Mounting file systems¶

Practical 4.4 mounting-file-systems-annex.pdf · PDF

You plug a USB stick into your laptop and, a second later, a window opens with your files. Who did that? Something had to take the stick's file system and attach it somewhere you could reach it.

On deb-lab that something is missing. Your second disk has three partitions, each with a file system and a label — and no command of yours can reach them. cd /dev/vdb1 answers Not a directory. Today you open the doors, by hand first, and then for good, so they are still open after a reboot.

Three rooms with no door¶

lsblk -f

Three partitions, three file systems, three labels — and an empty MOUNTPOINTS column. The disk is connected. The file systems exist. Nobody can go in.

In the pantry of File systems, each room got shelves and an inventory. They still have no door onto the kitchen. Mounting cuts that door.

Mounting is attaching a file system to an ordinary, empty directory of the tree, called the mount point. From then on, behind that directory you see the shelves of that file system.

• The room = the file system on /dev/vdb1.
• The door = the mount point, for example /srv/tramuntana/dev.

Remember from the directory tree: Linux has one tree. There is no D:. Every file system hangs from a folder of that tree.

flowchart TB
    subgraph TREE ["The one tree of deb-lab (system disk vda)"]
        ROOT["<b>/</b>"] --> SRV["<b>/srv/tramuntana</b>"]
        ROOT --> MNT["<b>/mnt</b>"]
        SRV --> DEVD["<b>dev</b><br>mount point"]
        SRV --> DESD["<b>design</b><br>mount point"]
        MNT --> EXD["<b>exchange</b><br>mount point"]
    end
    subgraph DISK ["Second disk vdb"]
        V1["<b>vdb1</b> · ext4 · dev"]
        V2["<b>vdb2</b> · xfs · design"]
        V3["<b>vdb3</b> · exfat · EXCHANGE"]
    end
    V1 -->|"mounted on"| DEVD
    V2 -->|"mounted on"| DESD
    V3 -->|"mounted on"| EXD

Mount and unmount by hand¶

Two commands, and nothing else:

sudo mount DEVICE DIRECTORY
sudo umount DIRECTORY

Note the spelling: umount, without the n.

Mount dev¶

Make the mount point, and leave a note inside it before mounting:

sudo mkdir -p /srv/tramuntana/dev
sudo touch /srv/tramuntana/dev/i-was-here.txt
ls /srv/tramuntana/dev

mkdir -p also creates the parent directories that are missing. Now mount, and look again:

student@deb-lab:~$ sudo mount /dev/vdb1 /srv/tramuntana/dev
student@deb-lab:~$ ls -a /srv/tramuntana/dev
.  ..  lost+found
student@deb-lab:~$ findmnt /srv/tramuntana/dev
TARGET              SOURCE    FSTYPE OPTIONS
/srv/tramuntana/dev /dev/vdb1 ext4   rw,relatime

Your note is gone from view, and there is a lost+found folder (mkfs.ext4 makes it: the disk checker puts there the pieces it recovers after a crash). findmnt shows what is mounted where, and df -h /srv/tramuntana/dev says the size: 2.0G, almost nothing used.

Unmount it¶

student@deb-lab:~$ cd /srv/tramuntana/dev
student@deb-lab:/srv/tramuntana/dev$ sudo umount /srv/tramuntana/dev
umount: /srv/tramuntana/dev: target is busy.
student@deb-lab:/srv/tramuntana/dev$ cd ~
student@deb-lab:~$ sudo umount /srv/tramuntana/dev
student@deb-lab:~$ ls /srv/tramuntana/dev
i-was-here.txt

The first umount failed because you were inside: you cannot remove a door while somebody stands in it. From your home it works in silence. And the note is back.

The exchange stick¶

EXCHANGE is the room every system can read. Use it like a USB stick: mount, copy, unmount.

sudo mkdir /mnt/exchange
sudo mount /dev/vdb3 /mnt/exchange
df -hT /mnt/exchange
sudo cp ~/tramuntana-share/admin/staff.csv /mnt/exchange/
ls -l /mnt/exchange
sudo umount /mnt/exchange
ls /mnt/exchange

df -hT shows type exfat, about 1021M. The last ls shows nothing: the stick is "safely removed". That is exactly what Safely remove does on a desktop: unmount before you pull the stick out, so every pending write reaches it.

