File systems and mounting: XFS, ext4, mount and df

What a file system is

A partition is just raw space. A file system organises that space: it keeps a list of files and folders, their owners and permissions, and which blocks hold their data. Creating a file system is called formatting, and it erases whatever was on the partition before.

The one fact to remember: XFS cannot be shrunk. If you may need to make a file system smaller later, plan for that (or use ext4).

# mkfs.xfs /dev/vdb1
meta-data=/dev/vdb1              isize=512    agcount=4, agsize=131008 blks
data     =                       bsize=4096   blocks=524032, imaxpct=25
...
# mkfs.ext4 -L backups /dev/vdc1        # -L gives a label
mke2fs 1.46.5 (30-Dec-2021)
Creating filesystem with 262144 4k blocks and 65536 inodes
...
# lsblk -f /dev/vdb /dev/vdc
NAME   FSTYPE LABEL   UUID                                 MOUNTPOINTS
vdb
└─vdb1 xfs            5b2f3c1e-8d7a-4f0b-a9e1-3c6d2b8f9a10
vdc
└─vdc1 ext4   backups 0e9c7d6b-1a2f-4c3d-8e5f-6a7b8c9d0e1f

Mounting: attaching to the tree

To use a file system you mount it on a directory, called the mount point. After that, everything you put in that directory is stored on that file system. umount detaches it again (note the spelling: umount, no n).

# mkdir -p /data
# mount /dev/vdb1 /data
# df -h /data
Filesystem      Size  Used Avail Use% Mounted on
/dev/vdb1       2.0G   47M  2.0G   3% /data
# findmnt /data
TARGET SOURCE    FSTYPE OPTIONS
/data  /dev/vdb1 xfs    rw,relatime,seclabel,attr2,inode64,logbufs=8,logbsize=32k,noquota
# umount /data

mount without options mounts read-write with default options. Useful options: -o ro (read-only), -o noexec (do not allow running programs from here), -o remount,rw (change options of a mounted file system). A mount made with the mount command is lost at reboot; the next topic makes it permanent.

How much space? df and du

$ df -h                         # every mounted file system
Filesystem             Size  Used Avail Use% Mounted on
/dev/mapper/rhel-root   17G  4.1G   13G  25% /
/dev/vda1             1014M  298M  717M  30% /boot
/dev/vdb1              2.0G   47M  2.0G   3% /data
$ df -i /                       # inodes: number of files, can run out before space does
$ du -sh /var/log               # size of one directory tree
612M    /var/log
$ du -h --max-depth=1 /var | sort -h | tail -3     # biggest folders under /var

df asks the file system how full it is. du adds up file sizes by walking the tree. They can disagree: if a program still has a deleted file open, df counts the space but du cannot see the file. Restarting that program frees the space. lsof +L1 lists such deleted-but-open files.

Common mistakes

• "umount: /data: target is busy": some process, often your own shell, is using it. cd out of it, then check with lsof /data or fuser -vm /data.
• Mounting on a directory that already has files. They are hidden (not deleted) while something is mounted on top, which confuses people. Mount on empty directories.
• Running mkfs on a partition that has data, or on the whole disk instead of the partition (/dev/vdb instead of /dev/vdb1).
• Planning to shrink an XFS file system later. It is not possible; back up, recreate smaller, and restore.