-
-Chapter 10. RAIT (Redundant Array of Inexpensive Tape) Support
-Prev Part II. About Tapes and Changers Next
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-
-Chapter 10. RAIT (Redundant Array of Inexpensive Tape) Support
-
-
-Marc Mengel
-
-Original text<mengel@fnal.gov>
-
-Stefan G. Weichinger
-
-XML-conversion;Updates
-AMANDA Core Team
-<sgw@amanda.org>
-Table of Contents
-
-
- What_is_a_RAIT?
-
- Using_a_RAIT
-
- Disaster_Recovery
-
-Currently it is only integrated with the chg-manual changer script;
-collaboration on integrating it with the other tape changers is needed.
-
- What is a RAIT?
-
-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.
-
- Using a RAIT
-
-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.
-
- Disaster Recovery
-
-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.
-
-Note
-
-Refer to http://www.amanda.org/docs/rait.html for the current version of this
-document.
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