btrfs-progs/Documentation/mkfs.btrfs.asciidoc

mkfs.btrfs(8)
=============

NAME
----
mkfs.btrfs - create a btrfs filesystem

SYNOPSIS
--------
*mkfs.btrfs* [options] <device> [<device>...]

DESCRIPTION
-----------
*mkfs.btrfs* is used to create the btrfs filesystem on a single or multiple
devices.  <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 *btrfs device scan* or using the *device*
mount option. See section *MULTIPLE DEVICES* for more details.

OPTIONS
-------
*-A|--alloc-start <offset>*::
(An option to help debugging chunk allocator.)
Specify the (physical) offset from the start of the device at which allocations
start.  The default value is zero.

*-b|--byte-count <size>*::
Specify the size of the filesystem. If this option is not used,
mkfs.btrfs uses the entire device space for the filesystem.

*-d|--data <profile>*::
Specify the profile for the data block groups.  Valid values are 'raid0',
'raid1', 'raid5', 'raid6', 'raid10' or 'single' or dup (case does not matter).
+
See 'DUP PROFILES ON A SINGLE DEVICE' for more.

*-m|--metadata <profile>*::
Specify the profile for the metadata block groups.
Valid values are 'raid0', 'raid1', 'raid5', 'raid6', 'raid10', 'single' or
'dup', (case does not matter).
+
A single device filesystem will default to 'DUP', unless a SSD is detected. Then
it will default to 'single'. The detection is based on the value of
`/sys/block/DEV/queue/rotational`, where 'DEV' is the short name of the device.
+
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 etc). Use the options '--data/--metadata' to avoid confusion.
+
See 'DUP PROFILES ON A SINGLE DEVICE' for more details.

*-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.

*-l|--leafsize <size>*::
Alias for --nodesize. Deprecated.

*-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, but not larger than 64KiB (65536).  Leafsize always
equals nodesize and the options are aliases.
+
Smaller node size increases fragmentation but lead to higher 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 4k.

*-s|--sectorsize <size>*::
Specify the sectorsize, the minimum data block allocation unit.
+
The default value is the page size and is autodetected. If the sectorsize
differs from the page size, the created filesystem may not be mountable by the
kernel. Therefore it is not recommended to use this option unless you are going
to mount it on a system with the appropriate 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 `btrfs`(5).

*-r|--rootdir <rootdir>*::
Populate the toplevel subvolume with files from 'rootdir'.  This does not
require root permissions and does not mount the filesystem.

*-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 *FILESYSTEM FEATURES* for more details.  To see all available
features that mkfs.btrfs supports run:
+
+mkfs.btrfs -O list-all+

*-f|--force*::
Forcibly overwrite the block devices when an existing filesystem is detected.
By default, mkfs.btrfs will utilize 'libblkid' to check for any known
filesystem on the devices. Alternatively you can use the `wipefs` utility
to clear the devices.

*-q|--quiet*::
Print only error or warning messages. Options --features or --help are unaffected.

*-U|--uuid <UUID>*::
Create the filesystem with the given 'UUID'. The UUID must not exist on any
filesystem currently present.

*-V|--version*::
Print the *mkfs.btrfs* version and exit.

*--help*::
Print help.

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.

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 *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.

 # mount -o device=/dev/sdb,device=/dev/sdc /dev/sda /mnt

NOTE: that 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.

As of kernel 4.9, RAID5/6 is still considered experimental and shouldn't be
employed for production use.

FILESYSTEM FEATURES
-------------------

Features that can be enabled during creation time. See also `btrfs`(5) section
'FILESYSTEM FEATURES'.

*mixed-bg*::
(kernel support since 2.6.37)
+
mixed data and metadata block groups, also set by option '--mixed'

*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

*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

*no-holes*::
(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

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 'PROFILES' for more details

PROFILES
--------

There are the following block group types available:

[ cols="^,^,^,^,^",width="60%" ]
|=============================================================
.2+^.<h| Profile   3+^.^h| Redundancy           .2+^.<h| Min/max devices
      ^.^h| Copies   ^.^h| Parity     ^.<h| Striping
| single  | 1            |                |            | 1/any
| DUP     | 2 / 1 device |                |            | 1/any ^(see note 1)^
| RAID0   |              |                | 1 to N     | 2/any
| RAID1   | 2            |                |            | 2/any
| RAID10  | 2            |                | 1 to N     | 4/any
| RAID5   | 1            | 1              | 2 to N - 1 | 2/any ^(see note 2)^
| RAID6   | 1            | 2              | 3 to N - 2 | 3/any ^(see note 3)^
|=============================================================

WARNING: It's not recommended to build btrfs 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, *mkfs.btrfs* will let you create DUP
on multiple devices.

'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).
N-copies RAID1 is not implemented.

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 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:

  # 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
------------
*mkfs.btrfs* is part of btrfs-progs.
Please refer to the btrfs wiki http://btrfs.wiki.kernel.org for
further details.

SEE ALSO
--------
`btrfs`(5),
`btrfs`(8),
`wipefs`(8)