541 lines
22 KiB
ReStructuredText
541 lines
22 KiB
ReStructuredText
mkfs.btrfs(8)
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=============
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SYNOPSIS
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--------
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**mkfs.btrfs** [options] <device> [<device>...]
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DESCRIPTION
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-----------
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:command:`mkfs.btrfs` is used to create the btrfs filesystem on a single or multiple
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devices. The *device* is typically a block device but can be a file-backed image
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as well. Multiple devices are grouped by UUID of the filesystem.
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Before mounting such filesystem, the kernel module must know all the devices
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either via preceding execution of :command:`btrfs device scan` or using the *device*
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mount option. See section *MULTIPLE DEVICES* for more details.
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The default block group profiles for data and metadata depend on number of
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devices and possibly other factors. It's recommended to use specific profiles
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but the defaults should be OK and allowing future conversions to other profiles.
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Please see options *-d* and *-m* for further details and :doc:`btrfs-balance(8)<btrfs-balance>` for
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the profile conversion post mkfs.
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OPTIONS
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-------
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-b|--byte-count <size>
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Specify the size of each device as seen by the filesystem. If not set,
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the entire device size is used. The total filesystem size will be sum
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of all device sizes, for a single device filesystem the option
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effectively specifies the size of the filesystem.
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--csum <type>, --checksum <type>
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Specify the checksum algorithm. Default is *crc32c*. Valid values are *crc32c*,
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*xxhash*, *sha256* or *blake2*. To mount such filesystem kernel must support the
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checksums as well. See *CHECKSUM ALGORITHMS* in :doc:`btrfs(5)<btrfs-man5>`.
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-d|--data <profile>
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Specify the profile for the data block groups. Valid values are *raid0*,
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*raid1*, *raid1c3*, *raid1c4*, *raid5*, *raid6*, *raid10* or *single* or *dup*
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(case does not matter).
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See *DUP PROFILES ON A SINGLE DEVICE* for more details.
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On multiple devices, the default was *raid0* until version 5.7, while it is
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*single* since version 5.8. You can still select *raid0* manually, but it was not
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suitable as default.
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-m|--metadata <profile>
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Specify the profile for the metadata block groups.
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Valid values are *raid0*, *raid1*, *raid1c3*, *raid1c4*, *raid5*, *raid6*,
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*raid10*, *single* or *dup* (case does not matter).
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Default on a single device filesystem is *DUP* and is recommended for metadata
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in general. The duplication might not be necessary in some use cases and it's
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up to the user to changed that at mkfs time or later. This depends on hardware
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that could potentially deduplicate the blocks again but this cannot be detected
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at mkfs time.
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.. note::
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Up to version 5.14 there was a detection of a SSD device (more precisely
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if it's a rotational device, determined by the contents of file
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*/sys/block/DEV/queue/rotational*) that used to select *single*. This has
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changed in version 5.15 to be always *dup*.
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Note that the rotational status can be arbitrarily set by the underlying block
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device driver and may not reflect the true status (network block device, memory-backed
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SCSI devices, real block device behind some additional device mapper layer,
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etc). It's recommended to always set the options *--data/--metadata* to avoid
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confusion and unexpected results.
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See *DUP PROFILES ON A SINGLE DEVICE* for more details.
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On multiple devices the default is *raid1*.
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-M|--mixed
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Normally the data and metadata block groups are isolated. The *mixed* mode
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will remove the isolation and store both types in the same block group type.
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This helps to utilize the free space regardless of the purpose and is suitable
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for small devices. The separate allocation of block groups leads to a situation
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where the space is reserved for the other block group type, is not available for
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allocation and can lead to ENOSPC state.
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The recommended size for the mixed mode is for filesystems less than 1GiB. The
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soft recommendation is to use it for filesystems smaller than 5GiB. The mixed
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mode may lead to degraded performance on larger filesystems, but is otherwise
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usable, even on multiple devices.
