
Chapter 10. RAIT (Redundant Array of Inexpensive Tape) Support
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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


Note

Refer to http://www.amanda.org/docs/rait.html for the current version of this
document.
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.
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