Backups using rdiff-backup and rsnapshot


Using rsnapshot

The idea behind rsnapshot is clear and effective – snapshots of the data are created at specified times. Although there are no increments in rsnapshot, data is not simply copied across snapshots. Instead, the remaining files are hard linked. Hard links are a kind of reference that points to the filesystem's inode. Each file may have several such references, but it only occupies the storage space once.

In rsnapshot, you can check the use of hard links yourself as in Listing 3. The /etc/hosts file remained unchanged in the two snapshots: hourly.2 and hourly.1. They therefore share inode 28147. A new inode was allocated to the file in the latest backup hourly.0 because it had changed. For the data transfer itself, rsnapshot uses rsync, which synchronizes changes from A to B efficiently with its delta copy mechanism [2].

Listing 3

rsnapshot Hard Links

# ls -li hourly.*/localhost/etc/hosts
28589 -rw-r--r-- 1 root root 209 Mar 13 11:30 hourly.0/localhost/etc/hosts
28147 -rw-r--r-- 2 root root 186 Jul 10 2014 hourly.1/localhost/etc/hosts
28147 -rw-r--r-- 2 root root 186 Jul 10 2014 hourly.2/localhost/etc/hosts


The main configuration file in rsnapshot, /etc/rsnapshot.conf, has many configuration options – initially, you can limit the focus to the most important settings (Table 2).

Table 2

Important rsnapshot Backup Options

Option Purpose
snapshot_root Root directory for storing snapshots.
no_create_root If this option is 1, rsnapshot doesn't automatically create the root directory. This is useful, for example, if mounting snapshot_root failed.
retain The retain lines control which type of snapshot is retained for how long.
logfile Listed commands are logged in logfile.
backup The backup lines define the directories to be backed up. It is also possible to use a remote path (accessible via SSH).
cmd_ssh Path to the SSH program.

Parameters in the configuration are always separated by tab characters. You are best off running a configtest, as follows:

# rsnapshot configtest
 Syntax OK

to make sure you have set everything up correctly.

Regular Snapshots

The following example shows a rsnapshot configuration for backing up the /etc directory at hourly, daily, and weekly intervals:

retain    hourly       6
retain    daily        7
retain    weekly       4
backup    /etc/        localhost/

Using these settings, you still need to make sure rsnapshot is called regularly, because rsnapshot synchronizes and rotates data but doesn't run it regularly.

The cron service, which arranges continuous backups, is in charge of this task:

# vi /etc/cron.d/rsnapshot
0 */4 * * * root /usr/bin/rsnapshot hourly
30 3 * * * root /usr/bin/rsnapshot daily
0 3 * * 1 root /usr/bin/rsnapshot weekly

The entries shown here make sure that an "hourly backup" is performed every four hours (which means six snapshots are kept), a daily backup runs every day at 3:30am (seven snaphots are kept), and a weekly backup starts at 3:00am every Monday (four snapshots are available).

Regularly running backups using cron is equally suitable for rdiff-backup: just create a cron job that regularly ensures the rdiff-backup call.

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