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btrfs-progs/Documentation/mkfs.btrfs.rst
Mark Harmstone 93b27d715f btrfs-progs: mkfs: rework --subvol CLI option 
Change mkfs.btrfs --subvol so that instead of being of the form --subvol
DIR:FLAGS, it's instead --subvol MODIFIER:DIR, with MODIFIER being ro,
rw, default, or ro-default.

Signed-off-by: Mark Harmstone <maharmstone@fb.com>
2024-11-08 18:23:46 +01:00

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mkfs.btrfs(8)
=============
SYNOPSIS
--------
**mkfs.btrfs** [options] <device> [<device>...]
DESCRIPTION
-----------
:command:`mkfs.btrfs` is used to create the btrfs filesystem on a single or multiple
devices. The *device* is typically a block device but can be a file-backed image
as well. Multiple devices are grouped by UUID of the filesystem.
Before mounting such filesystem, the kernel module must know all the devices
either via preceding execution of :command:`btrfs device scan` or using the *device*
mount option. See section :ref:`MULTIPLE DEVICES<man-mkfs-multiple-devices>`
for more details.
The default block group profiles for data and metadata depend on number of
devices and possibly other factors. It's recommended to use specific profiles
but the defaults should be OK and allowing future conversions to other profiles.
Please see options *-d* and *-m* for further details and :doc:`btrfs-balance` for
the profile conversion post mkfs.
OPTIONS
-------
-b|--byte-count <size>
Specify the size of each device as seen by the filesystem. If not set,
the entire device size is used. The total filesystem size will be sum
of all device sizes, for a single device filesystem the option
effectively specifies the size of the filesystem.
--csum <type>, --checksum <type>
Specify the checksum algorithm. Default is *crc32c*. Valid values are *crc32c*,
*xxhash*, *sha256* or *blake2*. To mount such filesystem kernel must support the
checksums as well. See section :ref:`CHECKSUM ALGORITHMS<man-mkfs-checksum-algorithms>`
in :doc:`btrfs-man5`.
-d|--data <profile>
Specify the profile for the data block groups. Valid values are *raid0*,
*raid1*, *raid1c3*, *raid1c4*, *raid5*, *raid6*, *raid10* or *single* or *dup*
(case does not matter).
See section :ref:`DUP PROFILES ON A SINGLE DEVICE<man-mkfs-dup-profiles-on-a-single-device>`
for more details.
On multiple devices, the default was *raid0* until version 5.7, while it is
*single* since version 5.8. You can still select *raid0* manually, but it was not
suitable as default.
-m|--metadata <profile>
Specify the profile for the metadata block groups.
Valid values are *raid0*, *raid1*, *raid1c3*, *raid1c4*, *raid5*, *raid6*,
*raid10*, *single* or *dup* (case does not matter).
Default on a single device filesystem is *DUP* and is recommended for metadata
in general. The duplication might not be necessary in some use cases and it's
up to the user to changed that at mkfs time or later. This depends on hardware
that could potentially deduplicate the blocks again but this cannot be detected
at mkfs time.
.. note::
Up to version 5.14 there was a detection of a SSD device (more precisely
if it's a rotational device, determined by the contents of file
:file:`/sys/block/DEV/queue/rotational`) that used to select *single*. This has
changed in version 5.15 to be always *dup*.
Note that the rotational status can be arbitrarily set by the underlying block
device driver and may not reflect the true status (network block device, memory-backed
SCSI devices, real block device behind some additional device mapper layer,
etc). It's recommended to always set the options *--data/--metadata* to avoid
confusion and unexpected results.
See section :ref:`DUP PROFILES ON A SINGLE DEVICE<man-mkfs-dup-profiles-on-a-single-device>`
for more details.
On multiple devices the default is *raid1*.
.. _mkfs-feature-mixed-bg:
-M|--mixed
Normally the data and metadata block groups are isolated. The *mixed* mode
will remove the isolation and store both types in the same block group type.
This helps to utilize the free space regardless of the purpose and is suitable
for small devices. The separate allocation of block groups leads to a situation
where the space is reserved for the other block group type, is not available for
allocation and can lead to ENOSPC state.
The recommended size for the mixed mode is for filesystems less than 1GiB. The
soft recommendation is to use it for filesystems smaller than 5GiB. The mixed
mode may lead to degraded performance on larger filesystems, but is otherwise
usable, even on multiple devices.
