Holger Parplies wrote at about 01:46:03 +0100 on Thursday, December 11, 2014:
 > Hi,
 > 
 > Timothy J Massey wrote on 2014-12-10 11:53:38 -0500 [Re: [BackupPC-users] 
 > How to delete backups]:
 > > Holger Parplies <wbppc AT parplies DOT de> wrote on 12/10/2014 10:59:17 AM:
 > > > [...]
 > > > ***Never*** run BackupPC_nightly from the command line. ***Never*** 
 > > > advise
 > > > other to do so.
 > > > Really quite simple.
 > > 
 > > Why not, exactly?  I do it all the time.  I'm not saying you're wrong:  I 
 > > just want to know where the harm might be.  But you state the "never do 
 > > that" in such strong terms, I'm wondering where the disconnect it.
 > 
 > BackupPC_link and BackupPC_nightly shouldn't run concurrently because of
 > race conditions in the code. As far as I remember, BackupPC_nightly might
 > remove (or chain-renumber) a file BackupPC_link is about to create a link
 > to, which would result in the file missing from the backup or containing
 > incorrect data. The odds of this happening might not be high, but why take
 > the risk? There might be further problems I am unaware of. 

I believe you nailed it.

BackupPC is constructed so that only BackupPC_nightly and
BackupPC_link change pool entries by adding/deleting/renumbering chain
entries. Hence, the scheduler only needs to make sure that
BackupPC_nightly and BackupPC_link don't runs simultaneously... though
of course as you pointed out the risk on any given run is rather
small...

Note the second rsync run may change the content of an existing pool
entry by adding the rsync checksums but that doesn't create a conflict
since the name and actual uncompressed content of the pool entry
doesn't change.


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