Difference between revisions of "Fstab"

Revision as of 06:31, 14 September 2012

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The /etc/fstab file contains static filesystem information. It defines how storage devices and partitions are to be mounted and integrated into the overall system. It is read by the mount command to determine which options to use when mounting a specific device or partition.

File example

A simple /etc/fstab, using kernel name descriptors:

/etc/fstab

# <file system>        <dir>         <type>    <options>             <dump> <pass>

tmpfs                  /tmp          tmpfs     nodev,nosuid          0      0

/dev/sda1              /             ext4      defaults,noatime      0      1
/dev/sda2              none          swap      defaults              0      0
/dev/sda3              /home         ext4      defaults,noatime      0      2

Field definitions

The /etc/fstab file contains the following fields separated by a space or tab:

 <file system>        <dir>         <type>    <options>             <dump> <pass>

• <file system> - the partition or storage device to be mounted.
• <dir> - the mountpoint where <file system> is mounted to.
• <type> - the file system type of the partition or storage device to be mounted. Many different file systems are supported: ext2, ext3, ext4, reiserfs, xfs, jfs, smbfs, iso9660, vfat, ntfs, swap and auto. The auto type lets the mount command guess what type of file system is used. This is useful for optical media (CD/DVD).
• <options> - mount options of the filesystem to be used. Note that some mount options are filesystem specific. Some of the most common options are:

• auto - Mount automatically at boot, or when the command mount -a is issued.
• noauto - Mount only when you tell it to.
• exec - Allow execution of binaries on the filesystem.
• noexec - Disallow execution of binaries on the filesystem.
• ro - Mount the filesystem read-only.
• rw - Mount the filesystem read-write.
• user - Allow any user to mount the filesystem. This automatically implies noexec, nosuid, nodev, unless overridden.
• users - Allow any user in the users group to mount the filesystem.
• nouser - Allow only root to mount the filesystem.
• owner - Allow the owner of device to mount.
• sync - I/O should be done synchronously.
• async - I/O should be done asynchronously.
• dev - Interpret block special devices on the filesystem.
• nodev - Don't interpret block special devices on the filesystem.
• suid - Allow the operation of suid, and sgid bits. They are mostly used to allow users on a computer system to execute binary executables with temporarily elevated privileges in order to perform a specific task.
• nosuid - Block the operation of suid, and sgid bits.
• noatime - Don't update inode access times on the filesystem. Can help performance (see atime options).
• nodiratime - Do not update directory inode access times on the filesystem. Can help performance (see atime options).
• relatime - Update inode access times relative to modify or change time. Access time is only updated if the previous access time was earlier than the current modify or change time. (Similar to noatime, but doesn't break mutt or other applications that need to know if a file has been read since the last time it was modified.) Can help performance (see atime options).
• flush - The vfat option to flush data more often, thus making copy dialogs or progress bars to stay up until all data is written.
• defaults - the default mount options for the filesystem to be used. The default options for ext4 are: rw, suid, dev, exec, auto, nouser, async.

• <dump> - used by the dump utility to decide when to make a backup. Dump checks the entry and uses the number to decide if a file system should be backed up. Possible entries are 0 and 1. If 0, dump will ignore the file system; if 1, dump will make a backup. Most users will not have dump installed, so they should put 0 for the <dump> entry.

• <pass> - used by fsck to decide which order filesystems are to be checked. Possible entries are 0, 1 and 2. The root file system should have the highest priority 1 - all other file systems you want to have checked should have a 2. File systems with a value 0 will not be checked by the fsck utility.

Identifying filesystems

There are three ways to identify a partition or storage device in /etc/fstab: by its kernel name descriptor, label or UUID. The advantage of using UUIDs or labels is that they are not dependent on the disks order. This is useful if the storage device order in the BIOS is changed, or if you switch the storage device cabling. Also, sometimes the BIOS may occasionally change the order of storage devices. Read more about this in the Persistent block device naming article.

