+++ /dev/null
-RAIT (Redundant Array of Inexpensive Tape) Support
-
-Author: Marc Mengel<mengel@fnal.gov>.
-
-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 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.
-
-To use other than 3 or 5 tape RAIT sets, you need to change the blocksize
-in the tapetype to be divisble by the number of data stripes in your
-RAIT set (i.e. the number of devices minus one).
-
-Blocksizes and Performance:
-
-For better performance, and to get more flexibility with tape stripes,
-you want to build amanda with a very large maxtapeblocksize, something
-like:
- ./configure --with-maxtapeblocksize=640
-Then you can do something like:
- +------------------------------------
- | tapedev = "rait:/dev/rmt/tps0d{4,5,6,7}n"
- ...
- | tapetype DLT7000x4
- ...
- | define tapetype DLT700x4 {
- | comment "Exabyte DLT 4 drive stripe on decent machine"
- | length 115167 mbytes
- | filemark 120 kbytes
- | speed 4482 kbytes
- | blocksize 384 kbytes
- | }
- +------------------------------------
-which will give you 128k physical blocks on the tape; more generally for
-an n-tape stripe, you want (n-1)*32 kbytes for a tape that likes
-32k physical blocks, or (n-1)*128 kbytes for a tape that likes
-bigger blocks.
-
-Hardware Compression:
-the last stripe is the xor stripe, it is likely to compress poorly
-relative to the other data stripes if you use hardware compression.
-
-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.
projects / debian / amanda / blobdiff