Invalid performance data from check_disk on CentOS results in empty monitoring graphs

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Today I was stumbling across a very interesting problem: CentOS 7 machines would not show any disk usage graphs in our Grafana dashboard (which uses InfluxDB as data source and Icinga 2 as monitoring system).

Let's activate troubleshooting mode!

0 value shown in Icingaweb's performance data

The first thought was that this particular server may have a problem reporting back performance data. Could it be a buggy NRPE configuration? Or maybe even a bug in check_disk on this particular server?

But by looking at other CentOS 7 machines in Icingaweb 2, another discovery was made: Actually every CentOS 7 machine did not have any disk graphs!

Looking closer in Icingaweb, the value of the disk usage seems to be 0. On every partition. On every CentOS 7 server.

When looking at an Ubuntu Linux server, the values clearly show up:

Where does this 0.00 value on the CentOS machines come from? Is it the plugin check_disk itself? Or is NRPE the reason the data is not correctly landing in Icinga?

Manually running check_disk

By executing the nagios-plugin check_disk on the server, did not seem to trigger any errors. Certainly no 0.00 values, neither in the output nor in the performance data:

[nagios@centos7 ~]# /usr/lib/nagios/plugins/check_disk -w 15% -c 5% -W 10% -K 5% -p /
DISK OK - free space: / 8640 MiB (76.21% inode=95%);| /=2696MiB;10171;11368;0;11967

Even by executing this exact check command through NRPE seemed to work:

root@icinga:~# /usr/lib/nagios/plugins/check_nrpe -H nzz-wwapp01-d.nzzmg.local -c check_disk -a "15%" "5%" "/" "15%" "5%"
DISK OK - free space: / 8640 MiB (76.21% inode=95%);| /=2696MiB;10171;11368;0;11967

So both ways to execute the plugin (locally and via NRPE) seem to work. Maybe Icinga has a problem parsing the performance data?

Invalid perfdata!

A look into the Icinga logs quickly turned up a WARNING entry:

[2021-02-17 06:27:56 +0100] warning/InfluxdbWriter: Ignoring invalid perfdata for checkable 'centos7!Diskspace /' and command 'nrpe' with value: /=2696MiB;10171;11368;0;11967
        (0) Processing check result for 'centos7!Diskspace /'

So obviously Icinga somehow thinks that the received performance data is invalid and refuses to write the values into InfluxDB. But why?

Note: At this point I still didn't see the issue. Even for my trained monitoring eye using 15+ years of nagios/monitoring-plugins. ;-)

A research on the Internet led to a discussion in the Icinga forums and from there to the actual source of the problem: A bug in nagios-plugins!

Issue #518 in the nagios-plugins repositories describes a regression bug introduced by nagios-plugins 2.3.0. In the release notes of 2.3.0 the following change was applied to check_disk:

2.3.0 2019-12-04
Note: nagios-plugins 2.3.0 introduced a backward-compatibility issue for performance data graphs produced by check_icmp.
Anyone upgrading from an older version of nagios-plugins should upgrade directly to >=2.3.1.
    Added a plugin "remove_perfdata" to remove perfdata from another plugin's output
    Added a plugin "check_ssl_validity" which verifies a certificate through CRL or OCSP
    Added directory plugins-python containing three Python plugins
    check_apt: Add --only-critical switch
    check_apt: Add --packages-warning (Marc S├ínchez)
    check_disk: Add --combined-thresholds to alert on a percentage AND raw units.
    check_disk: Add --inode-perfdata to get extra performance data from inode statistics
    check_disk: Add support for base-10 units kB, MB, GB, TB; rename base-2 units to KiB, MiB, GiB, TiB


This change caused the performance output units to show MiB instead of MB. However "MiB" is not a valid UOM (Unit of measurement), even described in the Nagios plugin developer guidelines:

UOM (unit of measurement) is a string of zero or more characters, NOT including numbers, semicolons, or quotes. Some examples:
    no unit specified - assume a number (int or float) of things (eg, users, processes, load averages)
    s - seconds (also us, ms)
    % - percentage
    B - bytes (also KB, MB, TB)
    c - a continous counter (such as bytes transmitted on an interface)

The regression was fixed 3 months later in nagios-plugins 2.3.2:

2.3.2 2020-02-20
    build: Fix broken builds on some systems, including Homebrew (#508)
    check_disk: Change unit calculations to always use binary units for backward compatibility (#518)

It's fixed, all good, right?

Even though the bug was fixed in the upstream project "nagios-plugins" in 2.3.2, the bug is still around in the current nagios-plugins-disk package, installed from the EPEL 7 repositories. Interesting however is the fact that the package version is showing up as 2.3.3:

[root@centos7 ~]# yum info nagios-plugins-disk
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
 * base:
 * epel:
 * extras:
 * updates:
Installed Packages
Name        : nagios-plugins-disk
Arch        : x86_64
Version     : 2.3.3
Release     : 2.el7
Size        : 86 k
Repo        : installed
From repo   : epel
Summary     : Nagios Plugin - check_disk
URL         :
License     : GPLv2+
Description : Provides check_disk support for Nagios.

A bug report was opened in the Fedora EPEL project: Bugzilla 1929568 mentions these findings documented here in this article. Hopefully this bug will shortly be addressed and a new package in the EPEL repositories released.

However further analysis with the latest available upstream release nagios-plugins-2.3.3 reveals that this regression is still happening, although marked as fixed in the changelog:

# ./check_disk -V
check_disk v2.3.3 (nagios-plugins 2.3.3)

# ./check_disk -w 15% -c 5% -W 10% -K 5% -p /
DISK OK - free space: / 8447 MiB (89.03% inode=95%);| /=1040MiB;8512;9514;0;10015

Therefore it's actually still an upstream bug and issue #518 should be re-opened again.

A workaround, please?

Fortunately an immediate workaround is available: By using the -m parameter, the values will be shown in MegaBytes:

[root@centos7 ~]# /usr/lib/nagios/plugins/check_disk -w 15% -c 5% -W 10% -K 5% -p / -m
DISK OK - free space: / 8640 MB (76.21% inode=95%);| /=2696MB;10171;11368;0;1196

This corrects the UOM in the performance data and data can be parsed and written into InfluxDB again.

For NRPE setups, the check command needs to be adjusted accordingly:

[root@centos7 ~]# grep disk /etc/nagios/nrpe_local.cfg
command[check_disk]=/usr/lib/nagios/plugins/check_disk -w $ARG1$ -c $ARG2$ -W $ARG4$ -K $ARG5$ -p $ARG3$ -m

And what about monitoring-plugins?

A couple of years ago, the nagios-plugins project split into two projects: nagios-plugins and monitoring-plugins. Some distributions switched their package sources to monitoring-plugins (Debian and Ubuntu for example) but others remained at the "original" nagios-plugins (such as CentOS). The regression bug which caused the change in the UOM was only introduced in nagios-plugins, not in monitoring-plugins. However in EPEL, there are no monitoring-plugins packages available for installation.

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