The *nodesize* and *sectorsize* must be equal, and the block group types must
match.
.. note::
Versions up to 4.2.x forced the mixed mode for devices smaller than 1GiB.
This has been removed in 4.3+ as it caused some usability issues.
Mixed profile cannot be used together with other profiles. It can only
be set at creation time. Conversion to or from mixed profile is not
implemented.
-n|--nodesize <size>
Specify the nodesize, the tree block size in which btrfs stores metadata. The
default value is 16KiB (16384) or the page size, whichever is bigger. Must be a
multiple of the sectorsize and a power of 2, but not larger than 64KiB (65536).
Leafsize always equals nodesize and the options are aliases.
Smaller node size increases fragmentation but leads to taller b-trees which in
turn leads to lower locking contention. Higher node sizes give better packing
and less fragmentation at the cost of more expensive memory operations while
updating the metadata blocks.
.. note::
Versions up to 3.11 set the nodesize to 4KiB.
-s|--sectorsize <size>
Specify the sectorsize, the minimum data block allocation unit.
.. note::
Versions prior to 6.7 set the sectorsize matching the host CPU
page size, starting in 6.7 this is 4KiB for cross-architecture
compatibility. Please read more about the :doc:`subpage block size support<Subpage>`
and :ref:`its status<status-subpage-block-size>`.
By default, the value is 4KiB, but it can be manually set to match the
system page size (e.g. using command :command:`getconf PAGE_SIZE`).
However, if the sector size is different from the page
size, the resulting filesystem may not be mountable by the current
kernel, apart from the default 4KiB. Hence, using this option is not
advised unless you intend to mount it on a system with the suitable
page size.
-L|--label <string>
Specify a label for the filesystem. The *string* should be less than 256
bytes and must not contain newline characters.
-K|--nodiscard
Do not perform whole device TRIM operation on devices that are capable of that.
This does not affect discard/trim operation when the filesystem is mounted.
Please see the mount option *discard* for that in :doc:`btrfs-man5`.
-r|--rootdir <rootdir>
Populate the toplevel subvolume with files from *rootdir*. This does not
require root permissions to write the new files or to mount the filesystem.
.. note::
This option may enlarge the image or file to ensure it's big enough to
contain the files from *rootdir*. Since version 4.14.1 the filesystem size is
not minimized. Please see option *--shrink* if you need that functionality.
-u|--subvol <type>:<subdir>
Specify that *subdir* is to be created as a subvolume rather than a regular
directory. The option *--rootdir* must also be specified, and *subdir* must be an
existing subdirectory within it. This option can be specified multiple times.
*type* is an optional additional modifier. Valid choices are:
* *default*: create as default subvolume
* *ro*: create as read-only subvolume
* *rw*: create as read-write subvolume (the default)
* *default-ro*: create as read-only default subvolume
Only one of *default* and *default-ro* may be specified.
If you wish to create a subvolume with a name containing a colon and you don't
want this to be parsed as containing a modifier, you can prefix the path with `./`:
.. code-block:: bash
$ mkfs.btrfs --rootdir dir --subvol ./ro:subdir /dev/loop0
If there are hard links inside *rootdir* and *subdir* will split the
subvolumes, like the following case::
rootdir/
|- hardlink1
|- hardlink2
|- subdir/ <- will be a subvolume
|- hardlink3
In that case we cannot create `hardlink3` as hardlinks of
`hardlink1` and `hardlink2` because hardlink3 will be inside a new
subvolume.
--shrink
Shrink the filesystem to its minimal size, only works with *--rootdir* option.
If the destination block device is a regular file, this option will also
truncate the file to the minimal size. Otherwise it will reduce the filesystem
available space. Extra space will not be usable unless the filesystem is
mounted and resized using :command:`btrfs filesystem resize`.
.. note::
Prior to version 4.14.1, the shrinking was done automatically.
-O|--features <feature1>[,<feature2>...]
A list of filesystem features turned on at mkfs time. Not all features are
supported by old kernels. To disable a feature, prefix it with *^*.
See section :ref:`FILESYSTEM FEATURES<man-mkfs-filesystem-features>`
for more details. To see all available features that
:command:`mkfs.btrfs` supports run:
.. code-block:: bash
$ mkfs.btrfs -O list-all
-f|--force
Forcibly overwrite the block devices when an existing filesystem is detected.
