Centos7 fstab that 99% of people don't know

• 1、 Isn't fstab the only one?
• 2、 Demo: Mount with systemd
• 3、 Problem summary

centos7 fstab may not be what you want

The origin of the problem is that when I was researching docker, I searched for overlay mounts, and suddenly found that the execution results of /etc/fstab and mount commands are very different.

**1、 Isn't fstab the only one? **

fstab

Then the question is as follows:

• The contents of /etc/fstab and /proc/mount** are quite different, but the contents of /proc/mount will still be automatically mounted** after the system restarts. Why? (Everyone knows that /etc/fstab takes effect permanently, but why can other items take effect permanently?)
• The two formats type cgroup and overlay are not supported in the mount man document. Why can they be mounted normally?

In fact, **Starting with CentOS 7, a method for systemd to mount the file system is also provided. ** That is, /etc/fstab is no longer the only permanent mounting method. In CentOS 7&8, the content of /etc/fstab is very small, as we can also find in the figure above.

**In CentOS 7, we configure /etc/fstab, the system does not directly use this configuration, systemd actually converts the file into a mount file ending with mount. systemd is in the system initialization , It reads these files ending with .mount. **

• . The mount file is stored in /run/systemd/generator.

[ root@O2O-T-K8S-TEST4 generator]# ls -l /run/systemd/generator/
total 20-rw-r--r--1 root root 254 Apr 1418:55 boot.mount
- rw-r--r--1 root root 181 Apr 1418:55 dev-mapper-RootVG\x2dlv_swap.swap
..

In addition, systemd also provides commands for us to view the system mount

• View the file system being mounted

[ root@O2O-T-K8S-TEST4 generator]# systemctl -t mount
UNIT                                    LOAD   ACTIVE SUB     DESCRIPTION
- . mount                                 loaded active mounted /
boot.mount                              loaded active mounted /boot
dev-hugepages.mount                     loaded active mounted Huge Pages File System
dev-mqueue.mount                        loaded active mounted POSIX Message Queue File System
..

• systemd mount point location

[ root@O2O-T-K8S-TEST4 generator]# ls /usr/lib/systemd/system/*.mount
/usr/lib/systemd/system/dev-hugepages.mount
/usr/lib/systemd/system/dev-mqueue.mount
/usr/lib/systemd/system/proc-fs-nfsd.mount
/usr/lib/systemd/system/proc-sys-fs-binfmt_misc.mount
/usr/lib/systemd/system/sys-fs-fuse-connections.mount
.
.

2、 Demo: Mount with systemd

• Case: Mount /dev/sdb1 under /tmp_dir

[ root@rhel-8 system]# cat tmp_dir.mount
# This file is part of systemd.[Unit]
Description=Test Directory(/tmp_dir)
DefaultDependencies=no
Conflicts=umount.target
Before=local-fs.target umount.target
After=swap.target

[ Mount]
What=/dev/disk/by-uuid/cea0757d-6329-4bf8-abbf-03f9c313b07f
Where=/tmp_dir
Type=ext4
Options=defaults

[ Install]
WantedBy=multi-user.target

• Glossary

What=Indicates the absolute path of the device node, file or other device to be mounted.`man 8 mount`see more

Where=The device mount point needs to write an absolute path. This must not be a link address.

Type=The type of file system, usually a string

Options=Mount options, separated by commas, support multiple options in parallel.

• reload daemon makes systemd effective

systemctl daemon-reload

3、 Question summary

• The first question, starting with centos 7, after sysv is handed over to systemd for maintenance, /etc/fstab is no longer the only mount control item, systemd can also complete this work, and Systemd does not read the content of /etc/fstab, but converts it to a file ending in .mount. Looking at the trend, I feel that the follow-up may completely take over /etc/fstab. However, there is not much research on systemd, and the dominant family does not know whether it is good or bad. According to the style of linus, systmd may be strongly supported
• The second question is that man mount is not updated. centos 7 must support cgroup, overlay types.

Reference reading: https://www.golinuxcloud.com/mount-filesystem-without-fstab-systemd-rhel-8/