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.

> 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.

-- 
Bowie

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