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

John Goerzen wrote:
> 
> 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?
> 
> 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?

With the tar and smb backup methods, full runs transfer everything from 
the remote, incrementals transfer only files with timestamps newer than 
the last full.  With rsync, a full does a block checksum compare of all 
files, incrementals only files where the timestamp or length differ.  On 
the server side, fulls rebuild a complete tree of links, incrementals 
only have the differing files.

> 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?

The server doesn't run the stock version of rsync.  It has a perl 
version that understands the filename and compression conventions it 
uses for storage and can work with a stock rsync on the remote side.

-- 
   Les Mikesell
     lesmikesell AT gmail DOT com


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