Difference between revisions of "Fstab"
(→External devices: rm specific version clutter) |
m (wikipedia:Stat (system call)#Solutions: No Solutions section. Solutions -> Criticism of atime) |
||
| Line 187: | Line 187: | ||
: {{Note|{{ic|noatime}} already includes {{ic|nodiratime}}. [http://lwn.net/Articles/244941/ You do not need to specify both].}} | : {{Note|{{ic|noatime}} already includes {{ic|nodiratime}}. [http://lwn.net/Articles/244941/ You do not need to specify both].}} | ||
| − | * {{ic|relatime}} enables the writing of file access times only when the file is being modified (unlike {{ic|noatime}} where the file access time will never be changed and will be older than the modification time). The best compromise might be the use this option since programs like [[Mutt]] will continue to work, but you will still have a performance boost as the files will not get access times updated unless they are modified. This option is used when the {{ic|defaults}} keyword option, {{ic|atime}} option (which means to use the kernel default, which is relatime; see {{ic|man 8 mount}} and [[wikipedia:Stat (system call)# | + | * {{ic|relatime}} enables the writing of file access times only when the file is being modified (unlike {{ic|noatime}} where the file access time will never be changed and will be older than the modification time). The best compromise might be the use this option since programs like [[Mutt]] will continue to work, but you will still have a performance boost as the files will not get access times updated unless they are modified. This option is used when the {{ic|defaults}} keyword option, {{ic|atime}} option (which means to use the kernel default, which is relatime; see {{ic|man 8 mount}} and [[wikipedia:Stat (system call)#Criticism of atime]]) or no options at all are specified in ''fstab'' for a given mount point. |
The {{ic|noatime}} and {{ic|relatime}} options avoid writes to the file system when files are read but not modified, which results in better performance. | The {{ic|noatime}} and {{ic|relatime}} options avoid writes to the file system when files are read but not modified, which results in better performance. | ||
Revision as of 01:17, 23 July 2016
ro:Fstab zh-CN:Fstab zh-TW:Fstab
The /etc/fstab file can be 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, and when the configuration of the system manager is reloaded. 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 man 5 systemd.mount for details.
The mount command will use fstab, if just one of either directory or device is given, to fill in the value for the other parameter. When doing so, mount options which are listed in fstab will also be used.
File example
A simple /etc/fstab, using kernel name descriptors:
/etc/fstab
# <file system> <dir> <type> <options> <dump> <pass> /dev/sda1 / ext4 defaults,noatime 0 1 /dev/sda2 none swap defaults 0 0 /dev/sda3 /home ext4 defaults,noatime 0 2
Field definitions
Each line in the /etc/fstab file contains the following fields separated by spaces or tabs:
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,btrfs,reiserfs,xfs,jfs,smbfs,iso9660,vfat,ntfs,swapandauto. Theautotype 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. See the mount man page. Please note that some options are specific to filesystems; to discover them see below in the aforementioned mount man page.
- 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 (unless its type is btrfs, in which case this field should be 0) - 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 different ways to identify filesystems that will be mounted. /etc/fstab does support several methods: kernel name descriptor, label or UUID, and GPT labels and UUID for GPT disks. UUID must be privileged over kernel name descriptors and labels. See Persistent block device naming for more explanations. It is recommended to read that article first before continuing with this article.
In this section, we will describe how to mount filesystems using all the mount methods available via examples. The output of the commands lsblk -f and blkid used in the following examples are available in the article Persistent block device naming. If you have not read that article yet, please read it now.
Kernel name descriptors
Run lsblk -f to list the partitions and prefix the values in the NAME column with /dev/.
/etc/fstab
# <file system> <dir> <type> <options> <dump> <pass> /dev/sda1 /boot vfat rw,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro 0 2 /dev/sda2 / ext4 rw,relatime,discard,data=ordered 0 1 /dev/sda3 /home ext4 rw,relatime,discard,data=ordered 0 2 /dev/sda4 none swap defaults 0 0
Labels
Run lsblk -f to list the partitions, and prefix the values in the LABEL column with LABEL=:
/etc/fstab
# <file system> <dir> <type> <options> <dump> <pass> LABEL=EFI /boot vfat rw,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro 0 2 LABEL=SYSTEM / ext4 rw,relatime,discard,data=ordered 0 1 LABEL=DATA /home ext4 rw,relatime,discard,data=ordered 0 2 LABEL=SWAP none swap defaults 0 0
UUIDs
Run lsblk -f to list the partitions, and prefix the values in the UUID column with UUID=:
/etc/fstab
# <file system> <dir> <type> <options> <dump> <pass> UUID=CBB6-24F2 /boot vfat rw,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro 0 2 UUID=0a3407de-014b-458b-b5c1-848e92a327a3 / ext4 rw,relatime,discard,data=ordered 0 1 UUID=b411dc99-f0a0-4c87-9e05-184977be8539 /home ext4 rw,relatime,discard,data=ordered 0 2 UUID=f9fe0b69-a280-415d-a03a-a32752370dee none swap defaults 0 0
GPT labels
Run blkid to list the partitions, and use the PARTLABEL values without the quotes:
/etc/fstab
# <file system> <dir> <type> <options> <dump> <pass> PARTLABEL=EFI\040SYSTEM\040PARTITION /boot vfat rw,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro 0 2 PARTLABEL=GNU/LINUX / ext4 rw,relatime,discard,data=ordered 0 1 PARTLABEL=HOME /home ext4 rw,relatime,discard,data=ordered 0 2 PARTLABEL=SWAP none swap defaults 0 0
GPT UUIDs
Run blkid to list the partitions, and use the PARTUUID values without the quotes:
/etc/fstab
# <file system> <dir> <type> <options> <dump> <pass> PARTUUID=d0d0d110-0a71-4ed6-936a-304969ea36af /boot vfat rw,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro 0 2 PARTUUID=98a81274-10f7-40db-872a-03df048df366 / ext4 rw,relatime,discard,data=ordered 0 1 PARTUUID=7280201c-fc5d-40f2-a9b2-466611d3d49e /home ext4 rw,relatime,discard,data=ordered 0 2 PARTUUID=039b6c1c-7553-4455-9537-1befbc9fbc5b none swap defaults 0 0
Tips and tricks
Automount with systemd
If you have a large /home partition, it might be better to allow services that do not depend on /home to start while /home is checked by fsck. This can be achieved by adding the following options to the /etc/fstab entry of your /home partition:
noauto,x-systemd.automount
This will fsck and mount /home when it is first accessed, and the kernel will buffer all file access to /home until it is ready.