To list basic information about the partitions, run:

$ lsblk -f

NAME   FSTYPE LABEL      UUID                                 MOUNTPOINT
sd1                                                           
├─sda1 ext4   Arch_Linux 978e3e81-8048-4ae1-8a06-aa727458e8ff /
├─sda2 ntfs   Windows    6C1093E61093B594                     
└─sda3 ext4   Storage    f838b24e-3a66-4d02-86f4-a2e73e454336 /media/Storage
sdb                                                           
├─sdb1 ntfs   Games      9E68F00568EFD9D3                     
└─sdb2 ext4   Backup     14d50a6c-e083-42f2-b9c4-bc8bae38d274 /media/Backup
sdc                                                           
└─sdc1 vfat   Camera     47FA-4071                            /media/Camera

Kernel name

Run lsblk -f to list the partitions, and prefix them with /dev.

See the example.

Label

To label a device or partition, see this article. You can also install and use gparted, but renaming the root partition would have to be done from a "live" Linux distribution (Parted Magic, Ubuntu, etc) because the partition needs to be unmounted first.

Run lsblk -f to list the partitions, and prefix them with LABEL= :

/etc/fstab

# <file system>        <dir>         <type>    <options>             <dump> <pass>

tmpfs                  /tmp          tmpfs     nodev,nosuid          0      0
 
LABEL=Arch_Linux       /             ext4      defaults,noatime      0      1
LABEL=Arch_Swap        none          swap      defaults              0      0

UUID

All partitions and devices have a unique UUID. They are generated by filesystem utilities (e.g. mkfs.*) when you create or format a partition.

Run lsblk -f to list the partitions, and prefix them with UUID= :

$ lsblk -no UUID /dev/sda2

/etc/fstab

# <file system>                            <dir>     <type>    <options>             <dump> <pass>

tmpfs                                      /tmp      tmpfs     nodev,nosuid          0      0

UUID=24f28fc6-717e-4bcd-a5f7-32b959024e26  /         ext4      defaults,noatime      0      1
UUID=03ec5dd3-45c0-4f95-a363-61ff321a09ff  /home     ext4      defaults,noatime      0      2
UUID=4209c845-f495-4c43-8a03-5363dd433153  none      swap      defaults              0      0

Tips and tricks

Swap UUID

In case your swap partition doesn't have an UUID, you can add it manually. This happens when the UUID of the swap is not shown with the lsblk -f command. Here are some steps to assign a UUID to your swap:

Identify the swap partition:

# swapon -s

Disable the swap:

# swapoff /dev/sda7

Recreate the swap with a new UUID assigned to it:

# mkswap -U random /dev/sda7

Activate the swap:

# swapon /dev/sda7

Filepath spaces

If any mountpoint contains spaces, use the escape character \ followed by the 3 digit octal code 040 to emulate them:

/etc/fstab

UUID=47FA-4071     /home/username/Camera\040Pictures   vfat  defaults,noatime       0  2
/dev/sda7          /media/100\040GB\040(Storage)       ext4  defaults,noatime,user  0  0

External devices

External devices that are to be mounted when present but ignored if absent may require the nofail option. This prevents errors being reported at boot.

/etc/fstab

/dev/sdg1        /media/backup    jfs    defaults,nofail    0  2

atime options

The use of noatime, nodiratime or relatime can help disk performance for ext2, ext3, and ext4 filesystems. Linux by default keeps a record (writes to the disk) every time it reads from the disk atime. This was more purposeful when Linux was being used for servers; it doesn't have much value for desktop use. The worst thing about the default atime option is that even reading a file from the page cache (reading from memory instead of the disk) will still result in a disk write! Using the noatime option fully disables writing file access times to the disk every time you read a file. This works well for almost all applications, except for a rare few like Mutt that need the such information. For mutt, you should only use the relatime option. Using the relatime option enables the writing of file access times only when the file is being modified (unlike noatime where the file access time will never be changed and will be older than the modification time). The nodiratime option disables the writing of file access times only for directories while other files still get access times written. The best compromise might be the use of relatime in which case programs like Mutt will continue to work, but you'll still have a performance boost because files will not get access times updated unless they are modified.

tmpfs

tmpfs is a temporary filesystem that resides in memory and/or your swap partition(s), depending on how much you fill it up. Mounting directories as tmpfs can be an effective way of speeding up accesses to their files, or to ensure that their contents are automatically cleared upon reboot.

