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

On 10/29/2010 12:53 PM, Richard Shaw wrote:
>
>>> 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.
>
> So in effect, BackupPC doesn't do anything directly, but rather
> indirectly by proxy? (not the network meaning, but the literal
> meaning).

Yes, the program selected by the XferMethod is used to access the client.

>> 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.
>
> So with tar/smbtar for a full backup all files are transferred? In
> other words there's no checksum on the remote client to see if it
> already exists?

Yes. Then existing files are discarded on the server and replaced with 
links to the pooled copies.

>> 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.
>
> I assume that the checksuming that rsync does can create quite a CPU
> load on the client which is why on linux clients are often nice'd? (or
> ionice?)

It is more of an io load than CPU since disks are much slower than 
processors.  And rsync incrementals quickly skip files where the 
directory name/length/timestamp match the previous full.  Full runs add 
the --ignore-times option to rsync so all files are read for the 
checksum comparison but only the differences are sent over the network.

-- 
   Les Mikesell
    lesmikesell AT gmail DOT com



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