <tt>tape:/dev/rmt/tps0d6n</tt> are to be treated as a RAIT set. You can now mount three tapes, and label them with amlabel, etc.
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| 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:
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| define tapetype EXB-8500 {
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| comment "Exabyte EXB-8500 drive on decent machine"
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| length 4200 mbytes
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| filemark 48 kbytes
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| speed 474 kbytes
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| }
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| You would want to make a new one, with capacity and speed times N-1:
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| define tapetype EXB-8500x3 {
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| comment "Exabyte EXB-8500 3 drive stripe on decent machine"
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| length 8400 mbytes
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| filemark 200 kbytes
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| speed 948 kbytes
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| }
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| and change your tapetype entry to "<tt>tapetype EXB-8500x3</tt>" to tell Amanda about the multiple drive set.
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| = How To =
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| For instructions and suggestions for using the RAIT device, see [[How To:Set Up RAIT (Redundant Array of Independent Tapes)]].
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