2 Chapter 10. RAIT (Redundant Array of Inexpensive Tape) Support
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7 Chapter 10. RAIT (Redundant Array of Inexpensive Tape) Support
12 Original text<mengel@fnal.gov>
16 XML-conversion;Updates
28 Currently it is only integrated with the chg-manual changer script;
29 collaboration on integrating it with the other tape changers is needed.
33 RAIT is an acronym for "Redundant Array of Inexpensive Tapes", where data is
34 striped over several tape drives, with one drive writing an exclusive-or-sum of
35 the others which can be used for error recovery. Any one of the data streams
36 can be lost, and the data can still be recovered.
37 This means that a 3-drive RAIT set will write 2 "data" streams and one "parity"
38 stream, and give you twice the capacity, twice the throughput, and the square
39 of the failure rate (i.e. a 1/100 failure rate becomes 1/10,000, since a
40 double-tape failure is required to lose data).
41 Similarly, a 5-drive RAIT set will give you 4 times the capacity, 4 times the
42 throughput (with sufficient bus bandwidth), and the square of the failure rate.
43 This means you can back up partitions as large as four times your tape size
44 with Amanda, with higher reliability and speed.
48 If you have several tape devices on your system [currently either 3 or 5 drive
49 sets are supported] you tell Amanda to use them as a RAIT by listing them as a
50 single tape device using /bin/csh curly-brace-and-comma notation, as in:
52 tapedev = "rait:/dev/rmt/tps0d{4,5,6}n"
55 which means that /dev/rmt/tps0d4n, /dev/rmt/tps0d5n, and /dev/rmt/tps0d6n are
56 to be treated as a RAIT set. You can now mount three tapes, and label them with
58 Also, you want to create a new tape-type entry, which lists an n-drive RAIT
59 set, for this RAIT-set. So if you were using an entry like:
61 define tapetype EXB-8500 {
62 comment "Exabyte EXB-8500 drive on decent machine"
69 You would want to make a new one like:
72 define tapetype EXB-8500x3 {
73 comment "Exabyte EXB-8500 3 drive stripe on decent machine"
80 and change your tapetype entry to:
85 to tell Amanda about the multiple drive set.
89 To assist in disaster recovery (as well as changer scripts) the Amanda package
90 now also includes amdd, which is a simple dd(1) replacement which supports
91 (only) the "if=xxx", "of=xxx", "bs=nnn[kMb]" "skip=nnn" and "count=nnn"
92 options, but which can read and write RAIT tapesets.
93 Using amdd and your usual Amanda unpack instructions will suffice for disaster
94 recovery from RAIT tape-sets.
98 Refer to http://www.amanda.org/docs/rait.html for the current version of this
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