Linux – How to get nagios to return free diskspace

After you have installed it, I would recommend the following to get a quick head start:

• take a look at the checks that are available to you (usually found in the /usr/local/libexec directory, depending on the distribution) so you can see the wide range of options out of the box
• apply those checks to your servers based on recent issues you have had

This will let you get going right away with results. Once you've got the problem areas covered, begin applying the checks to all known services (http, https, ssl certificate checking, pop3, etc).

For long term trending, give serious consideration to a tool like Cacti. This is great for gathering SNMP info across Unix and Windows boxes (if using windows, make sure that you install the free SNMP Informant) and allowing to see how it changes over a period of time.

not an answer really, but some pointers:

• definitivly take a look at presentation about nagios @ goldman sachs. they faced problems you mention - redundancy, scalability: thousands of hosts, also automated configuration generation.

• i had redundant nagios setup but at much smaller scale - 80 servers, ~1k services in total. one dedicated master server, one slave server pulling configuration from master at regular intervals few times a day. both servers covered monitoring of the same machines, they had health cross-check between each other. i used nagios mostly as framework for invoking custom product specific checks [ bunch of cron jobs executing scripts doing 'artificial flow controls', results ware logged to sql, nrpe plugins ware checking for successful / failed executions of those in last x minutes ]. all worked very nicely.

• your quorum logic sounds good - a bit similar to my 'artificial flows' - basically go on, ipmplement your self ;-]. and have nrpe just check some kind of flag [ or sql db with timestamp-status ] how things are doing.

• you'll probably want to build some hierarchy to scale - you'll have some nodes that gather overview of other nodes, do look at presentation from first point. default nagios forking for every single check is overkill at higher number of monitored services.

to answer some questions:

• in my case environment monitored was typical master-slave setup [ primary sql or app server + hot standby ], no master-master.
• my setup involved 'human filtering factor' - resolver group who was a 'backup' for sms notification. there was already paid group of technicians who for other reasons had 24/5 shifts, they got 'checking nagios mails' as additional task not putting too much load on them. and they ware in charge of making sure that db-admins / it-ops / app-admins ware actually getting up and fixing problems ;-]
• i've heard lot's of good things about zabbix - for alerting and plotting trends, but never used it. for me munin does the trick, i have hacked simple nagios plugin checking if there is 'any red' [ critical ] color on munin list of servers - just an additional check. you can as well read values from munin rrd-files to decrease number of queries you send to monitored machine.