Re: [BackupPC-users] How does BackupPC work?

[Jeffrey J. K. <bac...@ko...>]

Bowie Bailey wrote at about 12:26:16 -0500 on Wednesday, February 18, 2009:
 > John Goerzen wrote:
 > > Hi,
 > > 
 > > I've been reading docs on BackupPC and I have a few questions about
 > > how it works.
 > > 
 > > First off, I gather that it keeps a hardlinked pool of data, so
 > > whenever a file changes on any host, on the backup device, it will be
 > > hardlinked to a file containing the same data, regardless of the host
 > > it came from, right?
 > 
 > Right.
 > 
 > > So, given that, I don't really understand why there is a distinction
 > > between a full and an incremental backup.  Shouldn't either one take
 > > up the same amount of space?  That is, if you've got few changes on
 > > the client, then on the server you're mostly just hardlinking things
 > > anyway, right?  So why is there a choice?
 > 
 > The result is basically the same, the difference is in implementation.
 > A full backup will compare the contents of every file on the system to
 > see if anything has changed.  An incremental will only check files whose
 > change date has been updated since the last backup.
 > 
 > I believe one of the main incremental backup issues is that they do not
 > detect deleted files.  Incremental backups are also usually MUCH faster
 > than full backups.  For example, one of my backups takes about 600
 > minutes for a full backup, but only 56 minutes for an incremental.

Deleted files are detected. The only issue is if an existing file changes but
the timestamp is pre-dated to before the last backup.

 > 
 > > Secondly, I gather that BackupPC mangles filenames.  That doesn't
 > > bother me, but how is it possible to use rsync in an efficient way
 > > with that?  rsync wouldn't be able to match up client-side filenames
 > > with the server-side names since the server names are different, so it
 > > wouldn't do its efficient transfers.  Either that or you're having to
 > > create temporary directory trees on the server, which sounds
 > > inefficient.  Or am I missing something?
 > 
 > BackupPC uses its own implementation of rsync which knows how to handle
 > the filenames and compression used by BackupPC.

More specifically, it uses a perl library to interface directly with
the rsync protocol.

 > 
 > -- 
 > Bowie
 > 
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