Shared Mount
I have Kubernetes at home. I use it for my own infrastructure and my own server. I will maybe start a series of post about it in the future. However, today I will speak about a really specific issue I ran into.
For my Kubernetes cluster I needed to deploy a monitoring stack. I am using the
now standard prometheus + grafana (and some others services) to handle this.
To monitor my host machine I am using the node exporter project in a daemon-set
and mount the entire host system in it so it can be monitored.
During the deployment the node exporter pod was crashing with the following error:
Error: failed to start container "node-exporter": Error response from daemon:
path / is mounted on / but it is not a shared or slave mount
Error: failed to start container "node-exporter": Error response from daemon:
path /sys is mounted on /sys but it is not a shared or slave mount
Little did I know about this kind of issue or even what was a slave mount or
a shared mount. I ended up checking the world wide web and found out a bunch
of explanation about it.
Here is the stack exchange page which describe it
the best in the most concise way.
Now that I knew about it, it was only one command away:
mount --make-rshared /
Since this is recursive and I will need the root directory for node exporter, I
only need to run it once (no need to go for /sys or /run as they are below /). However, there might be security considerations here
so use this line with caution. If I find better info I will amend this article.
Next step is to make me able to check if a specific directory is shared or not.
I wanted this check to be as simple as possible (so no extra command to install).
I ended up finding this information in the /proc directory.
cat /proc/self/mountinfo | grep <path_to_check>
Another thing is to make this change permanent. At first, I thought this was
some kind of a mount option. I ended up reading (not properly I have to confess)
/etc/fstab without finding any evidence on how to do this. It appeared that
you may have to run the --make-shared option at every boot phase.
Since I am using alpine linux I followed those instructions to
ensure my filesystem was mounted properly:
install -D -m 0755 /dev/stderr /etc/local.d/10-mount.start 2<<-EOF
#!/bin/sh
mount --make-rshared /
EOF
rc-update add local default
This conclude this small article, I hope it could be useful for someone else. All in all this is a good personal reminder of what I did and how I managed to make it work. Cheers!