Detecting read-only file system in Linux

Just for future reference as you already found your solution:

From looking at the util-linux package I understand that _if_ the /proc-filesystem is mounted the information is used from there. So it is ensured that up-to-date information is returned by `mount`.
Back in the old Linux-days before /proc existed (at the minimum pre kernel 2.4) `mount` relied on the information from `/etc/mtab` which was not necessarily up-to-date (in particular in case of a ro-mount of `/`).

I would expect almost every Linux system in service today to have /proc.
The behaviour of `mount` on other OSes (AIX, *BSD, …) might differ though.

Cheers
Thomas

On 09 Mar 2015, at 18:09, SebA <user-4631430d620a@xymon.invalid> wrote:

Thanks Thomas. Simulating this suggests your code will work on our server, but as you said, you are not 100% sure that the 'ro' will surface in the mount command. The output here suggests that it may not:
http://sisyphus.ru/en/srpm/Sisyphus/xymon/sources/8 <http://sisyphus.ru/en/srpm/Sisyphus/xymon/sources/8>;
#!/bin/sh

# Read data from /proc/mounts on linux and report back in the xymon client
# It gives more accurate data than the 'mount' command and can catch
# disks in a read-only state.

test -r /proc/mounts && exec cat /proc/mounts
But maybe that is only needed for some other version of linux... Our server is now fixed so I can't test it properly. But I do prefer a server-side extension, or even a patch to xymon-server, to client plugins that I have to install to every server. The issue with your code is that it only works for 1 named server ('bb.local'), so I would need to have something iterating over all hosts. Probably not hard, and probably something that someone here already does...
 In fact, I think I have found the place to add it and that is in this add-on: https://wiki.xymonton.org/doku.php/monitors:check-client <https://wiki.xymonton.org/doku.php/monitors:check-client>; - then it will work for all hosts (correct me if I am wrong David Baldwin?)
Kind regards,
SebA

From: Xymon [mailto:xymon-bounces at xymon.com] On Behalf Of Thomas Eckert
Sent: 09 March 2015 14:05
To: Xymon MailingList
Subject: Re: [Xymon] Detecting read-only file system in Linux

Hi,

On 09 Mar 2015, at 13:44, SebA <user-4631430d620a@xymon.invalid <mailto:user-4631430d620a@xymon.invalid>> wrote:

(…)

Although we have some Debian systems, I was looking for a solution for another Linux distro.
 If I was to write something myself to do it, I would check /proc/mounts and the best command I could find was:
awk '$4~/(^|,)ro($|,)/' /proc/mounts
which outputs:
/dev/root / ext3 ro,data=ordered 0 0
with sample line:
/dev/root / ext3 ro,data=ordered 0 0

You could use the reported “Client data” / “clientlog” with a server-side extension. I have to admit that I’m not 100% sure if error-or-remounts are reflected properly by this — but `mount` seems to use `/proc/self/mountinfo` as it’s datasource, so it _should_ be ok.

Extract the reported `mount`-data for host bb.local:

xymon 127.0.0.1 “clientlog bb.local section=mount”

Some testing of the output:

(export host=bb.local; xymon 127.0.0.1:1984 "clientlog $host section=mount" | gawk '/[(,]+ro[,)]+/ { print "filesystem " $1 " mounted RO!" } ')

This would not require installing and maintaining an extension on every client. One drawback would be increased latency as the ro-check would be “indirect”.

Cheers
Thomas