By default, :command:`mkfs.btrfs` will utilize *libblkid* to check for any known
filesystem on the devices. Alternatively you can use the :command:`wipefs` utility
to clear the devices.
-q|--quiet
Print only error or warning messages. Options *--features* or *--help* are unaffected.
Resets any previous effects of *--verbose*.
-U|--uuid <UUID>
Create the filesystem with the given *UUID*. For a single-device filesystem,
you can duplicate the UUID. However, for a multi-device filesystem, the UUID
must not already exist on any currently present filesystem.
--device-uuid <UUID>
Create the filesystem with the given device-uuid *UUID* (also known as
UUID_SUB in :command:`blkid`). For a single device filesystem, you can
duplicate the device-uuid. However, used for a multi-device filesystem
this option will not work at the moment.
-v|--verbose
Increase verbosity level, default is 1.
-V|--version
Print the :command:`mkfs.btrfs` version and exit.
--help
Print help.
-l|--leafsize <size>
Removed in 6.0, used to be alias for *--nodesize*.
-R|--runtime-features <feature1>[,<feature2>...]
Removed in 6.3, was used to specify features not affecting on-disk format.
Now all such features are merged into `-O|--features` option. The option
-R will stay for backward compatibility.
SIZE UNITS
----------
The default unit is *byte*. All size parameters accept suffixes in the 1024
base. The recognized suffixes are: *k*, *m*, *g*, *t*, *p*, *e*, both uppercase
and lowercase.
.. _man-mkfs-multiple-devices:
MULTIPLE DEVICES
----------------
Before mounting a multiple device filesystem, the kernel module must know the
association of the block devices that are attached to the filesystem UUID.
There is typically no action needed from the user. On a system that utilizes a
udev-like daemon, any new block device is automatically registered. The rules
call :command:`btrfs device scan`.
The same command can be used to trigger the device scanning if the btrfs kernel
module is reloaded (naturally all previous information about the device
registration is lost).
Another possibility is to use the mount options *device* to specify the list of
devices to scan at the time of mount.
.. code-block:: bash
# mount -o device=/dev/sdb,device=/dev/sdc /dev/sda /mnt
.. note::
This means only scanning, if the devices do not exist in the system,
mount will fail anyway. This can happen on systems without initramfs/initrd and
root partition created with RAID1/10/5/6 profiles. The mount action can happen
before all block devices are discovered. The waiting is usually done on the
initramfs/initrd systems.
.. warning::
RAID5/6 has known problems and should not be used in production.
.. _man-mkfs-filesystem-features:
FILESYSTEM FEATURES
-------------------
Features that can be enabled during creation time. See also :doc:`btrfs-man5` section
:ref:`FILESYSTEM FEATURES<man-btrfs5-filesystem-features>`.
mixed-bg
(kernel support since 2.6.37)
mixed data and metadata block groups, also set by option *--mixed*
.. _mkfs-feature-extended-refs:
extref
(default since btrfs-progs 3.12, kernel support since 3.7)
increased hardlink limit per file in a directory to 65536, older kernels
supported a varying number of hardlinks depending on the sum of all file name
sizes that can be stored into one metadata block
raid56
(kernel support since 3.9)
extended format for RAID5/6, also enabled if RAID5 or RAID6 block groups
are selected
.. _mkfs-feature-skinny-metadata:
skinny-metadata
(default since btrfs-progs 3.18, kernel support since 3.10)
reduced-size metadata for extent references, saves a few percent of metadata
.. _mkfs-feature-no-holes:
no-holes
(default since btrfs-progs 5.15, kernel support since 3.14)
improved representation of file extents where holes are not explicitly
stored as an extent, saves a few percent of metadata if sparse files are used
zoned
(kernel support since 5.12)
zoned mode, data allocation and write friendly to zoned/SMR/ZBC/ZNS devices,
see :ref:`ZONED MODE<man-btrfs5-zoned-mode>` in
:doc:`btrfs-man5`, the mode is automatically selected when a
zoned device is detected
quota
(kernel support since 3.4)
Enable quota support (qgroups). The qgroup accounting will be consistent,
can be used together with *--rootdir*. See also :doc:`btrfs-quota`.
.. _mkfs-feature-free-space-tree:
free-space-tree
(default since btrfs-progs 5.15, kernel support since 4.5)
Enable the free space tree (mount option *space_cache=v2*) for persisting the
free space cache in a b-tree. This is built on top of the COW mechanism
and has better performance than v1.
