Normally I used to have a backup-retention-script in place that will create a TAR-ball of the backup data (using Herakles). But this way I was not able to have a retention of longer then 3 days 
So I had to look into another solution, I could add a new harddrive in the server... but there should be something else possible. So I ended up by using LVM snapshots. So I created a Volume group of about 100GB. In that volume group I created a logical volume of about 30GB, which is enough (and if not, we can 'grow' the Filesystem thanks to LVM 
)
After having all that done, I've created a script located in /root/scripts/lvm-snapshot. This script runs every midnight and creates a snapshot.
#!/bin/bash
#
# Create LVM Snapshots
#
#
#---------------------------------------------------------------------------------------------------------------
CURRENT_SNAPNAME="snap-"$(date "+%Y%m%d%H%M%S")
VOLUME2SNAPSHOT="/dev/vol_backup/lvm0"
LVMSNAPSHOTCMD="/usr/sbin/lvcreate -L 2G -s -n $CURRENT_SNAPNAME $VOLUME2SNAPSHOT"
LINE="---------------------------------------------------------------------------------------------------------------------"
echo $LINE
df -h /mnt/data
echo $LINE
$LVMSNAPSHOTCMD 2> /dev/null
#---------------------------------------------------------------------------------------------------------------
SNAPSHOT_RETENTION=15
CURRENT_SNAPSHOT_COUNT=$(lvdisplay | grep "^ LV Name /dev/vol_backup/snap" | sort | awk '{ print $3 }' | wc -l)
OVERFLOW=$(echo $CURRENT_SNAPSHOT_COUNT - $SNAPSHOT_RETENTION | bc)
if [ $OVERFLOW -gt 0 ];
then
echo $LINE
for files in $(lvdisplay | grep "^ LV Name /dev/vol_backup/snap" | sort | awk '{ print $3 }' | head -n$OVERFLOW);
do
/usr/sbin/lvremove -f $files 2> /dev/null
done
fi
#---------------------------------------------------------------------------------------------------------------
echo $LINE
/usr/sbin/vgdisplay vol_backup
echo $LINE
/usr/sbin/lvdisplay $VOLUME2SNAPSHOT
And the crontab entry is:
# crontab -l
0 0 * /root/scripts/lvm-snapshot