Make it permanent: /etc/fstab¶

Restart deb-lab and your mounts are gone: mount lasts until the machine is switched off. The system needs a written list of what to mount at every start.

/etc/fstab (file systems table) is the text file that lists which file systems to mount at boot, where, and how.

Why a UUID, not /dev/vdb1¶

The names vda, vdb, vdc are handed out at every boot, in the order the kernel finds the disks. Add or remove a disk — in LVM and RAID, later in this unit, you add four — and vdb can become another disk.

The UUID you saw in File systems is written inside the file system by mkfs. It travels with it. In the pantry: /dev/vdb1 is "the second room on the left" — rebuild the corridor and it is another room. The UUID is the number engraved on its inventory.

The six fields¶

cat /etc/fstab shows the lines the installer wrote. Here is one for dev:

UUID=4b481e94-b109-4014-a26b-be4bdb6cd5ca  /srv/tramuntana/dev  ext4  defaults,nofail  0  2

Edit it safely¶

Get the UUIDs with blkid, and copy the UUID, not the PARTUUID (that one belongs to the partition table):

sudo blkid /dev/vdb1 /dev/vdb2

Then make the second mount point, keep a copy of the file, and open it in nano, a simple text editor that runs inside the terminal:

sudo mkdir /srv/tramuntana/design
sudo cp /etc/fstab /etc/fstab.bak
sudo nano /etc/fstab

In nano: arrows to move, Ctrl+Shift+V to paste, Ctrl+O then Enter to save, Ctrl+X to leave. Do not copy 36 characters by hand: select the UUID with the mouse and use Ctrl+Shift+C. Add two lines at the end, one for dev (ext4) and one for design (xfs), each with its own UUID.

Test before you reboot¶

sudo findmnt --verify
sudo systemctl daemon-reload
sudo findmnt --verify
sudo mount -a
findmnt /srv/tramuntana/dev
findmnt /srv/tramuntana/design

• The first findmnt --verify warns that your fstab has been modified, but systemd still uses the old version. systemctl daemon-reload makes systemd re-read it.
• The second one should end with Success, no errors or warnings detected.
• mount -a mounts everything in fstab that is not mounted yet. It prints nothing when all is well.

The two checks catch different mistakes:

So: both checks, then findmnt to see the mount with your own eyes. And then the real test, sudo reboot: after logging in, both findmnt must show the mounts without you typing mount. A mount that was not tested after a reboot is not finished.

df asks the disk, du asks the folder¶

The designers say the disk is full. Two questions: how full? and who did it? Two different commands.

df -hT
du -sh /srv/tramuntana/design

-h means human: 2.0G instead of a long number of blocks. -T adds the type, -s gives one total. Skip the lines that start with tmpfs and udev: they live in RAM, not on a disk. In the pantry: df reads the sign on the door (how many shelves are free); du goes in and weighs one box.

Find the hog¶

The system disk fills up one day. Where do you look? Measure one level of a folder and sort by size:

sudo du -h --max-depth=1 /usr | sort -h

sort -h understands human sizes like 12M and 1.3G, and puts the biggest at the bottom. Go down into the biggest folder and repeat until you find the culprit.

In Practical 4.4 you fill the rooms with the work of the dev and design teams and a fake 300 MB video, made with head -c 300M /dev/urandom > campaign.mp4 (300 MiB of random bytes), and then catch it with df and du.

Key takeaways¶

• Mounting attaches a file system to a directory of the one tree, the mount point. Nothing is copied: what was in that directory is only hidden.
• sudo mount DEVICE DIRECTORY and sudo umount DIRECTORY (no n). findmnt shows what is mounted where. You cannot unmount from inside.
• /etc/fstab makes mounts permanent: six fields, UUID first, because device names like vdb can change at boot.
• Always: back up the file, add nofail to extra disks, run findmnt --verify and mount -a, then reboot to prove it.
• df -h tells how full each file system is; du -sh tells how big one folder is. du ... | sort -h finds the hog.

Quick check¶

You partitioned, formatted and mounted a disk on Linux. Seen from Windows, the same ideas have other names. Next: Windows Disk Management.