systemd-fstab-generator - Man Page

/usr/lib/systemd/system-generators/systemd-fstab-generator

systemd-fstab-generator is a generator that translates /etc/fstab (see fstab(5) for details) into native systemd units early at boot and when configuration of the system manager is reloaded​. This will instantiate mount and swap units as necessary​.

The passno field is treated like a simple boolean, and the ordering information is discarded​. However, if the root file system is checked, it is checked before all the other file systems​.

See systemd.mount(5) and systemd.swap(5) for more information about special /etc/fstab mount options this generator understands​.

One special topic is handling of symbolic links​. Historical init implementations supported symlinks in /etc/fstab​. Because mount units will refuse mounts where the target is a symbolic link, this generator will resolve any symlinks as far as possible when processing /etc/fstab in order to enhance backwards compatibility​. If a symlink target does not exist at the time that this generator runs, it is assumed that the symlink target is the final target of the mount​.

systemd-fstab-generator implements systemd.generator(7)​.

systemd-fstab-generator understands the following kernel command line parameters:

fstab=, rd​.fstab=

Takes a boolean argument​. Defaults to "yes"​. If "no", causes the generator to ignore any mounts or swap devices configured in /etc/fstab​. rd​.fstab= is honored only in the initrd, while fstab= is honored by both the main system and the initrd​.

Added in version 186​.

root=

Configures the operating system's root filesystem to mount when running in the initrd​. This accepts a device node path (usually /dev/disk/by-uuid/​... or /dev/disk/by-label/​... or similar), or the special values "gpt-auto", "gpt-auto-force", "dissect", "dissect-force", "fstab", "tmpfs", "bind:​...", and "off"​.

Set to "gpt-auto", "gpt-auto-force", "dissect", "dissect-force" to explicitly request automatic root file system discovery, implemented in systemd-gpt-auto-generator(8)​.

Use "fstab" to explicitly request automatic root file system discovery via the initrd /etc/fstab rather than via kernel command line​.

Use "tmpfs" in order to mount a tmpfs(5) file system as root file system of the OS​. This is useful in combination with mount​.usr= (see below) in order to combine a volatile root file system with a separate, immutable /usr/ file system​. Also see systemd​.volatile= below​.

Use "bind:​..." to bind mount another directory as operating system root filesystems (added in v258)​. Expects an absolute path name referencing an existing directory within the initrd's file hierarchy to boot into​. Since the resulting root file system is supposed to behave like a regular OS root, the bind mount is established with the dev, suid and exec options (i​.e​. the nodev, nosuid and noexec flags that would otherwise be inherited from the file system the source directory resides on, such as /run/, are cleared), unless overridden via rootflags=​.

Set to "off" to turn off mounting of a root file system​.

Note that further root= values may be supported, implemented in additional packages​.

Added in version 217​.

rootfstype=

Takes the root filesystem type that will be passed to the mount command​. rootfstype= is honored by the initrd​.

Added in version 217​.

rootflags=

Takes the root filesystem mount options to use​. rootflags= is honored by the initrd​.

Note that unlike most kernel command line options this setting does not override settings made in configuration files (specifically: the mount option string in /etc/fstab)​. See systemd-remount-fs.service(8)​.

Added in version 217​.

mount​.usr=

Takes the /usr/ filesystem to be mounted by the initrd​. If mount​.usrfstype= or mount​.usrflags= is set, then the mount configured via mount​.usr= will default to the same value set in root=​.

Set to "dissect" to explicitly request automatic /usr/ file system discovery, implemented in systemd-gpt-auto-generator(8)​.

Set to "off" to turn off mounting of a separate /usr/ file system​.

mount​.usr= is honored by the initrd​.

Added in version 217​.

mount​.usrfstype=

Takes the /usr/ filesystem type that will be passed to the mount command​. If mount​.usr= or mount​.usrflags= is set, then mount​.usrfstype= will default to the value set in rootfstype=​.

Otherwise, this value will be read from the /usr/ entry in /etc/fstab on the root filesystem​.

mount​.usrfstype= is honored by the initrd​.

Added in version 217​.

mount​.usrflags=

Takes the /usr/ filesystem mount options to use​. If mount​.usr= or mount​.usrfstype= is set, then mount​.usrflags= will default to the value set in rootflags=​.

Otherwise, this value will be read from the /usr/ entry in /etc/fstab on the root filesystem​.

mount​.usrflags= is honored by the initrd​.

