= Integration

== Monitoring your disk layout with Relax-and-Recover (ReaR)

A crucial part to properly recreate the system is the disk layout,
i.e. the disk partitioning with filesystems and mount points.

When you use "rear mkbackup" to create the ReaR rescue/recovery system
together with a backup of all files and directories of your system
at the same time, you have both parts (recovery system and backup) in sync
which is a prerequirement to properly recreate the system with "rear recover".

But when you use "rear mkrescue" to create the ReaR rescue/recovery system
and call "rear mkbackuponly" separatedly to create the backup
or you call a third party backup tool separatedly,
you may not have both parts (recovery system and backup) in sync
so "rear recover" may not properly recreate the system
or it may even completely fail to do it.

In particular when the disk layout had changed the ReaR rescue/recovery system
must be created anew.

Caution:
There are zillions of other ways how the latest created ReaR rescue/recovery system
could become outdated or even invalid/useless in general.
Each change of the basic system setup (like disk layout, network environment,...) and
each change of a software that is used by ReaR (like 'parted', 'mkfs', 'tar',...) and
of course also each ReaR version upgrade requires that the ReaR rescue/recovery system
gets created anew together with a matching up-to-date backup as prerequirement
so that "rear recover" is able to properly recreate the current system.
Furthermore after such changes you must carefully and completely re-validate
that "rear recover" still works in your particular case/environment.

The disk layout information is stored in var/lib/rear/layout/disklayout.conf

ReaR provides two specific workflows regarding the disk layout:

=== Saving the current disk layout of the system

ReaR automatically saves the current disk layout of the system
when it creates a new ReaR rescue/recovery system.
However if you want to only save the current disk layout manually,
use "rear savelayout" (this does not update the recovery system).

=== Checking if the disk layout has changed

When you want to know if the current disk layout of the system
has changed compared to the latest saved disk layout,
use "rear checklayout".
If the disk layout has changed, "rear checklayout" results a non-zero return code
so you could use something like
----
# rear checklayout || rear mkrescue
----
to create a new ReaR recovery system when the disk layout has changed.

== Integration with Nagios and Opsview

If having current DR rescue images is important to your organization, but they
cannot be automated (eg. a tape or USB device needs inserting), we provide a
Nagios plugin that can send out a notification whenever there is a critical
change to the system that requires updating your rescue environment.

Changes to the system requiring an update are:

 - Changes to hardware RAID
 - Changes to software RAID
 - Changes to partitioning
 - Changes to DRBD configuration
 - Changes to LVM
 - Changes to filesystems

The integration is done using our own _check_rear_ plugin for Nagios.

[source,bash]
----
#!/bin/bash
#
# Purpose: Checks if disaster recovery usb stick is up to date

# Check if ReaR is installed
if [[ ! -x /usr/sbin/rear ]]; then
    echo "REAR IS NOT INSTALLED"
    exit 2
fi

# ReaR disk layout status can be identical or changed
# returncode: 0 = ok
if ! /usr/sbin/rear checklayout; then
    echo "Disk layout has changed. Please insert Disaster Recovery USB stick into system !"
    exit 2
fi
----

We also monitor the _/var/log/rear/rear-system.log_ file for +ERROR:+ and +BUG+
strings, so that in case of problems the operator is notified immediately.

Note that error messages may not come from ReaR itself but from programs that are called by ReaR
because stdout and stderr are redirected into ReaR's log file (see the section
"What to do with stdin, stdout, and stderr" in https://github.com/rear/rear/wiki/Coding-Style)
so in case of error messages one must check if that is actually an error or only false alarm.
