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Fstab

Revision as of 13:53, 19 June 2021 by Fabby (talk | contribs) (Made useful this time after some sleep)

fstab usage

The /etc/fstab file is used to define how disk partitions, various other block devices, or remote filesystems should be mounted into the filesystem.

Each filesystem is described in a separate line. These definitions will be converted into systemd mount units dynamically at boot, or when the configuration of the system manager is reloaded.

The mount --all command will mount all filesystems mentioned in fstab, (except for those whose line contains the noauto keyword). The filesystems are mounted following their order in fstab. The mount command compares filesystem source and target to detect already mounted filesystems. The kernel table with already mounted filesystems is cached during mount --all. This means that all duplicated fstab entries will be mounted.

Example File

Each line in the file describes a filesystem, and contain fields (specified in columns) used to provide information about its mountpoint, the options which should be used when mounting it etc. Each field can be separated from one another either by spaces or tabs: 

# /etc/fstab: static file system information.
# <file system>                           <mount point>  <type>  <options>                     <dump>  <pass>
LABEL=ESP                                 /boot/efi      vfat    umask=0077                         0       2
/dev/sda5                                 /              ext4    defaults,noatime,discard           0       1
UUID=18360b04-a96d-4a99-8323-b07717f36a31 swap           swap    defaults,noatime,discard           0       0
UUID=b4108631-e051-48d8-b2ff-a1d924a893f1 /home          ext4    defaults,noatime,discard           0       2
tmpfs                                     /tmp           tmpfs   defaults,noatime,mode=1777         0       0
UUID=634E43D367B0A4B1                     /media/Data    ntfs-3g noauto,x-systemd.automount,x-systemd.device-timeout=10,rw,inherit,permissions,streams_interface=windows,windows_names,compression,norecover,hide_dot_files,hide_hid_files,big_writes 0 2

Each column in the above are the fstab "fields" and will be explained in the next section.

Field definitions

Filesystem Specification

This field describes one of each of the following:

  • the block special device
  • remote filesystem
  • filesystem image for loop device to be mounted
  • swap file or swap partition to be enabled

This field generally takes the form of:

  • /dev/XdY
  • LABEL=<label>
  • UUID=<uuid>

to uniquely identify a filesystem you want to mount. As a UUID is necessarily unique, you will find that this is the onn in use on more modern systems whereas a device name and label are only in use on older systems, so we'll concentrate on UUIDs in this tutorial, but know that if you insist on using these order systems, they still work at the time of this writing.

To get the UUID for all of your devices, execute the command blkid elevated to root: 

sudo blkid

and you'll get someting like this: 

/dev/sda1: LABEL_FATBOOT="ESP" LABEL="ESP" UUID="2462-755F" BLOCK_SIZE="512" TYPE="vfat" PARTLABEL="EFI system partition" PARTUUID="b86c0cae-3055-4d9e-9e12-1fa1e2cd32d2"
/dev/sda2: PARTLABEL="Microsoft reserved partition" PARTUUID="b0679b89-007c-441b-88a3-74a39f1f8b2b"
/dev/sda3: LABEL="WINSYS" BLOCK_SIZE="512" UUID="029873F49873E497" TYPE="ntfs" PARTLABEL="Basic data partition" PARTUUID="f8d444b9-ec60-4ac8-b12a-52448119fad1"
/dev/sda4: BLOCK_SIZE="512" UUID="0EB23063B2305207" TYPE="ntfs" PARTLABEL="Basic data partition" PARTUUID="cb083fb5-d61c-48e3-a657-ceb363a983b1"
/dev/sda5: UUID="9b539186-41e9-46f0-a515-1ec6c3544367" BLOCK_SIZE="4096" TYPE="ext4" PARTUUID="7bd52fb7-4d7d-412c-bc01-e3c98c7f3bb1"
/dev/sda6: UUID="b4108631-e051-48d8-b2ff-a1d924a893f0" BLOCK_SIZE="4096" TYPE="ext4" PARTUUID="9683d669-e0fd-496a-bd9c-f0e51d0bd561"
/dev/sda7: UUID="18360b04-a96d-4a99-8323-b07717f36a30" TYPE="swap" PARTUUID="7434de88-dd5c-4ce5-a6e2-4bb65c16eab5"
/dev/sdb2: UUID="26055107-28cd-457e-9a31-46781de4065d" BLOCK_SIZE="4096" TYPE="ext4" PARTUUID="86bdde2c-b92c-43fc-811a-6d0cdc312be1"
/dev/sdb3: UUID="c9dd0f4c-5793-446e-90bb-d10e27bf4922" BLOCK_SIZE="4096" TYPE="ext4" PARTUUID="d02838bd-371c-4673-96ed-8aad73bb6d91"
/dev/sdc1: LABEL="Win-Data" BLOCK_SIZE="512" UUID="634E43D367B0A4B1" TYPE="ntfs" PARTUUID="ca8d0663-6a6e-4b09-a0d7-05b59d109ab1"

Any one of the output of blkid can be used as the first field in the fstab file.

