Hi all,
after having a private discussion with Serge E. Hallyn and then inspired by
the posting of Matto Fransen on the thread "read only rootfs" I was able to
realize an entry from my wish list, which may be useful for others, too:
To have a (read-only) access limited to "it's" branch of the cgroupfs
inside a container.
This might be useful to write some tool to be run inside a container in a
"canonical way", i.e. with a fixed path (disregarding of the mount point
prefix, which may be taken from /proc/mounts) and without knowing the name of
the current container running in. You may compare it to the mechanism of user
beancounters in openVZ at /proc/user_beancounters .
One may e.g. write something like a replacement for 'free', which will use the
values from the /cgroup/memory.*-entries to show up the "right" values. Because
at the moment, the "normal" syscall used by free and others will yield the
"wrong" values from the host.
At the moment too, one can't mount any subtrees of the cgroupfs. But I found
that this can be "emulated" by use of a bind-mount. And it can made read-only
by use of the same instrument, too. From that, it turns out that the feature in
discussion can be already set up without including new features into lxc.
Using lxc-0.7.4.1, I just had to add a "dynamical" config option (lxc-start -s
...) to my lxc maintenance masterscript, in particular
-s lxc.mount.entry="/cgroup/$CONTAINER cgroup none ro,bind 0 0"
where $CONTAINER is the shell variable holding the name of the container to
start. I my case, at the host i'm using a single cgroupfs holding all
subsystems at /cgroup. Notice, that with 0.7.4.1 the destination mount point --
it will also be /cgroup with respect to the container's rootfs -- have to be
relative.
After booting the container, you'll find it's cgroup subtree mounted read-only
at /cgroup.
with greetings
Guido
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