On Mon, Aug 07, 2006 at 02:37:17PM -0400, Ian Turner wrote:
> Marilyn,
> 
> Amanda's tape selection policy is as follows.
> 
> 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, and P is a weak subset of 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 two sets as follows:
> -- Any labeled but unused tapes are in I and P. This includes unlabeled tapes 
> if the label_new_tapes option is set.
> -- The most recently used "tapecycle" 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'.
> 
> What all of this means from a slightly less mathematical perspective is that 
> Amanda will not consider overwriting the tapecycle most recent tapes. If you 
> want to relax this restruction, just reduce tapecycle, and Amanda will 
> countenance the use of newer (more recently used) tapes.
> 
> Alternatively, if you have a specific tape that you want Amanda to reuse, 
> just 
> relabel it, and it will be treated as a new tape.
> 
> --Ian
> 
> On Monday 07 August 2006 13:10, HUGHES Marilyn F wrote:
> > We have a situation where the next  Amanda tapes that it is asking for
> > are currently offsite.  It costs $75 for them to be retrieved so we
> > don't want to do that.
> >
> >
> > Besides we have available tapes here onsite.  Is there a way (a command
> > or other way?) to force Amanda to select one of the tapes that we have
> > onsite?  Does Amanda select from the top of the tapelist on down?


Ian,

Thanks for your description.  I was thinking of trying to put together
a description of the tape selection algorithm myself.  But I didn't
know some(most?) of the detail.  Certainly not in mathematical sets.

One thing still up in the air (to me anyway) is final tape selection
from within the tapelist and physical tape changer.  Your description
gets to which tapes are "eligible" to be selected, but not which tape
(or runtape number of tapes) among that set is ultimately chosen.

Let me try to use my own terminology to descibe your algorithm so
that I'm sure I understand it.  I'd appreciate corrections to any
mis-statements, in fact or in timing.  And, perhaps you could extend
it by describing the final tape selection.

>From the entire tapelist, those marked "no-reuse" are eliminated
from further consideration.  Only those marked "reuse" are considered.
(is there any other tapelist classification?)

The reuse"able" list is divided into "previously used" and "never used"
(have a valid date stamp or have a 0 date stamp respectively).

The previously used tapes are date-sorted and the most recently used
tapecycle-1 of those are eliminated from further consideration.  This
would be your "active" set that are reusable but can't be overwritten
at this time as they fall within a tapecycle's number of tapes.

Any remaining, previously used tapes, plus the labeled but never used
tapes (if any), constitute the set of tapes eligible to be used for
the next run.

If what I've described is reasonably accurate, none of it is dependent
on tapelist order nor availability in the tape changer.  So the physical
device must be accessed before the final selection of a tape to use.

Does amanda at this point look specifically for the next tape based on
the order in the tapelist file?  (i.e. the last tape in the tapelist
file that is on the eligible list)  Will it scan the entire changer
looking for that specific tape?  Or will it start to scan the changer
looking for any tape from the eligible list?  Or something else?

Thanks.

jl
-- 
Jon H. LaBadie                  jon AT jgcomp DOT com
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