Hi,
backuppc AT kosowsky DOT org wrote on 2015-06-02 20:03:04 -0400 [Re:
[BackupPC-users] BackupPC Archive Failing]:
> Mark Campbell wrote at about 10:53:04 -0700 on Tuesday, June 2, 2015:
> > [...]
> > 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...
I don't want to imagine what could happen if BackupPC decided to start a
second instance of BackupPC_nightly (for the same part of the pool).
> 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.
I would just like to add that this is dependent on the version of BackupPC. In
earlier versions, the conflict used to be between BackupPC_nightly and
BackupPC_dump (and, possibly, BackupPC_link). In future versions it might be
yet different. The point being that the daemon is programmed to do the right
thing for the version of BackupPC it belongs to. As Jeffrey said:
> BackupPC_nightly was never meant to be run by users.
So it may not always be clearly documented when it would be safe to do so,
just like it is not clearly documented how you should manipulate attrib files
or the on-disk metadata of your favorite file system. You just shouldn't.
> > 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...
You mean, they shut it down gracefully *after* having pulled the plug? :-)
Regards,
Holger
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