425 lines
14 KiB
ReStructuredText
425 lines
14 KiB
ReStructuredText
btrfs-man5(5)
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=============
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DESCRIPTION
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-----------
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This document describes topics related to BTRFS that are not specific to the
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tools. Currently covers:
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#. mount options
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#. filesystem features
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#. checksum algorithms
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#. compression
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#. filesystem exclusive operations
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#. filesystem limits
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#. bootloader support
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#. file attributes
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#. zoned mode
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#. control device
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#. filesystems with multiple block group profiles
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#. seeding device
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#. raid56 status and recommended practices
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#. storage model, hardware considerations
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MOUNT OPTIONS
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-------------
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.. include:: ch-mount-options.rst
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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 or checksum
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algorithm, see ``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 /dev/sdx** 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 is stored
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inside the file. The value *1* is for enabled and active, while *0* means the
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feature was enabled at mount time but turned off afterwards.
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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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the filesystem uses *nodesize* for metadata blocks, this can be bigger than the
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page size
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compress_lzo
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(since: 2.6.38)
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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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compress_zstd
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(since: 4.14)
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the *zstd* 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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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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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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free_space_tree
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(since: 4.5)
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free space representation using a dedicated b-tree, successor of v1 space cache
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metadata_uuid
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(since: 5.0)
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the main filesystem UUID is the metadata_uuid, which stores the new UUID only
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in the superblock while all metadata blocks still have the UUID set at mkfs
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time, see ``btrfstune(8)`` for more
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mixed_backref
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(since: 2.6.31)
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the last major disk format change, improved backreferences, now default
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mixed_groups
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(since: 2.6.37)
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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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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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raid1c34
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(since: 5.5)
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extended RAID1 mode with copies on 3 or 4 devices respectively
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raid56
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(since: 3.9)
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the filesystem contains or contained a raid56 profile of block groups
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rmdir_subvol
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(since: 4.18)
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indicate that ``rmdir(2)`` syscall can delete an empty subvolume just like an
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ordinary directory. Note that this feature only depends on the kernel version.
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skinny_metadata
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(since: 3.10)
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reduced-size metadata for extent references, saves a few percent of metadata
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send_stream_version
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(since: 5.10)
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number of the highest supported send stream version
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supported_checksums
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(since: 5.5)
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list of checksum algorithms supported by the kernel module, the respective
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modules or built-in implementing the algorithms need to be present to mount
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the filesystem, see *CHECKSUM ALGORITHMS*
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supported_sectorsizes
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(since: 5.13)
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list of values that are accepted as sector sizes (**mkfs.btrfs --sectorsize**) by
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the running kernel
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supported_rescue_options
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(since: 5.11)
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list of values for the mount option *rescue* that are supported by the running
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kernel, see ``btrfs(5)``
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zoned
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(since: 5.12)
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zoned mode is allocation/write friendly to host-managed zoned devices,
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allocation space is partitioned into fixed-size zones that must be updated
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sequentially, see *ZONED MODE*
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SWAPFILE SUPPORT
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----------------
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.. include:: ch-swapfile.rst
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CHECKSUM ALGORITHMS
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-------------------
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.. include:: ch-checksumming.rst
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COMPRESSION
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-----------
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.. include:: ch-compression.rst
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FILESYSTEM EXCLUSIVE OPERATIONS
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-------------------------------
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There are several operations that affect the whole filesystem and cannot be run
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in parallel. Attempt to start one while another is running will fail (see
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exceptions below).
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Since kernel 5.10 the currently running operation can be obtained from
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*/sys/fs/UUID/exclusive_operation* with following values and operations:
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* balance
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* balance paused (since 5.17)
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* device add
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* device delete
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* device replace
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* resize
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* swapfile activate
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* none
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Enqueuing is supported for several btrfs subcommands so they can be started
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at once and then serialized.
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There's an exception when a paused balance allows to start a device add
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operation as they don't really collide and this can be used to add more space
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for the balance to finish.
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FILESYSTEM LIMITS
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-----------------
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.. include:: ch-fs-limits.rst
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BOOTLOADER SUPPORT
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------------------
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.. include:: ch-bootloaders.rst
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FILE ATTRIBUTES
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---------------
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.. include:: ch-file-attributes.rst
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ZONED MODE
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----------
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.. include:: ch-zoned-intro.rst
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CONTROL DEVICE
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--------------
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There's a character special device */dev/btrfs-control* with major and minor
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numbers 10 and 234 (the device can be found under the 'misc' category).
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.. code-block:: none
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$ ls -l /dev/btrfs-control
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crw------- 1 root root 10, 234 Jan 1 12:00 /dev/btrfs-control
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The device accepts some ioctl calls that can perform following actions on the
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filesystem module:
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* scan devices for btrfs filesystem (ie. to let multi-device filesystems mount
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automatically) and register them with the kernel module
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* similar to scan, but also wait until the device scanning process is finished
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for a given filesystem
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* get the supported features (can be also found under */sys/fs/btrfs/features*)
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The device is created when btrfs is initialized, either as a module or a
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built-in functionality and makes sense only in connection with that. Running
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eg. mkfs without the module loaded will not register the device and will
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probably warn about that.
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In rare cases when the module is loaded but the device is not present (most
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likely accidentally deleted), it's possible to recreate it by
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.. code-block:: bash
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# mknod --mode=600 /dev/btrfs-control c 10 234
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or (since 5.11) by a convenience command
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.. code-block:: bash
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# btrfs rescue create-control-device
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The control device is not strictly required but the device scanning will not
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work and a workaround would need to be used to mount a multi-device filesystem.