Offline conversion from filesystems that don't have this feature
enabled at *mkfs* time is possible, see :doc:`btrfstune`.
Online conversion can be done by mounting with ``space_cache=v2``, this
is sufficient to be done one time.
.. _mkfs-feature-block-group-tree:
block-group-tree
(kernel support since 6.1)
Enable a dedicated b-tree for block group items, this greatly reduces
mount time for large filesystems due to better data locality that
avoids seeking. On rotational devices the *large* size is considered
starting from the 2-4TiB. Can be used on other types of devices (SSD,
NVMe, ...) as well.
Offline conversion from filesystems that don't have this feature
enabled at *mkfs* time is possible, see :doc:`btrfstune`. Online
conversion is not possible.
.. _mkfs-feature-raid-stripe-tree:
raid-stripe-tree
(kernel support since 6.7, CONFIG_BTRFS_DEBUG)
Separate tree for logical file extent mapping where the physical mapping
may not match on multiple devices. This is now used in zoned mode to
implement RAID0/RAID1* profiles, but can be used in non-zoned mode as
well. The support for RAID56 is in development and will eventually
fix the problems with the current implementation. This is a backward
incompatible feature and has to be enabled at mkfs time.
.. note::
Due to the status of implementation it is enabled only in
builds with CONFIG_BTRFS_DEBUG. Support by the kernel module
can be found in the sysfs feature list.
squota
(kernel support since 6.7)
Enable simple quota accounting (squotas). This is an alternative to
qgroups with a smaller performance impact but no notion of shared vs.
exclusive usage.
.. _mkfs-section-profiles:
BLOCK GROUPS, CHUNKS, RAID
--------------------------
The highlevel organizational units of a filesystem are block groups of three types:
data, metadata and system.
DATA
store data blocks and nothing else
METADATA
store internal metadata in b-trees, can store file data if they fit into the
inline limit
SYSTEM
store structures that describe the mapping between the physical devices and the
linear logical space representing the filesystem
Other terms commonly used:
block group, chunk
a logical range of space of a given profile, stores data, metadata or both;
sometimes the terms are used interchangeably
A typical size of metadata block group is 256MiB (filesystem smaller than
50GiB) and 1GiB (larger than 50GiB), for data it's 1GiB. The system block group
size is a few megabytes.
RAID
a block group profile type that utilizes RAID-like features on multiple
devices: striping, mirroring, parity
profile
when used in connection with block groups refers to the allocation strategy
and constraints, see the section :ref:`PROFILES<man-mkfs-profiles>` for more details
.. _man-mkfs-profiles:
PROFILES
--------
There are the following block group types available:
+----------+---------------+------------+------------+-------------------+--------------------+
| Profiles | Redundancy | Redundancy | Redundancy | Space utilization | Min/max devices |
| | | | | | |
| | Copies | Parity | Striping | | |
+==========+===============+============+============+===================+====================+
| single | 1 | | | 100% | 1/any |
+----------+---------------+------------+------------+-------------------+--------------------+
| DUP | 2 / 1 device | | | 50% | 1/any (see note 1) |
+----------+---------------+------------+------------+-------------------+--------------------+
| RAID0 | 1 | | 1 to N | 100% | 1/any (see note 5) |
+----------+---------------+------------+------------+-------------------+--------------------+
| RAID1 | 2 | | | 50% | 2/any |
+----------+---------------+------------+------------+-------------------+--------------------+
| RAID1C3 | 3 | | | 33% | 3/any |
+----------+---------------+------------+------------+-------------------+--------------------+
| RAID1C4 | 4 | | | 25% | 4/any |
+----------+---------------+------------+------------+-------------------+--------------------+
| RAID10 | 2 | | 1 to N | 50% | 2/any (see note 5) |
+----------+---------------+------------+------------+-------------------+--------------------+
| RAID5 | 1 | 1 | 2 to N-1 | (N-1)/N | 2/any (see note 2) |
+----------+---------------+------------+------------+-------------------+--------------------+
| RAID6 | 1 | 2 | 3 to N-2 | (N-2)/N | 3/any (see note 3) |
+----------+---------------+------------+------------+-------------------+--------------------+
.. warning::
It's not recommended to create filesystems with RAID0/1/10/5/6
profiles on partitions from the same device. Neither redundancy nor
performance will be improved.
