On 10/29/2010 8:16 AM, Richard Shaw wrote:
> There is surprisingly little info on how BackupPC really works, at
> least with the google searches I've tried. I'm just looking for a
> concise overview of how the different backup methods work and how they
> are different from one another.
>
> The reason I'm looking for this information is I will be giving a
> presentation of BackupPC to my local LUG.
>
> I'm looking for something as close as I can get to:
>
> Rsync:
> 1. BackupPC does x.
> 2. Then this.
> 3. Then this.
> 4. etc.
>
> Rsyncd:
> 1. Same as rsync except x.
>
> Tar:
> ...
>
> Smb:
> ...
>
> and so forth.

First backuppc uses a native tool to transfer files.  Then it checks the 
file contents against the pool with a hashing mechanism, and replaces 
any exact matches with a link to the existing pool copy.

> For instance: I can understand how tar over SSH transfers files. But
> who decides what files to transfer?

Your file include or exclude lists are mapped into the options 
appropriate for the xfer program with the 'share' as the top of the tree.

> Does BackupPC crawl the share
> first and build a filelist? Or is there an option for tar that takes
> care of if for BackupPC?

With tar and smb, backuppc doesn't know much about the remote side - it 
just passes the options to the program.  Tar runs over ssh entirely on 
the remote side.

> For SMB I understand that it would just look like a local filesystem
> after the share is mapped to the BackupPC server. Then what? Does it
> use rsync against the network share?

The smb method actually uses the smbtar program so it looks more like 
tar than a mapped file system.

The rsync method runs a native rsync via ssh on the remote side, using a 
perl implementation on the server.   Rsyncd is similar, but talks to a 
standalone rsync running in daemon mode that must be set up on the 
target.  Rsync sends the entire directory tree you request from the 
remote, then both sides walk the list to find and send differences.

The practical difference is that rsync uses less bandwidth and is better 
at catching every change in incrementals.   The smb and tar methods go 
strictly by file timestamps and will miss moved or copied files that 
keep their old timestamps, where rsync will find them with the directory 
comparison against your previous full tree.

-- 
   Les Mikesell
    lesmikesell AT gmail DOT com



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