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@lists.sourceforge.net List: https://lists.sourceforge.net/lists/listinfo/backuppc-users Wiki: http://backuppc.wiki.sourceforge.net Project: http://backuppc.sourceforge.net/