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The *nodesize* and *sectorsize* must be equal, and the block group types must
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match.
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.. note::
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Versions up to 4.2.x forced the mixed mode for devices smaller than 1GiB.
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This has been removed in 4.3+ as it caused some usability issues.
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Mixed profile cannot be used together with other profiles. It can only
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be set at creation time. Conversion to or from mixed profile is not
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implemented.
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-n|--nodesize <size>
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Specify the nodesize, the tree block size in which btrfs stores metadata. The
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default value is 16KiB (16384) or the page size, whichever is bigger. Must be a
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multiple of the sectorsize and a power of 2, but not larger than 64KiB (65536).
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Leafsize always equals nodesize and the options are aliases.
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Smaller node size increases fragmentation but leads to taller b-trees which in
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turn leads to lower locking contention. Higher node sizes give better packing
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and less fragmentation at the cost of more expensive memory operations while
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updating the metadata blocks.
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.. note::
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Versions up to 3.11 set the nodesize to 4KiB.
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-s|--sectorsize <size>
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Specify the sectorsize, the minimum data block allocation unit.
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The default value is the page size and is autodetected. If the sectorsize
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differs from the page size, the created filesystem may not be mountable by the
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running kernel. Therefore it is not recommended to use this option unless you
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are going to mount it on a system with the appropriate page size.
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-L|--label <string>
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Specify a label for the filesystem. The *string* should be less than 256
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bytes and must not contain newline characters.
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-K|--nodiscard
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Do not perform whole device TRIM operation on devices that are capable of that.
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This does not affect discard/trim operation when the filesystem is mounted.
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Please see the mount option *discard* for that in :doc:`btrfs(5)<btrfs-man5>`.
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-r|--rootdir <rootdir>
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Populate the toplevel subvolume with files from *rootdir*. This does not
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require root permissions to write the new files or to mount the filesystem.
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.. note::
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This option may enlarge the image or file to ensure it's big enough to
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contain the files from *rootdir*. Since version 4.14.1 the filesystem size is
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not minimized. Please see option *--shrink* if you need that functionality.
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--shrink
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Shrink the filesystem to its minimal size, only works with *--rootdir* option.
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If the destination block device is a regular file, this option will also
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truncate the file to the minimal size. Otherwise it will reduce the filesystem
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available space. Extra space will not be usable unless the filesystem is
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mounted and resized using :command:`btrfs filesystem resize`.
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.. note::
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Prior to version 4.14.1, the shrinking was done automatically.
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-O|--features <feature1>[,<feature2>...]
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A list of filesystem features turned on at mkfs time. Not all features are
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supported by old kernels. To disable a feature, prefix it with *^*.
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See section *FILESYSTEM FEATURES* for more details. To see all available
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features that :command:`mkfs.btrfs` supports run:
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.. code-block:: bash
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$ mkfs.btrfs -O list-all
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-f|--force
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Forcibly overwrite the block devices when an existing filesystem is detected.
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By default, :command:`mkfs.btrfs` will utilize *libblkid* to check for any known
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filesystem on the devices. Alternatively you can use the :command:`wipefs` utility
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to clear the devices.
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-q|--quiet
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Print only error or warning messages. Options *--features* or *--help* are unaffected.
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Resets any previous effects of *--verbose*.
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-U|--uuid <UUID>
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Create the filesystem with the given *UUID*. The UUID must not exist on any
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filesystem currently present.
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-v|--verbose
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Increase verbosity level, default is 1.
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-V|--version
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Print the :command:`mkfs.btrfs` version and exit.
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--help
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Print help.
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-l|--leafsize <size>
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Removed in 6.0, used to be alias for *--nodesize*.
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-R|--runtime-features <feature1>[,<feature2>...]
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Removed in 6.3, was used to specify features not affecting on-disk format.
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Now all such features are merged into `-O|--features` option. The option
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-R will stay for backward compatibility.
