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Storage & infrastructureCloud / DevSecOps series

Anatomy of an /etc/fstab line

/etc/fstab decides which disks get mounted at boot and how. RHCSA tests it, and one wrong line can stop a server from booting, so it is worth reading field by field. Each line has six, separated by spaces or tabs.

Quick reference

text
UUID=3f2a... /data ext4 defaults,nofail 0 2
PartNameWhat it means
UUID=3f2a...What to mountThe device, by UUID. Stable, unlike /dev/sdb1.
/dataWhereThe mount point. The folder it appears at.
ext4TypeThe filesystem: ext4, xfs, vfat, nfs.
defaults,nofailOptionsMount flags. nofail lets the server boot if it is missing.
0DumpBackup flag for the old dump tool. Almost always 0.
2fsck orderBoot-time check order. 1 for root, 2 for others, 0 to skip.

Explained

Field 1, what to mount. The device. Use UUID= from blkid, not /dev/sdb1, because device names can change when disks are added or reordered, while a UUID never does.

Field 2, where. The mount point: the existing folder where the filesystem appears, like /data. Create the folder first.

Field 3, type. The filesystem type: ext4, xfs (the RHEL default), vfat for EFI, or nfs for a network share. auto lets the system guess, but naming it is clearer.

Field 4, options. Mount flags, comma-separated. defaults covers the common set. The one worth knowing is nofail: with it, a missing or failed disk does not stop the boot, which is exactly what you want for a secondary data disk.

Field 5, dump. A flag for the ancient dump backup tool. On modern systems it is almost always 0.

Field 6, fsck order. The order filesystems are checked at boot. 1 for the root filesystem, 2 for other permanent disks, and 0 to skip the check, which you use for network or removable mounts.

The rule that saves you: after editing fstab, never reboot blindly. Run mount -a to mount everything the way boot would, and findmnt --verify to catch mistakes. Fix them now, not from a rescue console.

In one line

Test it before you reboot. A bad fstab line drops the server to emergency mode.

The safe check

mount -a && findmnt --verify

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