Taper scan algorithm
The Taper scan algorithm is the algorithm used to determine which volume, if any, is the next to be written by taper. It applies to a specific amanda configuration, and parameter values are taken from that amanda.conf file. See amanda.conf(5) for individual parameter definitions.
Overview
Those tapes (or volumes) that have been labeled with amlabel and whose labels match the expression in the parameter “labelstr” may be used by this configuration. In addition, if the parameter “label_new_tapes” has been set, any unlabeled tapes (OR non amanda tapes!) may also be used and will be labeled when used.
Among those tapes that may be used by this configuration, those that have already been used, and are among the most recent “tapecycle”-1 to have been used, are considered active. Any tapes that have been used less recently are considered inactive and are elegable to be used again.
The Taper scan algorithm will first select any available labeled but unused tape (or unlabeled tape if “label_new_tapes” has been set). If the algorithm finds no unused tapes, it will select the least recently used tape next, as long as it is not active. If neither of those choices is available, it will select an inactive, used tape. If none of those choices are available (all available tapes are active), then amanda will go into degraded mode.
This algorithm is applied from scratch any time a new tape is needed during a backup run. You can run the algorithm without running Amanda by doing `amtape <conf> taper`.
Details
Consider the set of tapes T. We can partition the set into two disjoint subsets A (the set of active tapes) and I (the set of inactive tapes). Assuming I is nonempty, there exists a subset P of I, called the set of preferred tapes. Note that T = A ∪ I, P ⊆ I, and A ∩ I = ∅.
Amanda will only use tapes from I; active tapes are not considered for overwriting. Also, tapes from P are preferred to other tapes in I; a tape not in P (but in I) will be used only if no tapes in P are available. If no tapes from I are available, then no tapes are used and Amanda will go into degraded mode.
Tapes are assigned to each of the sets as follows:
- Any labeled but unused tapes are in both I and P. This includes unlabeled tapes if the label_new_tapes option is set.
- The most recently used "tapecycle" - 1 number of tapes is in A.
- Any remaining tapes are in I. The single least recently used of these is also in P.
This algorithm is applied from scratch any time a new tape is needed during a backup run. You can run the algorithm without running Amanda by doing 'amtape taper'.
Proposed Manpage
I am working on a manpage to document the taperscan algorithm, as well as make the algorithm selectable in amanda.conf. Here's a draft. --Dustin 10:27, 12 April 2009 (PDT)
AMANDA-TAPERSCAN(7) Miscellanea AMANDA-TAPERSCAN(7)
NAME
amanda-taperscan - Amanda Taperscan Algorithms
DESCRIPTION
Amanda uses a taperscan algorithm to select volumes on which to store
dumps. Historically Amanda has provided only one, fairly complex
taperscan algorithm, but this algorithm did not suit the needs of all
users. Now Amanda offers a wide array of algorithms to suit varied
needs. Adding new algorithms is not difficult.
The taperscan algorithm is specified with the taperscan parameter,
which has a default value of traditional.
Taperscan algorithms are implemented as perl packages with the prefix
Amanda::Taper::Scan::. See the perl documentation for
Amanda::Taper::Scan for more information.
TAPERSCAN ALGORITHMS
In general, these volumes will only select reusable volumes. These are
volumes which are listed in the tapelist(5) with the reuse flag, and
which are not among the tapecycle-1 most recent volumes in the list.
Put another way, reusable volumes do not contain data that must be
retained. Note that if fewer than tapecycle-1 volumes have been written
then there are no reusable volumes. Newly labeled volumes (volumes that
have been labeled with amlabel but never used) are considered reusable.
Many of these algorithms look for the oldest reusable volume. In most
cases, this is the best volume to overwrite, as the data it contains is
older than that on any other volume. If there are no reusable volumes,
then there is no oldest reusable volume. Newly labeled volumes are not
considered when calculating the oldest reusable volume.
traditional
This algorithm duplicates Amanda's historical behavior, and it operates
in two stages.
First, if there is an oldest reusable volume, the algorithm uses the
changer to search for that volume. Note that, because newly labled
volumes are not included in the calculation of the oldest reusable
volume, this search prefers volumes which have been used before to
newly-labeled volumes.
Second, if there is no oldest reusable volume, or if that volume is not
available in the changer, then the algorithm begins a sequential scan
of the changer, starting at the current slot. It selects the first
suitable volume it finds: a reusable volume (perhaps newly-labeled) or,
if label_new_tapes is true, a blank volume.
Note
This algorithm shows an undue preference for volumes already
containing data, by omitting newly-labeled volumes from its first
stage. Historically, many Amanda changer scripts were not
searchable (including chg-disk and, if havereader=0, chg-zd-mtx),
and thus skipped the first stage. Starting with Amanda 2.6.1, all
changers are searchable, so both stages of the algorithm are used.
If this causes an undesirable change in behavior, consider one of
the other taperscan algorithms..sp .5v
SEE ALSO
amanda(8), amanda.conf(5), tapelist(5), amanda-changers(7)
The Amanda Wiki: : http://wiki.zmanda.com/
AUTHOR
Dustin J. Mitchell <[email protected]>
Zmanda, Inc. (http://www.zmanda.com)
Amanda 2.6.2alpha 04/12/2009 AMANDA-TAPERSCAN(7)