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SIZE UNITS
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----------
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The default unit is *byte*. All size parameters accept suffixes in the 1024
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base. The recognized suffixes are: *k*, *m*, *g*, *t*, *p*, *e*, both uppercase
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and lowercase.
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MULTIPLE DEVICES
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----------------
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Before mounting a multiple device filesystem, the kernel module must know the
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association of the block devices that are attached to the filesystem UUID.
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There is typically no action needed from the user. On a system that utilizes a
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udev-like daemon, any new block device is automatically registered. The rules
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call :command:`btrfs device scan`.
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The same command can be used to trigger the device scanning if the btrfs kernel
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module is reloaded (naturally all previous information about the device
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registration is lost).
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Another possibility is to use the mount options *device* to specify the list of
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devices to scan at the time of mount.
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.. code-block:: bash
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# mount -o device=/dev/sdb,device=/dev/sdc /dev/sda /mnt
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.. note::
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This means only scanning, if the devices do not exist in the system,
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mount will fail anyway. This can happen on systems without initramfs/initrd and
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root partition created with RAID1/10/5/6 profiles. The mount action can happen
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before all block devices are discovered. The waiting is usually done on the
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initramfs/initrd systems.
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.. warning::
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RAID5/6 has known problems and should not be used in production.
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FILESYSTEM FEATURES
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-------------------
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Features that can be enabled during creation time. See also :doc:`btrfs(5)<btrfs-man5>` 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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mixed data and metadata block groups, also set by option *--mixed*
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extref
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(default since btrfs-progs 3.12, kernel support 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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raid56
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(kernel support since 3.9)
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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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skinny-metadata
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(default since btrfs-progs 3.18, kernel support since 3.10)
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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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(default since btrfs-progs 5.15, kernel support 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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zoned
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(kernel support since 5.12)
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zoned mode, data allocation and write friendly to zoned/SMR/ZBC/ZNS devices,
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see *ZONED MODE* in :doc:`btrfs(5)<btrfs-man5>`, the mode is automatically selected when
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a zoned device is detected
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quota
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(kernel support since 3.4)
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Enable quota support (qgroups). The qgroup accounting will be consistent,
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can be used together with *--rootdir*. See also :doc:`btrfs-quota(8)<btrfs-quota>`.
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free-space-tree
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(default since btrfs-progs 5.15, kernel support since 4.5)
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Enable the free space tree (mount option *space_cache=v2*) for persisting the
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free space cache.
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block-group-tree
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(kernel support since 6.1)
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Enable the block group tree to greatly reduce mount time for large filesystems.
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BLOCK GROUPS, CHUNKS, RAID
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--------------------------
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The highlevel organizational units of a filesystem are block groups of three types:
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data, metadata and system.
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DATA
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store data blocks and nothing else
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METADATA
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store internal metadata in b-trees, can store file data if they fit into the
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inline limit
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SYSTEM
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store structures that describe the mapping between the physical devices and the
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linear logical space representing the filesystem
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Other terms commonly used:
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block group, chunk
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a logical range of space of a given profile, stores data, metadata or both;
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sometimes the terms are used interchangeably
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A typical size of metadata block group is 256MiB (filesystem smaller than
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50GiB) and 1GiB (larger than 50GiB), for data it's 1GiB. The system block group
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size is a few megabytes.
