Jon LaBadie wrote:
> 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
If Amanda's behavior hasn't changed (I'm still using 2.4.5), it will
use the first non-active tape it finds in the changer.  It appears
to just load the next tape until it finds a non-active one, not look
for an unused one or find the oldest.

Frank

-- 
Frank Smith                                      fsmith AT hoovers DOT com
Sr. Systems Administrator                       Voice: 512-374-4673
Hoover's Online                                   Fax: 512-374-4501