Some directories where tmpfs is commonly used are /tmp, /var/lock and /var/run. Do NOT use it on /var/tmp, because that folder is meant for temporary files that are preserved across reboots. Arch uses a tmpfs /run directory, and /var/run and /var/lock are already simply symlinks to it.

By default, a tmpfs partition has its maximum size set to half your total RAM, but this can be customized. Note that the actual memory/swap consumption depends on how much you fill it up, as tmpfs partitions do not consume any memory until it is actually needed.

To use tmpfs for /tmp, add this line to /etc/fstab:

/etc/fstab

tmpfs   /tmp         tmpfs   nodev,nosuid                  0  0

You may or may not want to specify the size here, but you should leave the mode option alone in these cases to ensure that they have the correct permissions (1777). In the example above, /tmp will be set to use up to half of your total RAM. To explicitly set a maximum size, use the size mount option:

/etc/fstab

tmpfs   /tmp         tmpfs   nodev,nosuid,size=2G          0  0

Here is a more advanced example showing how to add tmpfs mounts for users. This is useful for websites, mysql tmp files, ~/.vim/, and more. It's important to try and get the ideal mount options for what you are trying to accomplish. The goal is to have as secure settings as possible to prevent abuse. Limiting the size, and specifying uid and gid + mode is very secure. More info.

/etc/fstab

tmpfs   /www/cache    tmpfs  rw,size=1G,nr_inodes=5k,noexec,nodev,nosuid,uid=648,gid=648,mode=1700   0  0

See the mount command man page for more information. One useful mount option in the man page is the default option, understand that at least.

Reboot for the changes to take effect. Note that although it may be tempting to simply run mount -a to make the changes effective immediately, this will make any files currently residing in these directories inaccessible (this is especially problematic for running programs with lockfiles, for example). However, if all of them are empty, it should be safe to run mount -a instead of rebooting (or mount them individually).

After applying changes, you may want to verify that they took effect by looking at /proc/mounts and using findmnt:

$ findmnt --target /tmp

TARGET SOURCE FSTYPE OPTIONS
/tmp   tmpfs  tmpfs  rw,nosuid,nodev,relatime

Usage

Generally speaking, IO intensive tasks that benefit from fast, No-HDD-read/write-space, such as video encoding, gaming, etc. can make extensive use out of tmpfs. However, that's not to say that simpler applications can't recieve substantial gains from offloading data onto shared memory. Firefox, for example, shows that having its profile relocated into ram makes a big difference.

Improving compile times

You can tell makepkg to use a tmpfs filesystem for the build directory (this can also be set in makepkg.conf):

$ BUILDDIR=/tmp/makepkg makepkg

Writing to FAT32 as Normal User

To write on a FAT32 partition, you must make a few changes to your /etc/fstab file.

/etc/fstab

/dev/sdxY    /mnt/some_folder  vfat   user,rw,umask=000              0  0

The user flag means that any user (even non-root) can mount and unmount the partition /dev/hdX. rw gives read-write access; umask option removes selected rights - for example umask=111 remove executable rights. The problem is that this entry removes executable rights from directories too, so we must correct it by dmask=000. And why using these options? Without these options all files will be executable. You can use the option showexec instead of the umask and dmask options, which shows all Windows executables (com, exe, bat) in executable colours.

For example, if your FAT32 partition is on /dev/hda9, and you wish to mount it to /mnt/fat32, then you would use:

/etc/fstab

/dev/sda9    /mnt/fat32        vfat   user,rw,umask=111,dmask=000    0  0

See also

• tuxfiles - How to edit and understand /etc/fstab
• Full device listing including block device
• Filesystem Hierarchy Standard
• 30x Faster Web-Site Speed (Detailed tmpfs)