[CentOS] Rsync and differential Backups

Tue Nov 10 12:24:17 UTC 2015
Ole Holm Nielsen <Ole.H.Nielsen at fysik.dtu.dk>

Alessandro Baggi <alessandro.baggi at gmail.com> wrote:
> how to perform a differential backup using rsync?
> On web there is a great confusion about diff backup concept when
> searched with rsync.

I think the answer to this question is Rsnapshot, which is an old and 
well proven tool: http://rsnapshot.org/.  To quote the homepage:
> rsnapshot is a filesystem snapshot utility based on rsync. rsnapshot makes it easy to make periodic snapshots of local machines, and remote machines over ssh. The code makes extensive use of hard links whenever possible, to greatly reduce the disk space required.

It's very easy to set up Rsnapshot with a simple configuration file. You 
can define hourly, daily, weekly and monthly backups.  You run Rsnapshot 
from crontab and it's all automatic.

We run Rsnapshot on the backup server which NFS-mounts the source file 
system as read-only (NFS read operations are low overhead).  You may 
also use SSH.

The backup server has XFS filesystems with the inode64 mount option (no 
inode problems :-). Our backup filesystems varies from 1-40 TB in size 
with 100k to 25M inodes. For very large source filesystems you may want 
to use 10 Gbit/s links to speed up the backups.

The only issue I see is with source filesystems containing tens of 
millions of files, since new inodes have to be allocated on the backup 
server to "snapshot" a filesystem.  The time for Rsnapshot to do 
"snapshots" can become substantial, depending on the speed of the 
underlying filesystem.

Ole Holm Nielsen
Department of Physics, Technical University of Denmark,