Holger Parplies wrote at about 18:57:05 +0100 on Friday, November 1, 2013:
 > 3.) finding "unnecessary duplicates" can have a normal explanation: if at 
 > some
 >     point you had more than 31999 copies of one file (content) in your
 >     backups, BackupPC would have created a pool duplicate. Some of the 
 > backups
 >     linking to the first copy would have expired over time, leaving behind a
 >     link count < 31999. Further rsync backups would tend to link to the 
 > second
 >     copy, at least for unchanging existing files (in full backups). In other
 >     cases, the first copy might be reused, but there's no guarantee the link
 >     count would be exactly 31999 (though it would probably tend to be).

Interesting... I think this depends on the transfer method. The rsync
method looks to the immediately prior full for comparison, so new hard
links will be made to the same chain as the last full. Thus, if
earlier elements in the chain have reduced link count, they will tend
not to be filled back in.

It seems like the other transfer methods directly reference the
PoolWrite package which always crawls up the chain looking for
matches... If true, it does seem that one could in general, speed up
fulls for the other algorithms by putting a matching candidate from
the previous full (if any) first in the candidate match list... rather
than matching them in chain order (or several simultaneously).

In any case, if my quick reading of the code is correct, then the
other methods will tend to fill in earlier chains elements first so
that the link count will march back up to 31999.

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