E.G. all of the following lines are valid entries for the first line of the output of the above example: 

# <file system>                           <mount point>  <type>  <options>                     <dump>  <pass>
LABEL=ESP                                 /boot/efi      vfat    umask=0077                         0       2
UUID=2462-755F                            /boot/efi      vfat    umask=0077                         0       2
LABEL=EFI\040system\040partition          /boot/efi      vfat    umask=0077                         0       2
UUID=b86c0cae-3055-4d9e-9e12-1fa1e2cd32d2 /boot/efi      vfat    umask=0077                         0       2

I.E. only one of the above lines should be present in your fstab file to mount the ESP partition, but any of them will work! However it is strongly recommended to use the UUID as that is globally unique!. This means that if your move a disk to another machine, it will keep its UUID and there is no opportunity for surprises like the computer you've moved the existing disk to suddenly:

  • booting from the EFI System Partition on the new disk instead of its own ESP
  • failing to mount its own Windows partition because you just added another one and this takes precedence (or not: depending on the exact kernel version and filesystem driver resulting in a boot only working 50% of the time and other weird problems)
  • failing to mount a disk because the disk you just inserted into M2 slot #1 gets named /dev/sda and the one in slot #2 which used to be called /dev/sda now suddenly becomes /deb/sdb.
  • etc, etc.

Mount Point

This field describes the mount point (target) inside your filesystem. For swap partitions, this field should be specified as `none'. If the name of the mount point contains spaces or tabs these can be escaped as `\040' and '\011' respectively.

For more information on the Linux Filesystem Hierarchy standard refer to the #See Also section, but in a nutshell you should specify an existing directory on your silesystem where you want to mount a partition.

In the example fstab file above, the following line in it's blkid output: 

/dev/sdc1: LABEL="Win-Data" BLOCK_SIZE="512" UUID="634E43D367B0A4B1" TYPE="ntfs" PARTUUID="ca8d0663-6a6e-4b09-a0d7-05b59d109ab1"
  • is the Windows D: Drive
  • has an NTFS label of "Win-Data"
  • has a Windows UUID of "634E43D367B0A4B1" (different from Linux UUIDs)
  • contains the NTFS File System
  • has a partition UUID of "ca8d0663-6a6e-4b09-a0d7-05b59d109ab1"

and gets mounted on the /media/Data according to this line: 

UUID=634E43D367B0A4B1                     /media/Data    ntfs-3g noauto,x-systemd.automount,x-systemd.device-timeout=10,rw,inherit,permissions,streams_interface=windows,windows_names,compression,norecover,hide_dot_files,hide_hid_files,big_writes 0 2

Filesystem Type

This field describes the type of the filesystem. Linux supports many filesystem types: ext4, xfs, btrfs, f2fs, vfat, ntfs-3g (for Windows compatibility), hfsplus (for Mac Compatibility), tmpfs, sysfs, proc, iso9660 (to allow you to mount CD/DVD ISO files as Read-Only directories), udf, squashfs, nfs, cifs, and many more.

An entry swap denotes a file or partition to be used for swapping. An entry none is useful for bind or move mounts.

More than one type may be specified in a comma-separated list.

Using the same example fstab file and "Win-Data" line as above, ntfs-3g is the file system we're mounting this one under although the blkid shows ntfs. This is simply because the ntfs driver is a read-only driver (allowing you to see your Windows files, but not modify them, whereas the ntfs-3g (for 3rd generation) allows most NTFS options (except encryption) thus also read-write capabilities: 

UUID=634E43D367B0A4B1                     /media/Data    ntfs-3g noauto,x-systemd.automount,x-systemd.device-timeout=10,rw,inherit,permissions,streams_interface=windows,windows_names,compression,norecover,hide_dot_files,hide_hid_files,big_writes 0 2

Mount options

This field describes the mount options associated with the filesystem.

It is formatted as a comma-separated list of options. It contains at least the type of mount (ro or rw), plus any additional options appropriate to the filesystem type (including performance-tuning options).

Basic filesystem-independent options are:

  • defaults: use default options: rw, suid, dev, exec, auto, nouser, and async.
  • noauto: do not mount when mount -a is given (I.E. at boot time)
  • user: allow any user to mount (I.E. no sudo needed)
  • owner: Only allow device owner to mount
  • comment: or x-<name> for use by fstab-maintaining programs
  • nofail: do not report errors for this device if it does not exist.

EXT2/3/4 Mount options

TBD

NTFS-3G Mount options

TBD

Dump?

This field must be 1 or 0 and is used by dump to determine which filesystems need to be dumped in case of a coredump (=crash). Defaults to zero (don’t dump) if not present.

Pass number

This field is used by fsck(8) to determine the order in which filesystem checks are done at boot time. The root filesystem should be specified with a fs_passno of 1. Other filesystems should have a fs_passno of 2. Filesystems within a drive will be checked sequentially, but filesystems on different drives will be checked at the same time to utilize parallelism available in the hardware.

Defaults to zero (don’t check the filesystem) if not present.

Tips and tricks

The default setup will automatically `fsck` and `mount` filesystems before starting services that need them to be mounted. For example, systemd automatically makes sure that remote filesystem mounts like NFS or Samba are only started after the network has been set up. Therefore, local and remote filesystem mounts specified in `/etc/fstab` should work out of the box. See Template:Man 5 systemd.mount for details.

See Also

https://refspecs.linuxfoundation.org/FHS_3.0/index.html

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