MKFS.BTRFS(8) Btrfs Manual 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:
+--------+------------------------------------+------------+
| | | |
|Profile | Redundancy | Min/max |
| +--------------+--------+------------+ devices |
| | | | | |
| | Copies | Parity | 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)
Btrfs v4.9.1 08/06/2017 MKFS.BTRFS(8)
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