Why Can't I Mount a Disk on an Old Mount Point by Modifying fstab in CentOS 7?

Symptom

File /etc/fstab was modified to allow a new disk to be mounted on an old mount point. The mounting failed when queried using the df command.

The operations described in this section apply to ECSs running CentOS or EulerOS.

Possible Cause

Run the following command to query the involved mount unit:
systemctl list-units --type=mount |grep failed
```
test1.mount             loaded failed failed  /test1
```

Run the following command to query the unit status:

systemctl status test1.mount

Information similar to the following is displayed:

● test1.mount - /test1
Loaded: loaded (/etc/fstab; bad; vendor preset: disabled)

Active: failed (Result: exit-code) since Wed 2019-08-28 15:32:53 CST; 3min 27s ago
Where: /test1
What: /dev/vdb1
Docs: man:fstab(5)
man:systemd-fstab-generator(8)
Process: 4601 ExecUnmount=/bin/umount /test1 (code=exited, status=0/SUCCESS)
Process: 3129 ExecMount=/bin/mount /dev/vdb1 /test1 -t ext4 (code=exited, status=0/SUCCESS)
... ...
Warning: test1.mount changed on disk. Run 'systemctl daemon-reload' to reload units.

In the command output, mount unit test1.mount has changed. Run systemctl daemon-reload to reload units.

The changes in fstab do not get applied automatically. You must run the systemctl daemon-reload command to update the mount units generated for each entry in fstab.