Added in version 217​.

roothash=, usrhash=

These options are primarily read by systemd-veritysetup-generator(8)​. When set this indicates that the root file system (or /usr/) shall be mounted from Verity volumes with the specified hashes​. If these kernel command line options are set the root (or /usr/) file system is thus mounted from a device mapper volume /dev/mapper/root (or /dev/mapper/usr)​.

Added in version 251​.

systemd​.volatile=

Controls whether the system shall boot up in volatile mode​. Takes a boolean argument or the special values "state" and "overlay"​.

If "no" (the default), this generator makes no changes to the mount tree and the system is booted up in normal mode​.

If "yes" the generator ensures systemd-volatile-root.service(8) is run in the initrd​. This service changes the mount table before transitioning to the host system, so that a volatile memory file system ("tmpfs") is used as root directory, with only /usr/ mounted into it from the configured root file system, in read-only mode​. This way the system operates in fully stateless mode, with all configuration and state reset at boot and lost at shutdown, as /etc/ and /var/ will be served from the (initially unpopulated) volatile memory file system​.

If set to "state" the generator will leave the root directory mount point unaltered, however will mount a "tmpfs" file system to /var/​. In this mode the normal system configuration (i​.e​. the contents of "/etc/") is in effect (and may be modified during system runtime), however the system state (i​.e​. the contents of "/var/") is reset at boot and lost at shutdown​.

If this setting is set to "overlay" the root file system is set up as "overlayfs" mount combining the read-only root directory with a writable "tmpfs", so that no modifications are made to disk, but the file system may be modified nonetheless with all changes being lost at reboot​.

Note that in none of these modes the root directory, /etc/, /var/ or any other resources stored in the root file system are physically removed​. It's thus safe to boot a system that is normally operated in non-volatile mode temporarily into volatile mode, without losing data​.

Note that with the exception of "overlay" mode, enabling this setting will only work correctly on operating systems that can boot up with only /usr/ mounted, and are able to automatically populate /etc/, and also /var/ in case of "systemd​.volatile=yes"​.

Also see root=tmpfs above, for a method to combine a "tmpfs" file system with a regular /usr/ file system (as configured via mount​.usr=)​. The main distinction between systemd​.volatile=yes, and root=tmpfs in combination mount​.usr= is that the former operates on top of a regular root file system and temporarily obstructs the files and directories above its /usr/ subdirectory, while the latter does not hide any files, but simply mounts a unpopulated tmpfs as root file system and combines it with a user picked /usr/ file system​.

Added in version 233​.

systemd​.swap=

Takes a boolean argument or enables the option if specified without an argument​. If disabled, causes the generator to ignore any swap devices configured in /etc/fstab​. Defaults to enabled​.

Added in version 246​.

systemd​.mount-extra=WHAT:WHERE[:FSTYPE[:OPTIONS]],  rd​.systemd​.mount-extra=WHAT:WHERE[:FSTYPE[:OPTIONS]]

Specifies the mount unit​. Takes at least two and at most four fields separated with a colon (":")​. Each field is handled as the corresponding fstab field​. This option can be specified multiple times​. rd​.systemd​.mount-extra= is honored only in the initrd, while systemd​.mount-extra= is honored by both the main system and the initrd​. In the initrd, the mount point (and also source path if the mount is bind mount) specified in systemd​.mount-extra= is prefixed with /sysroot/​.

Example:

systemd​.mount-extra=/dev/sda1:/mount-point:ext4:rw,noatime

Added in version 254​.

systemd​.swap-extra=WHAT[:OPTIONS],  rd​.systemd​.swap-extra=WHAT[:OPTIONS]

Specifies the swap unit​. Takes the block device to be used as a swap device, and optionally takes mount options followed by a colon (":")​. This option can be specified multiple times​. rd​.systemd​.swap-extra= is honored only in the initrd, while systemd​.swap-extra= is honored by both the main system and the initrd​.

Example:

systemd​.swap-extra=/dev/sda2:x-systemd​.makefs

Added in version 254​.

fstab​.extra

This credential may contain addition mounts to establish, in the same format as fstab(5), with one mount per line​. It is read in addition to /etc/fstab​.

Added in version 254​.

systemd(1), fstab(5), systemd.mount(5), systemd.swap(5), systemd-cryptsetup-generator(8), systemd-gpt-auto-generator(8), kernel-command-line(7), Known Environment Variables​^[1]

1.

Known Environment Variables
https://systemd.io/ENVIRONMENT/