Where do mount options go?

On the command line they follow -o as a comma-separated list, e.g. mount -o ro,noexec,nosuid /dev/sdb1 /mnt. In /etc/fstab they occupy the fourth field of the entry. The same option names work in both places.

What do noexec, nosuid and nodev do?

They are hardening options. noexec blocks executing binaries from the filesystem, nosuid ignores set-user-ID and set-group-ID bits, and nodev ignores device special files. Applying all three to mounts like /tmp and removable media limits common privilege-escalation vectors.

What is the difference between noatime and relatime?

Both reduce writes by limiting access-time updates. noatime never updates the access time, maximising performance. relatime updates it only when older than the modify/change time or more than a day stale — the modern default that keeps tools like mutt working while still cutting writes.

How is a bind mount different?

A bind mount makes an existing directory tree appear at a second location with mount --bind src dst or the bind option in fstab. It does not involve a filesystem driver; it re-exposes the same inodes. Add rbind to include submounts and ro for a read-only view of the bound tree.

How do I size a tmpfs?

tmpfs lives in RAM/swap and grows on demand up to a limit set with the size option, e.g. -o size=512m or size=50%. Without a size it defaults to half of physical RAM. Also set mode to control permissions and nr_inodes to cap the file count.

Linux mount Options Reference

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What mount options control

mount options tune how a filesystem is attached: read-only vs read-write, security restrictions, access-time behaviour and filesystem-specific tuning. They are supplied after -o on the command line or in the fourth field of an /etc/fstab entry. Some are generic and work on any VFS filesystem; others are specific to ext4, tmpfs or bind mounts. This reference groups them by scope.

How it works

Options are a comma-separated list. The kernel applies generic VFS flags first, then hands filesystem-specific options to the driver:

# command line
mount -o ro,noexec,nosuid,nodev /dev/sdb1 /mnt/usb

# /etc/fstab (device  mountpoint  fstype  options  dump  pass)
tmpfs   /tmp   tmpfs   rw,nosuid,nodev,noexec,size=2g,mode=1777   0  0
/data   /srv   none    bind,ro                                    0  0

defaults is shorthand for rw,suid,dev,exec,auto,nouser,async. You override individual flags by listing their negation afterward, e.g. defaults,noexec.

Tips and notes

Harden untrusted or removable mounts with noexec,nosuid,nodev.
Prefer relatime (the default) over noatime unless you have benchmarked the write savings.
A bind mount re-exposes the same inodes; add rbind to carry submounts.
For tmpfs, always set size= so a runaway process cannot exhaust RAM.