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RAID
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a block group profile type that utilizes RAID-like features on multiple
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devices: striping, mirroring, parity
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profile
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when used in connection with block groups refers to the allocation strategy
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and constraints, see the section *PROFILES* for more details
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PROFILES
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--------
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There are the following block group types available:
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+----------+---------------+------------+------------+-------------------+--------------------+
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| Profiles | Redundancy | Redundancy | Redundancy | Space utilization | Min/max devices |
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| | | | | | |
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| | Copies | Parity | Striping | | |
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+==========+===============+============+============+===================+====================+
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| single | 1 | | | 100% | 1/any |
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+----------+---------------+------------+------------+-------------------+--------------------+
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| DUP | 2 / 1 device | | | 50% | 1/any (see note 1) |
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+----------+---------------+------------+------------+-------------------+--------------------+
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| RAID0 | 1 | | 1 to N | 100% | 1/any (see note 5) |
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+----------+---------------+------------+------------+-------------------+--------------------+
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| RAID1 | 2 | | | 50% | 2/any |
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+----------+---------------+------------+------------+-------------------+--------------------+
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| RAID1C3 | 3 | | | 33% | 3/any |
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+----------+---------------+------------+------------+-------------------+--------------------+
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| RAID1C4 | 4 | | | 25% | 4/any |
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+----------+---------------+------------+------------+-------------------+--------------------+
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| RAID10 | 2 | | 1 to N | 50% | 2/any (see note 5) |
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+----------+---------------+------------+------------+-------------------+--------------------+
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| RAID5 | 1 | 1 | 2 to N-1 | (N-1)/N | 2/any (see note 2) |
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+----------+---------------+------------+------------+-------------------+--------------------+
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| RAID6 | 1 | 2 | 3 to N-2 | (N-2)/N | 3/any (see note 3) |
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+----------+---------------+------------+------------+-------------------+--------------------+
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.. warning::
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It's not recommended to create filesystems with RAID0/1/10/5/6
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profiles on partitions from the same device. Neither redundancy nor
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performance will be improved.
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*Note 1:* DUP may exist on more than 1 device if it starts on a single device and
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another one is added. Since version 4.5.1, :command:`mkfs.btrfs` will let you create DUP
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on multiple devices without restrictions.
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*Note 2:* It's not recommended to use 2 devices with RAID5. In that case,
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parity stripe will contain the same data as the data stripe, making RAID5
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degraded to RAID1 with more overhead.
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*Note 3:* It's also not recommended to use 3 devices with RAID6, unless you
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want to get effectively 3 copies in a RAID1-like manner (but not exactly that).
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*Note 4:* Since kernel 5.5 it's possible to use RAID1C3 as replacement for
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RAID6, higher space cost but reliable.
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*Note 5:* Since kernel 5.15 it's possible to use (mount, convert profiles)
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RAID0 on one device and RAID10 on two devices.
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PROFILE LAYOUT
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^^^^^^^^^^^^^^
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For the following examples, assume devices numbered by 1, 2, 3 and 4, data or
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metadata blocks A, B, C, D, with possible stripes e.g. A1, A2 that would be
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logically A, etc. For parity profiles PA and QA are parity and syndrome,
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associated with the given stripe. The simple layouts single or DUP are left
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out. Actual physical block placement on devices depends on current state of
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the free/allocated space and may appear random. All devices are assumed to be
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present at the time of the blocks would have been written.
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RAID1
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"""""
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======== ======== ======== ========
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device 1 device 2 device 3 device 4
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======== ======== ======== ========
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A D
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B C
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C
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D A B
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======== ======== ======== ========
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RAID1C3
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"""""""
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======== ======== ======== ========
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device 1 device 2 device 3 device 4
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======== ======== ======== ========
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A A D
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B B
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C A C
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D D C B
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======== ======== ======== ========
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RAID0
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"""""
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======== ======== ======== ========
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device 1 device 2 device 3 device 4
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======== ======== ======== ========
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A2 C3 A3 C2
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B1 A1 D2 B3
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C1 D3 B4 D1
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D4 B2 C4 A4
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======== ======== ======== ========
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RAID5
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"""""
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======== ======== ======== ========
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device 1 device 2 device 3 device 4
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======== ======== ======== ========
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A2 C3 A3 C2
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B1 A1 D2 B3
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C1 D3 PB D1
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PD B2 PC PA
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======== ======== ======== ========
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RAID6
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"""""
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======== ======== ======== ========
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device 1 device 2 device 3 device 4
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======== ======== ======== ========
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A2 QC QA C2
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B1 A1 D2 QB
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C1 QD PB D1
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PD B2 PC PA
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======== ======== ======== ========
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DUP PROFILES ON A SINGLE DEVICE
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-------------------------------
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The mkfs utility will let the user create a filesystem with profiles that write
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the logical blocks to 2 physical locations. Whether there are really 2
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physical copies highly depends on the underlying device type.
