btrfs-progs: docs: describe filesystem features
Signed-off-by: David Sterba <dsterba@suse.com>
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David Sterba
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@ -12,9 +12,12 @@ tools. Currently covers:
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1. mount options
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2. file attributes
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2. filesystem features
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3. file attributes
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4. control device
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3. control device
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MOUNT OPTIONS
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-------------
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@ -406,6 +409,91 @@ NOTE: This option has replaced 'recovery'.
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Allow subvolumes to be deleted by their respective owner. Otherwise, only the
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root user can do that.
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FILESYSTEM FEATURES
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-------------------
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The basic set of filesystem features gets extended over time. The backward
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compatibility is maintained and the features are optional, need to be
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explicitly asked for so accidental use will not create incompatibilities.
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There are several classes and the respective tools to manage the features:
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at mkfs time only::
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This is namely for core structures, like the b-tree nodesize, see
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`mkfs.btrfs`(8) for more details.
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after mkfs, on an unmounted filesystem::
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Features that may optimize internal structures or add new structures to support
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new functionality, see `btrfstune`(8). The command *btrfs inspect-internal
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dump-super device* will dump a superblock, you can map the value of
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'incompat_flags' to the features listed below
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after mkfs, on a mounted filesystem::
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The features of a filesystem (with a given UUID) are listed in
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`/sys/fs/btrfs/UUID/features/`, one file per feature. The status of is stored
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insid the file. The value '1' is for enabled, '0' means the feature was had
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been enabled at the mount time and turned off afterwards.
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+
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Whether a particular feature can be turned on a mounted filesystem can be found
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in the directory `/sys/fs/btrfs/features/`, one file per feature. The value '1'
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means the feature can be enabled.
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List of features (see also `mkfs.btrfs`(8) section 'FILESYSTEM FEATURES'):
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big_metadata::
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(since: 3.4)
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+
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the filesystem uses 'nodesize' bigger than the page size
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compress_lzo::
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(since: 2.6.38)
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+
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the 'lzo' compression has been used on the filesystem, either as a mount option
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or via *btrfs filesystem defrag*.
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default_subvol::
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(since: 2.6.34)
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+
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the default subvolume has been set on the filesystem
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extended_iref::
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(since: 3.7)
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+
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increased hardlink limit per file in a directory to 65536, older kernels
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supported a varying number of hardlinks depending on the sum of all file name
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sizes that can be stored into one metadata block
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mixed_backref::
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(since: 2.6.31)
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+
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the last major disk format change, improved backreferences
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mixed_groups::
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(since: 2.6.37)
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+
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mixed data and metadata block groups, ie. the data and metadata are not
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separated and occupy the same block groups, this mode is suitable for small
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volumes as there are no constraints how the remaining space should be used
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(compared to the split mode, where empty metadata space cannot be used for data
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and vice versa)
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+
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on the other hand, the final layout is quite unpredictable and possibly highly
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fragmented, which means worse performance
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no_holes::
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(since: 3.14)
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improved representation of file extents where holes are not explicitly
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stored as an extent, saves a few percent of metadata if sparse files are used
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raid56::
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(since: 3.9)
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+
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the filesystem contains or contained a raid56 profile of block groups
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+
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skinny_metadata::
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(since: 3.10)
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+
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reduced-size metadata for extent references, saves a few percent of metadata
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FILE ATTRIBUTES
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---------------
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The btrfs filesystem supports setting the following file attributes using the
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@ -17,7 +17,11 @@ parameters. The filesystem must be unmounted.
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The common usecase is to enable features that were not enabled at mkfs time.
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Please make sure that you have kernel support for the features. You can find a
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complete list of features and kernel version of their introduction at
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https://btrfs.wiki.kernel.org/index.php/Changelog#By_feature .
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https://btrfs.wiki.kernel.org/index.php/Changelog#By_feature . Also, the
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manual page `mkfs.btrfs`(8) contains more details about the features.
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Some of the features could be enabled on a mounted filesytem. Please refer to
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the respective section in `btrfs`(5).
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OPTIONS
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-------
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@ -26,14 +30,24 @@ Enable seeding on a given device. Value 1 will enable seeding, 0 will disable it
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A seeding filesystem is forced to be mounted read-only. A new device can be added
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to the filesystem and will capture all writes keeping the seeding device intact.
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-r::
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(since kernel: 3.7)
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+
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Enable extended inode refs (hardlink limit per file in a directory is 65536),
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enabled by mkfs feature 'extref'. Since kernel 3.7.
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enabled by mkfs feature 'extref'.
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-x::
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(since kernel: 3.10)
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+
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Enable skinny metadata extent refs (more efficient representation of extents),
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enabled by mkfs feature 'skinny-metadata'. Since kernel 3.10.
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enabled by mkfs feature 'skinny-metadata'.
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+
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All newly created extents will use the new representation. To completely switch
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the entire filesystem, run a full balance of the metadata. Please refer to
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`btrfs-balance`(8).
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-n::
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(since kernel: 3.14)
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+
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Enable no-holes feature (more efficient representation of file holes), enabled
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by mkfs feature 'no-holes'. Since kernel 3.14.
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by mkfs feature 'no-holes'.
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-f::
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Allow dangerous changes, e.g. clear the seeding flag or change fsid. Make sure
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that you are aware of the dangers.
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@ -72,4 +86,6 @@ will become deprecated and removed afterwards.
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SEE ALSO
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--------
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`btrfs`(5),
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`btrfs-balance`(8),
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`mkfs.btrfs`(8)
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@ -185,7 +185,12 @@ initramfs/initrd systems.
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FILESYSTEM FEATURES
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-------------------
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Features that can be enabled during creation time. See also `btrfs`(5) section
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'FILESYSTEM FEATURES'.
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*mixed-bg*::
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(kernel support since 2.6.37)
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+
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mixed data and metadata block groups, also set by option '--mixed'
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*extref*::
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@ -196,6 +201,8 @@ supported a varying number of hardlinks depending on the sum of all file name
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sizes that can be stored into one metadata block
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*raid56*::
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(kernel support since 3.9)
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+
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extended format for RAID5/6, also enabled if raid5 or raid6 block groups
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are selected
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@ -205,6 +212,8 @@ are selected
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reduced-size metadata for extent references, saves a few percent of metadata
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*no-holes*::
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(kernel support since 3.14)
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+
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improved representation of file extents where holes are not explicitly
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stored as an extent, saves a few percent of metadata if sparse files are used
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@ -351,4 +360,6 @@ further details.
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SEE ALSO
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--------
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`btrfs`(8), `wipefs`(8)
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`btrfs`(5),
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`btrfs`(8),
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`wipefs`(8)
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