/home filesystem type autofs, which is ignored by mlocate by default. The speedup of automounting /home may not be more than a second or two, depending on your system, so this trick may not be worth it.The same applies to remote filesystem mounts. If you want them to be mounted only upon access, you will need to use the noauto,x-systemd.automount parameters. In addition, you can use the x-systemd.device-timeout=# option to specify a timeout in case the network resource is not available.
exec flag with automount, you should remove the user flag for it to work properly as found in the course of a Fedora Bug ReportIf you have encrypted filesystems with keyfiles, you can also add the noauto parameter to the corresponding entries in /etc/crypttab. systemd will then not open the encrypted device on boot, but instead wait until it is actually accessed and then automatically open it with the specified keyfile before mounting it. This might save a few seconds on boot if you are using an encrypted RAID device for example, because systemd does not have to wait for the device to become available. For example:
/etc/crypttab
data /dev/md0 /root/key noauto
You may also specify an idle timeout for a mount with the x-systemd.idle-timeout flag. For example:
noauto,x-systemd.automount,x-systemd.idle-timeout=1min
This will make systemd unmount the mount after it has been idle for 1 minute.
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. For example:
/etc/fstab
/dev/sdg1 /media/backup jfs defaults,nofail,x-systemd.device-timeout=1 0 2
The nofail option is best combined with the x-systemd.device-timeout option. This is because the default device timeout is 90 seconds, so a disconnected external device with only nofail will make your boot take 90 seconds longer, unless you reconfigure the timeout as shown. Make sure not to set the timeout to 0, as this translates to infinite timeout.
If your external device requires another systemd unit to be loaded (for example the network for a network share) you can use x-systemd.requires=x combined with x-systemd.automountto postpone automounting until after the unit is available. For example:
/etc/fstab
//host/share /net/share cifs noauto,nofail,x-systemd.automount,x-systemd.requires=network-online.target,x-systemd.device-timeout=10,workgroup=workgroup,credentials=/foo/credentials 0 0
Filepath spaces
Since spaces are used in fstab to delimit fields, if any field (PARTLABEL, LABEL or the mount point) contains spaces, these spaces must be replaced by escape characters \ followed by the 3 digit octal code 040:
/etc/fstab
UUID=47FA-4071 /home/username/Camera\040Pictures vfat defaults,noatime 0 0 /dev/sda7 /media/100\040GB\040(Storage) ext4 defaults,noatime,user 0 2
atime options
Below atime options can impact drive performance.
- The
strictatimeoption updates the atime of the files every time they are accessed. This is more purposeful when Linux is used for servers; it does not have much value for desktop use. The drawback about thestrictatimeoption is that even reading a file from the page cache (reading from memory instead of the drive) will still result in a write!
- The
noatimeoption fully disables writing file access times to the drive every time you read a file. This works well for almost all applications, except for a rare few like Mutt that needs such information. For mutt, you should only use therelatimeoption. The write time information to a file will continue to be updated anytime the file is written to with this option enabled.
- The
nodiratimeoption disables the writing of file access times only for directories while other files still get access times written.
relatimeenables the writing of file access times only when the file is being modified (unlikenoatimewhere the file access time will never be changed and will be older than the modification time). The best compromise might be the use this option since programs like Mutt will continue to work, but you will still have a performance boost as the files will not get access times updated unless they are modified. This option is used when thedefaultskeyword option,atimeoption (which means to use the kernel default, which is relatime; seeman 8 mountand wikipedia:Stat (system call)#Criticism of atime) or no options at all are specified in fstab for a given mount point.
The noatime and relatime options avoid writes to the file system when files are read but not modified, which results in better performance.
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/sdX. 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. See also Umask.
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/sda9, 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
Now, any user can mount it with:
$ mount /mnt/fat32
And unmount it with:
$ umount /mnt/fat32
Remounting the root partition
If for some reason the root partition has been improperly mounted read only, remount the root partition with read-write access with the following command:
# mount -o remount,rw /
bind mounts
rm -r * will also erase any content from the original location. So softlinks should be the preferable way in most cases. If you need permission to a directory on a Btrfs and softlinks are not sufficient its subvolumes faciliate extended capabilities like mount options compared to bind mountingSometimes programs or users cannot access one specific directory due to insufficient permissions. One feasable possibility to give the program access to this directory is bind mounting it to a location the program can access. If a program has permission to access directory bar but not to directory foo, under some circumstances the access can be granted without any permission hassle by adding an entry to /etc/fstab:
/etc/fstab
/<path to foo> /<path to bar> none bind 0 0