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For example, a SSD drive can remap the blocks internally to a single copy--thus
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deduplicating them. This negates the purpose of increased redundancy and just
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wastes filesystem space without providing the expected level of redundancy.
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The duplicated data/metadata may still be useful to statistically improve the
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chances on a device that might perform some internal optimizations. The actual
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details are not usually disclosed by vendors. For example we could expect that
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not all blocks get deduplicated. This will provide a non-zero probability of
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recovery compared to a zero chance if the single profile is used. The user
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should make the tradeoff decision. The deduplication in SSDs is thought to be
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widely available so the reason behind the mkfs default is to not give a false
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sense of redundancy.
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As another example, the widely used USB flash or SD cards use a translation
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layer between the logical and physical view of the device. The data lifetime
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may be affected by frequent plugging. The memory cells could get damaged,
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hopefully not destroying both copies of particular data in case of DUP.
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The wear levelling techniques can also lead to reduced redundancy, even if the
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device does not do any deduplication. The controllers may put data written in
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a short timespan into the same physical storage unit (cell, block etc). In case
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this unit dies, both copies are lost. BTRFS does not add any artificial delay
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between metadata writes.
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The traditional rotational hard drives usually fail at the sector level.
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In any case, a device that starts to misbehave and repairs from the DUP copy
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should be replaced! **DUP is not backup**.
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KNOWN ISSUES
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------------
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**SMALL FILESYSTEMS AND LARGE NODESIZE**
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The combination of small filesystem size and large nodesize is not recommended
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in general and can lead to various ENOSPC-related issues during mount time or runtime.
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Since mixed block group creation is optional, we allow small
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filesystem instances with differing values for *sectorsize* and *nodesize*
|
|
to be created and could end up in the following situation:
|
|
|
|
.. code-block:: none
|
|
|
|
# mkfs.btrfs -f -n 65536 /dev/loop0
|
|
btrfs-progs v3.19-rc2-405-g976307c
|
|
See http://btrfs.wiki.kernel.org for more information.
|
|
|
|
Performing full device TRIM (512.00MiB) ...
|
|
Label: (null)
|
|
UUID: 49fab72e-0c8b-466b-a3ca-d1bfe56475f0
|
|
Node size: 65536
|
|
Sector size: 4096
|
|
Filesystem size: 512.00MiB
|
|
Block group profiles:
|
|
Data: single 8.00MiB
|
|
Metadata: DUP 40.00MiB
|
|
System: DUP 12.00MiB
|
|
SSD detected: no
|
|
Incompat features: extref, skinny-metadata
|
|
Number of devices: 1
|
|
Devices:
|
|
ID SIZE PATH
|
|
1 512.00MiB /dev/loop0
|
|
|
|
# mount /dev/loop0 /mnt/
|
|
mount: mount /dev/loop0 on /mnt failed: No space left on device
|
|
|
|
The ENOSPC occurs during the creation of the UUID tree. This is caused
|
|
by large metadata blocks and space reservation strategy that allocates more
|
|
than can fit into the filesystem.
|
|
|
|
|
|
AVAILABILITY
|
|
------------
|
|
|
|
**btrfs** is part of btrfs-progs. Please refer to the documentation at
|
|
`https://btrfs.readthedocs.io <https://btrfs.readthedocs.io>`_.
|
|
|
|
SEE ALSO
|
|
--------
|
|
|
|
:doc:`btrfs(5)<btrfs-man5>`,
|
|
:doc:`btrfs(8)<btrfs>`,
|
|
:doc:`btrfs-balance(8)<btrfs-balance>`,
|
|
``wipefs(8)``
|