XFS, ext4 and VFAT: create, label and mount permanently

The idea

A new partition or logical volume is raw space. To store files it needs a file system: the on-disk structure of names, directories and data. On RHEL you will meet three:

File system	Created with	Properties
XFS	mkfs.xfs	RHEL default. Fast with big files and many files, grows online, cannot shrink
ext4	mkfs.ext4	Mature and flexible, can be shrunk (offline), good for small volumes
VFAT	mkfs.vfat (package dosfstools)	Works on every operating system; no Unix owners or permissions

mkfs.* erases whatever was on the device. Check the device name twice, and use lsblk -f first: a device that already shows a file system type has data.

Labels and UUIDs

Device names such as /dev/vdb1 can change from one boot to the next when disks are added or detected in a different order. Two stable identifiers fix that:

• UUID: a unique ID generated when the file system is created. Always unique. blkid shows it.
• Label: a name you choose, for example webdata. Easy to read; you must ensure it is unique yourself.

Set a label at creation (mkfs.xfs -L webdata, mkfs.vfat -n EXCHANGE), or later with xfs_admin -L (unmounted), e2label DEV NAME (ext4) or fatlabel DEV NAME (VFAT).

The fstab line

/etc/fstab lists what to mount at boot. Six fields:

UUID=3f1c... /srv/ext4 ext4 defaults 0 0
│            │         │    │        │ └─ fsck order (0 = skip; XFS always 0)
│            │         │    │        └─── dump flag (always 0)
│            │         │    └──────────── mount options
│            │         └───────────────── file system type
│            └─────────────────────────── mount point (must exist)
└──────────────────────────────────────── device: UUID=, LABEL= or a path

defaults means rw,suid,dev,exec,auto,nouser,async. Useful additions: nofail (boot continues if the device is missing), noauto (do not mount at boot), ro, noexec, _netdev (network file systems, wait for the network).

A mistake in fstab can stop a machine from booting normally, so test before you reboot: mount -a (mounts everything listed that is not mounted, and reports errors now) and findmnt --verify (checks syntax, targets and sources).

Try it

sudo mkfs.xfs -L webdata /dev/vdb1
sudo blkid /dev/vdb1

meta-data=/dev/vdb1              isize=512    agcount=4, agsize=32000 blks
data     =                       bsize=4096   blocks=128000, imaxpct=25
log      =internal log           bsize=4096   blocks=16384, version=2
/dev/vdb1: LABEL="webdata" UUID="e0d2c1c4-2b1b-4a6e-9c59-6d8c8e3e4f10" TYPE="xfs" PARTUUID="6f1d8f3b-01"

sudo mkdir -p /srv/webdata
echo 'LABEL=webdata /srv/webdata xfs defaults 0 0' | sudo tee -a /etc/fstab
sudo systemctl daemon-reload
sudo mount -a
findmnt /srv/webdata

TARGET        SOURCE    FSTYPE OPTIONS
/srv/webdata  /dev/vdb1 xfs    rw,relatime,seclabel,attr2,inode64,logbufs=8,logbsize=32k,noquota

ext4 by UUID, built without typing the UUID

sudo mkfs.ext4 /dev/vdb2
uuid=$(sudo blkid -s UUID -o value /dev/vdb2)
echo "UUID=$uuid /srv/ext4 ext4 defaults 0 0" | sudo tee -a /etc/fstab

UUID=8a5d6a1b-7c0e-4c3d-b6f1-0f8c8a9a1b44 /srv/ext4 ext4 defaults 0 0

Copying a 36-character UUID by hand invites typos. Command substitution does not.

VFAT has no permissions of its own

sudo mkfs.vfat -n EXCHANGE /dev/vdb3
echo 'LABEL=EXCHANGE /mnt/exchange vfat defaults,umask=000 0 0' | sudo tee -a /etc/fstab
sudo mkdir -p /mnt/exchange && sudo systemctl daemon-reload && sudo mount -a
ls -ld /mnt/exchange

mkfs.fat 4.2 (2021-01-31)
drwxrwxrwx. 2 root root 16384 Jan  1  1970 /mnt/exchange

umask=000 removes no permission bits, giving rwx to everyone. Because VFAT cannot store owners or modes, chmod and chown fail or do nothing; ownership comes only from uid=, gid=, umask=, fmask= and dmask= options.

Check before you reboot

sudo findmnt --verify
sudo mount -a

Success, no errors or warnings detected

With a wrong UUID in the file, findmnt --verify reports the source as unreachable, and mount -a prints can't find UUID=.... At boot, the same mistake on a normal line makes systemd wait and then drop to emergency mode (a root-password prompt). Add nofail for non-essential mounts.

Busy file systems

umount /srv/webdata fails with target is busy when a process has a file open or its shell is inside it. Find the cause with sudo fuser -vm /srv/webdata or lsof +f -- /srv/webdata, cd out, and try again.

Going further

xfs_info /srv/webdata shows geometry and tune2fs -l DEV the ext4 superblock. xfs_repair and fsck repair unmounted file systems; XFS is never fsck-ed at boot, which is why its fstab pass is 0. mount -o remount,ro /srv/webdata changes options in place. systemd also creates .mount units from fstab lines; systemctl list-units --type=mount shows them, and x-systemd.automount mounts on first access. Prefer UUID for anything non-removable and a label when a human has to recognise the volume.