*Note 1:* DUP may exist on more than 1 device if it starts on a single device and
another one is added. Since version 4.5.1, :command:`mkfs.btrfs` will let you create DUP
on multiple devices without restrictions.
*Note 2:* It's not recommended to use 2 devices with RAID5. In that case,
parity stripe will contain the same data as the data stripe, making RAID5
degraded to RAID1 with more overhead.
*Note 3:* It's also not recommended to use 3 devices with RAID6, unless you
want to get effectively 3 copies in a RAID1-like manner (but not exactly that).
*Note 4:* Since kernel 5.5 it's possible to use RAID1C3 as replacement for
RAID6, higher space cost but reliable.
*Note 5:* Since kernel 5.15 it's possible to use (mount, convert profiles)
RAID0 on one device and RAID10 on two devices.
PROFILE LAYOUT
^^^^^^^^^^^^^^
For the following examples, assume devices numbered by 1, 2, 3 and 4, data or
metadata blocks A, B, C, D, with possible stripes e.g. A1, A2 that would be
logically A, etc. For parity profiles PA and QA are parity and syndrome,
associated with the given stripe. The simple layouts single or DUP are left
out. Actual physical block placement on devices depends on current state of
the free/allocated space and may appear random. All devices are assumed to be
present at the time of the blocks would have been written.
RAID1
"""""
======== ======== ======== ========
device 1 device 2 device 3 device 4
======== ======== ======== ========
A D
B C
C
D A B
======== ======== ======== ========
RAID1C3
"""""""
======== ======== ======== ========
device 1 device 2 device 3 device 4
======== ======== ======== ========
A A D
B B
C A C
D D C B
======== ======== ======== ========
RAID0
"""""
======== ======== ======== ========
device 1 device 2 device 3 device 4
======== ======== ======== ========
A2 C3 A3 C2
B1 A1 D2 B3
C1 D3 B4 D1
D4 B2 C4 A4
======== ======== ======== ========
RAID5
"""""
======== ======== ======== ========
device 1 device 2 device 3 device 4
======== ======== ======== ========
A2 C3 A3 C2
B1 A1 D2 B3
C1 D3 PB D1
PD B2 PC PA
======== ======== ======== ========
RAID6
"""""
======== ======== ======== ========
device 1 device 2 device 3 device 4
======== ======== ======== ========
A2 QC QA C2
B1 A1 D2 QB
C1 QD PB D1
PD B2 PC PA
======== ======== ======== ========
.. _man-mkfs-dup-profiles-on-a-single-device:
DUP PROFILES ON A SINGLE DEVICE
-------------------------------
The mkfs utility will let the user create a filesystem with profiles that write
the logical blocks to 2 physical locations. Whether there are really 2
physical copies highly depends on the underlying device type.
For example, a SSD drive can remap the blocks internally to a single copy--thus
deduplicating them. This negates the purpose of increased redundancy and just
wastes filesystem space without providing the expected level of redundancy.
The duplicated data/metadata may still be useful to statistically improve the
chances on a device that might perform some internal optimizations. The actual
details are not usually disclosed by vendors. For example we could expect that
not all blocks get deduplicated. This will provide a non-zero probability of
recovery compared to a zero chance if the single profile is used. The user
should make the tradeoff decision. The deduplication in SSDs is thought to be
widely available so the reason behind the mkfs default is to not give a false
sense of redundancy.
As another example, the widely used USB flash or SD cards use a translation
layer between the logical and physical view of the device. The data lifetime
may be affected by frequent plugging. The memory cells could get damaged,
hopefully not destroying both copies of particular data in case of DUP.
The wear levelling techniques can also lead to reduced redundancy, even if the
device does not do any deduplication. The controllers may put data written in
a short timespan into the same physical storage unit (cell, block etc). In case
this unit dies, both copies are lost. BTRFS does not add any artificial delay
between metadata writes.
The traditional rotational hard drives usually fail at the sector level.
In any case, a device that starts to misbehave and repairs from the DUP copy
should be replaced! **DUP is not backup**.
KNOWN ISSUES
------------
**SMALL FILESYSTEMS AND LARGE NODESIZE**
The combination of small filesystem size and large nodesize is not recommended
in general and can lead to various ENOSPC-related issues during mount time or runtime.
Since mixed block group creation is optional, we allow small
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 https://btrfs.readthedocs.io 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-man5`,
:doc:`btrfs`,
:doc:`btrfs-balance`,
:manref:`wipefs(8)`
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