> I don't see the issue here.
> - New files are created only when a new file is added to the
>   pool. Since this happens coincident with the need for a new database
>   entry, these two operations can be synchronized

Unless there's a database problem.  Or the executable crashes.  Or a
programming bug.  Or a database bug.  Or the database runs out of space. 
Or the database crashes.  Or somebody runs any number of processes on the
database that can interfere with inserts.  (I can be more specific with
any given database.)

> - Files are deleted or moved (i.e. renamed) only as part of
>   BackupPC_Nightly. Since this happens only once a day the database
>   can be locked appropriately when this process is running to make
>   sure that no files are deleted or renamed without being checked
>   against or synchronized with the database

And this works, until the database is out of sync, and now work needs to
be done to recover database orphans before files can be deleted...

> - Potential race conditions might exist if multiple copies of
>   BackuPC_dump are running but these would be caught first at the
>   database level where collisions can be prevented within the database
>   itself assuming that file creation is made to follow database entry.

This is kind of in left field, a database really isn't necessary to
prevent what I guess you're calling "race conditions," and since separate
processes can insert simultaneously in virtually every database, this
doesn't actually solve the problem without explicit locking.

> - Otherwise, I don't see how BackupPC could possibly change files out
>   of synch with the database except for external events such as
>   crashes, disk errors, or malicious intervention - but all of these
>   apply also to the existing BackupPC implementation.

There's a yawning gap between the ability to envision what can go wrong
and what can actually go wrong.  Database/file hybrid systems aren't
exactly untrodden ground, and the phenonena of potential desynchronization
requires substantial code and effort to overcome, and even then it's
problematic if the data isn't wholly redundant between the two systems.

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