[arch-general] systemd adds entries to /etc/mtab

[Rodrigo Rivas]

On Sat, Oct 20, 2012 at 3:01 AM, Janna M. <janna.martl109 at gmail.com> wrote:

> I just switched to systemd, and was poking around a bit when I noticed
> many new entries in /etc/mtab:
>
> ...
> securityfs /sys/kernel/security securityfs rw,nosuid,nodev,noexec,**relatime
> 0 0
> tmpfs /sys/fs/cgroup tmpfs rw,nosuid,nodev,noexec,mode=**755 0 0
> cgroup /sys/fs/cgroup/systemd cgroup
> rw,nosuid,nodev,noexec,**relatime,release_agent=/usr/**
> lib/systemd/systemd-cgroups-**agent,name=systemd 0 0
> cgroup /sys/fs/cgroup/cpuset cgroup rw,nosuid,nodev,noexec,**relatime,cpuset
> 0 0
> cgroup /sys/fs/cgroup/cpu,cpuacct cgroup rw,nosuid,nodev,noexec,**relatime,cpuacct,cpu
> 0 0
> cgroup /sys/fs/cgroup/devices cgroup rw,nosuid,nodev,noexec,**relatime,devices
> 0 0
> cgroup /sys/fs/cgroup/freezer cgroup rw,nosuid,nodev,noexec,**relatime,freezer
> 0 0
> cgroup /sys/fs/cgroup/net_cls cgroup rw,nosuid,nodev,noexec,**relatime,net_cls
> 0 0
> cgroup /sys/fs/cgroup/blkio cgroup rw,nosuid,nodev,noexec,**relatime,blkio
> 0 0
> systemd-1 /proc/sys/fs/binfmt_misc autofs rw,relatime,fd=21,pgrp=1,**
> timeout=300,minproto=5,**maxproto=5,direct 0 0
> hugetlbfs /dev/hugepages hugetlbfs rw,relatime 0 0
> debugfs /sys/kernel/debug debugfs rw,relatime 0 0
> mqueue /dev/mqueue mqueue rw,relatime 0 0
> tmpfs /tmp tmpfs rw 0 0
> fusectl /sys/fs/fuse/connections fusectl rw,relatime 0 0
>
>
> In particular, I notice that /dev/hugepages, /dev/mqueue, and
> /sys/kernel/debug seem to correspond to unit files
> dev-hugepages.mount, dev-mqueue.mount, and sys-kernel-debug.mount,
> respectively. As I'm trying to pare down the output of systemctl
> list-units, I'd like to know if these things can safely be removed.
>

Note that recent systemd versions include a link to the documentation of
some units. For example:

$ systemctl status dev-mqueue.mount | grep Docs
     Docs: man:mq_overview(7)

So doing:

$ man 7 mq_overview

will give you an overview of what it is used for.

Generally speaking, these are sometimes called "API filesystems" because
they povide kernel services to applications instead of file storage. They
can be removed only if none of your running applications use them, but that
could be difficult to assert. And since they use almost no resources for
just being mounted, there is little reason for not mounting them at system
startup. That's why systemd does it automatically by default.

However, you can try disabling them and see what happens. In particular
disabling the debugfs thing should do no harm.

PS: There is a very thorough and interesting series about the hugepages at
LWN: [1] [2] [3] [4] [5].

[1]: http://lwn.net/Articles/374424/
[2]: http://lwn.net/Articles/375096/
[3]: http://lwn.net/Articles/376606/
[4]: http://lwn.net/Articles/378641/
[5]: http://lwn.net/Articles/379748/

-- 
Rodrigo