Exercise: Mount at boot, then on demand

Mount at boot, then on demand

• Write and test an NFS fstab entry with _netdev.
• Create an autofs master entry and an indirect map.
• Observe on-demand mounting and idle unmounting.

Start the exercise

On workstation, start the exercise. It installs nfs-utils, exports /srv/shared from serverb to the lab network (NFS server running, firewall open) and creates the mount point /mnt/shared on servera.

[student@workstation ~]$ lab start sa-nfs-persistent

Later tasks assume that you are logged in to the server they name: ssh student@servera from workstation, then sudo -i for a root shell.

An fstab entry

On servera as root, back up fstab, add a line that mounts serverb:/srv/shared on /mnt/shared as nfs with defaults,_netdev, reload, mount everything, and check.

[student@servera ~]$ sudo -i
[root@servera ~]# cp /etc/fstab /root/fstab.bak
[root@servera ~]# mkdir -p /mnt/shared
[root@servera ~]# echo "serverb:/srv/shared  /mnt/shared  nfs  defaults,_netdev  0 0" >> /etc/fstab
[root@servera ~]# systemctl daemon-reload; findmnt --verify | tail -1
0 parse errors, 0 errors, 1 warning
[root@servera ~]# mount -a; df -hT /mnt/shared | tail -1
serverb:/srv/shared nfs4   20G  1.7G   18G   9% /mnt/shared

Prove it with a reboot

Reboot servera, log in, and check that the share is mounted without any manual step.

[root@servera ~]# systemctl reboot
[student@workstation ~]$ ssh student@servera
[student@servera ~]$ findmnt /mnt/shared -o TARGET,SOURCE,FSTYPE
TARGET      SOURCE              FSTYPE
/mnt/shared serverb:/srv/shared nfs4
[student@servera ~]$ ls /mnt/shared
hello.txt

Switch to autofs

Remove the fstab line (restore the backup) and unmount. Install autofs, create a master entry for /mnt/auto with a 20 second timeout and a map with the key data for the share, enable the service, and verify that nothing is mounted yet.

[student@servera ~]$ sudo -i
[root@servera ~]# umount /mnt/shared; cp /root/fstab.bak /etc/fstab; systemctl daemon-reload
[root@servera ~]# dnf install -y autofs > /dev/null
[root@servera ~]# echo '/mnt/auto  /etc/auto.shared  --timeout=20' > /etc/auto.master.d/shared.autofs
[root@servera ~]# echo 'data  -rw,sync  serverb:/srv/shared' > /etc/auto.shared
[root@servera ~]# systemctl enable --now autofs
Created symlink /etc/systemd/system/multi-user.target.wants/autofs.service → /usr/lib/systemd/system/autofs.service.
[root@servera ~]# mount | grep -c /mnt/auto/data
0

Watch it mount and unmount

Use the path to trigger the mount, inspect it, wait longer than the timeout, and check again.

[root@servera ~]# ls /mnt/auto/data
hello.txt
[root@servera ~]# findmnt -t autofs,nfs4 -o TARGET,SOURCE,FSTYPE | tail -2
/mnt/auto              /etc/auto.shared    autofs
└─/mnt/auto/data         serverb:/srv/shared nfs4
[root@servera ~]# cat /mnt/auto/data/hello.txt
shared file
[root@servera ~]# sleep 30; mount | grep -c /mnt/auto/data
0

The share was mounted by the first ls, used, and unmounted after the idle timeout. Repeat the ls and it comes back.

A wrong map line

Add a line broken -rw serverb:/srv/nothere to /etc/auto.shared, reload autofs, and try to use it. What do you see? Remove the line again.

[root@servera ~]# echo 'broken  -rw  serverb:/srv/nothere' >> /etc/auto.shared
[root@servera ~]# systemctl reload autofs
[root@servera ~]# ls /mnt/auto/broken
ls: cannot access '/mnt/auto/broken': No such file or directory
[root@servera ~]# sed -i '/^broken/d' /etc/auto.shared; systemctl reload autofs

The map entry exists, but the server has no such export, so the mount fails and the path does not appear. The error is "No such file or directory", not an NFS message, which is why showmount, exportfs -v on the server and journalctl -u autofs are the next stops.

Grade and finish

Check your work with lab grade sa-nfs-persistent, then put the exercise away with lab finish sa-nfs-persistent.