Mark Campbell wrote at about 10:53:04 -0700 on Tuesday, June 2, 2015:
 > Apologies if I'm hijacking this thread, but there's two things I don't 
 > understand...  
 > 
 > Firstly, what is the difference between running nightly manually vs it being 
 > run by the daemon?  It performs the same tasks either way, no?  So, 
 > shouldn't the inverse be true as well?  In other words, if it's not safe to 
 > run nightly manually, wouldn't it also be unsafe being run by the daemon?  
 > What makes it safe for the daemon to run it?

Because there is a (very small) chance of collisions if BackupPC_link and
BackupPC_nightly run concurrently...
BackupPC is beautifully constructed so that almost all operations are
atomic and non-interfering.
However, an error could occur if BackupPC_nightly deletes or chain
renumbers a pool file that BackupPC_link is simultaneously trying to link
to.

The daemon makes sure that BackupPC_nightly won't run if BackupPC_link
is running. Conversely, the daemon holds off starting new
BackupPC_link processes while BackupPC_nightly is running.

The beauty of the architecture is that BackupPC_backup does not
interfere and can continue to create backups. Only the final linking
to pool entries needs to be deferred.

Thus, BackupPC_nightly is only safe to run manually if you have shut down the
BackupPC service (daemon).

 > 
 > Secondly, if it's always been a terrible idea to do so, how did the idea get 
 > started to run it manually in the first place, and why is it so pervasive?

People do a lot of stupid things pervasively. Like pulling the power
cord on a PC before shutting it down gracefully...

BackupPC_nightly was never meant to be run by users. It's merely an
executable sitting inside the BackupPC bin directory. Sometimes users
think they are smarter than they really are and just start running
programs that they think will solve their problems... only to risk
creating worse problems... as evidenced by this user who figured that
if BackupPC_nightly did not automatically free up the amount of disk
space he wanted, then perhaps running it manually multiple times would
convince the program to try harder and find heretofore unfreeable
space. Sort of liking praying repeatedly if your initial prayer is not
answered...

------------------------------------------------------------------------------
_______________________________________________
BackupPC-users mailing list
BackupPC-users AT lists.sourceforge DOT net
List:    https://lists.sourceforge.net/lists/listinfo/backuppc-users
Wiki:    http://backuppc.wiki.sourceforge.net
Project: http://backuppc.sourceforge.net/