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The mount option *device* can trigger the device scanning during mount, see
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also **btrfs device scan**.
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FILESYSTEM WITH MULTIPLE PROFILES
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---------------------------------
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It is possible that a btrfs filesystem contains multiple block group profiles
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of the same type. This could happen when a profile conversion using balance
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filters is interrupted (see ``btrfs-balance(8)``). Some **btrfs** commands perform
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a test to detect this kind of condition and print a warning like this:
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.. code-block:: none
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WARNING: Multiple block group profiles detected, see 'man btrfs(5)'.
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WARNING: Data: single, raid1
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WARNING: Metadata: single, raid1
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The corresponding output of **btrfs filesystem df** might look like:
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.. code-block:: none
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WARNING: Multiple block group profiles detected, see 'man btrfs(5)'.
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WARNING: Data: single, raid1
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WARNING: Metadata: single, raid1
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Data, RAID1: total=832.00MiB, used=0.00B
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Data, single: total=1.63GiB, used=0.00B
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System, single: total=4.00MiB, used=16.00KiB
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Metadata, single: total=8.00MiB, used=112.00KiB
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Metadata, RAID1: total=64.00MiB, used=32.00KiB
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GlobalReserve, single: total=16.25MiB, used=0.00B
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There's more than one line for type *Data* and *Metadata*, while the profiles
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are *single* and *RAID1*.
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This state of the filesystem OK but most likely needs the user/administrator to
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take an action and finish the interrupted tasks. This cannot be easily done
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automatically, also the user knows the expected final profiles.
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In the example above, the filesystem started as a single device and *single*
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block group profile. Then another device was added, followed by balance with
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*convert=raid1* but for some reason hasn't finished. Restarting the balance
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with *convert=raid1* will continue and end up with filesystem with all block
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group profiles *RAID1*.
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.. note::
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If you're familiar with balance filters, you can use
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*convert=raid1,profiles=single,soft*, which will take only the unconverted
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*single* profiles and convert them to *raid1*. This may speed up the conversion
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as it would not try to rewrite the already convert *raid1* profiles.
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Having just one profile is desired as this also clearly defines the profile of
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newly allocated block groups, otherwise this depends on internal allocation
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policy. When there are multiple profiles present, the order of selection is
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RAID6, RAID5, RAID10, RAID1, RAID0 as long as the device number constraints are
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satisfied.
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Commands that print the warning were chosen so they're brought to user
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attention when the filesystem state is being changed in that regard. This is:
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**device add**, **device delete**, **balance cancel**, **balance pause**. Commands
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that report space usage: **filesystem df**, **device usage**. The command
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**filesystem usage** provides a line in the overall summary:
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.. code-block:: none
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Multiple profiles: yes (data, metadata)
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SEEDING DEVICE
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--------------
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.. include:: ch-seeding-device.rst
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RAID56 STATUS AND RECOMMENDED PRACTICES
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---------------------------------------
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The RAID56 feature provides striping and parity over several devices, same as
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the traditional RAID5/6. There are some implementation and design deficiencies
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that make it unreliable for some corner cases and the feature **should not be
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used in production, only for evaluation or testing**. The power failure safety
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for metadata with RAID56 is not 100%.
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Metadata
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^^^^^^^^
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Do not use *raid5* nor *raid6* for metadata. Use *raid1* or *raid1c3*
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respectively.
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The substitute profiles provide the same guarantees against loss of 1 or 2
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devices, and in some respect can be an improvement. Recovering from one
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missing device will only need to access the remaining 1st or 2nd copy, that in
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general may be stored on some other devices due to the way RAID1 works on
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btrfs, unlike on a striped profile (similar to *raid0*) that would need all
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devices all the time.
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The space allocation pattern and consumption is different (eg. on N devices):
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for *raid5* as an example, a 1GiB chunk is reserved on each device, while with
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*raid1* there's each 1GiB chunk stored on 2 devices. The consumption of each
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1GiB of used metadata is then *N * 1GiB* for vs *2 * 1GiB*. Using *raid1*
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is also more convenient for balancing/converting to other profile due to lower
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requirement on the available chunk space.
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Missing/incomplete support
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^^^^^^^^^^^^^^^^^^^^^^^^^^
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When RAID56 is on the same filesystem with different raid profiles, the space
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reporting is inaccurate, eg. **df**, **btrfs filesystem df** or **btrfs filesystem
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usage**. When there's only a one profile per block group type (eg. raid5 for data)
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the reporting is accurate.
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When scrub is started on a RAID56 filesystem, it's started on all devices that
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degrade the performance. The workaround is to start it on each device
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separately. Due to that the device stats may not match the actual state and
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some errors might get reported multiple times.
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The *write hole* problem. An unclean shutdown could leave a partially written
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stripe in a state where the some stripe ranges and the parity are from the old
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writes and some are new. The information which is which is not tracked. Write
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journal is not implemented. Alternatively a full read-modify-write would make
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sure that a full stripe is always written, avoiding the write hole completely,
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but performance in that case turned out to be too bad for use.
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The striping happens on all available devices (at the time the chunks were
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allocated), so in case a new device is added it may not be utilized
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immediately and would require a rebalance. A fixed configured stripe width is
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not implemented.
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STORAGE MODEL, HARDWARE CONSIDERATIONS
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--------------------------------------
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.. include:: ch-hardware-considerations.rst
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SEE ALSO
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--------
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``acl(5)``,
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``btrfs(8)``,
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``chattr(1)``,
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``fstrim(8)``,
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``ioctl(2)``,
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``mkfs.btrfs(8)``,
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``mount(8)``,
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``swapon(8)``
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