BTRFS-MAN5(5)BTRFS-MAN5(5)
NAME
btrfs-man5 - topics about the BTRFS filesystem (mount options,
supported file attributes and other)
DESCRIPTION
This document describes topics related to BTRFS that are not specific
to the tools. Currently covers:
1. mount options
2. filesystem features
3. checksum algorithms
4. filesystem limits
5. bootloader support
6. file attributes
7. control device
8. filesystems with multiple block group profiles
MOUNT OPTIONS
This section describes mount options specific to BTRFS. For the generic
mount options please refer to mount(8) manpage. The options are sorted
alphabetically (discarding the no prefix).
Note
most mount options apply to the whole filesystem and only options
in the first mounted subvolume will take effect. This is due to
lack of implementation and may change in the future. This means
that (for example) you can't set per-subvolume nodatacow,
nodatasum, or compress using mount options. This should eventually
be fixed, but it has proved to be difficult to implement correctly
within the Linux VFS framework.
Mount options are processed in order, only the last occurence of an
option takes effect and may disable other options due to constraints
(see eg. nodatacow and compress). The output of mount command shows
which options have been applied.
acl, noacl
(default: on)
Enable/disable support for Posix Access Control Lists (ACLs). See
the acl(5) manual page for more information about ACLs.
The support for ACL is build-time configurable (BTRFS_FS_POSIX_ACL)
and mount fails if acl is requested but the feature is not compiled
in.
autodefrag, noautodefrag
(since: 3.0, default: off)
Enable automatic file defragmentation. When enabled, small random
writes into files (in a range of tens of kilobytes, currently it's
64K) are detected and queued up for the defragmentation process.
Not well suited for large database workloads.
The read latency may increase due to reading the adjacent blocks
that make up the range for defragmentation, successive write will
merge the blocks in the new location.
Warning
Defragmenting with Linux kernel versions < 3.9 or >= 3.14-rc2
as well as with Linux stable kernel versions >= 3.10.31, >=
3.12.12 or >= 3.13.4 will break up the reflinks of COW data
(for example files copied with cp --reflink, snapshots or
de-duplicated data). This may cause considerable increase of
space usage depending on the broken up reflinks.
barrier, nobarrier
(default: on)
Ensure that all IO write operations make it through the device
cache and are stored permanently when the filesystem is at its
consistency checkpoint. This typically means that a flush command
is sent to the device that will synchronize all pending data and
ordinary metadata blocks, then writes the superblock and issues
another flush.
The write flushes incur a slight hit and also prevent the IO block
scheduler to reorder requests in a more effective way. Disabling
barriers gets rid of that penalty but will most certainly lead to a
corrupted filesystem in case of a crash or power loss. The ordinary
metadata blocks could be yet unwritten at the time the new
superblock is stored permanently, expecting that the block pointers
to metadata were stored permanently before.
On a device with a volatile battery-backed write-back cache, the
nobarrier option will not lead to filesystem corruption as the
pending blocks are supposed to make it to the permanent storage.
check_int, check_int_data, check_int_print_mask=value
(since: 3.0, default: off)
These debugging options control the behavior of the integrity
checking module (the BTRFS_FS_CHECK_INTEGRITY config option
required). The main goal is to verify that all blocks from a given
transaction period are properly linked.
check_int enables the integrity checker module, which examines all
block write requests to ensure on-disk consistency, at a large
memory and CPU cost.
check_int_data includes extent data in the integrity checks, and
implies the check_int option.
check_int_print_mask takes a bitmask of BTRFSIC_PRINT_MASK_* values
as defined in fs/btrfs/check-integrity.c, to control the integrity
checker module behavior.
See comments at the top of fs/btrfs/check-integrity.c for more
information.
clear_cache
Force clearing and rebuilding of the disk space cache if something
has gone wrong. See also: space_cache.
commit=seconds
(since: 3.12, default: 30)
Set the interval of periodic transaction commit when data are
synchronized to permanent storage. Higher interval values lead to
larger amount of unwritten data, which has obvious consequences
when the system crashes. The upper bound is not forced, but a
warning is printed if it's more than 300 seconds (5 minutes). Use
with care.
compress, compress=type[:level], compress-force,
compress-force=type[:level]
(default: off, level support since: 5.1)
Control BTRFS file data compression. Type may be specified as zlib,
lzo, zstd or no (for no compression, used for remounting). If no
type is specified, zlib is used. If compress-force is specified,
then compression will always be attempted, but the data may end up
uncompressed if the compression would make them larger.
Both zlib and zstd (since version 5.1) expose the compression level
as a tunable knob with higher levels trading speed and memory
(zstd) for higher compression ratios. This can be set by appending
a colon and the desired level. Zlib accepts the range [1, 9] and
zstd accepts [1, 15]. If no level is set, both currently use a
default level of 3. The value 0 is an alias for the default level.
Otherwise some simple heuristics are applied to detect an
incompressible file. If the first blocks written to a file are not
compressible, the whole file is permanently marked to skip
compression. As this is too simple, the compress-force is a
workaround that will compress most of the files at the cost of some
wasted CPU cycles on failed attempts. Since kernel 4.15, a set of
heuristic algorithms have been improved by using frequency
sampling, repeated pattern detection and Shannon entropy
calculation to avoid that.
Note
If compression is enabled, nodatacow and nodatasum are
disabled.
datacow, nodatacow
(default: on)
Enable data copy-on-write for newly created files. Nodatacow
implies nodatasum, and disables compression. All files created
under nodatacow are also set the NOCOW file attribute (see
chattr(1)).
Note
If nodatacow or nodatasum are enabled, compression is disabled.
Updates in-place improve performance for workloads that do frequent
overwrites, at the cost of potential partial writes, in case the
write is interrupted (system crash, device failure).
datasum, nodatasum
(default: on)
Enable data checksumming for newly created files. Datasum implies
datacow, ie. the normal mode of operation. All files created under
nodatasum inherit the "no checksums" property, however there's no
corresponding file attribute (see chattr(1)).
Note
If nodatacow or nodatasum are enabled, compression is disabled.
There is a slight performance gain when checksums are turned off,
the corresponding metadata blocks holding the checksums do not need
to updated. The cost of checksumming of the blocks in memory is
much lower than the IO, modern CPUs feature hardware support of the
checksumming algorithm.
degraded
(default: off)
Allow mounts with less devices than the RAID profile constraints
require. A read-write mount (or remount) may fail when there are
too many devices missing, for example if a stripe member is
completely missing from RAID0.
Since 4.14, the constraint checks have been improved and are
verified on the chunk level, not an the device level. This allows
degraded mounts of filesystems with mixed RAID profiles for data
and metadata, even if the device number constraints would not be
satisfied for some of the profiles.
Example: metadata -- raid1, data -- single, devices -- /dev/sda,
/dev/sdb
Suppose the data are completely stored on sda, then missing sdb
will not prevent the mount, even if 1 missing device would normally
prevent (any) single profile to mount. In case some of the data
chunks are stored on sdb, then the constraint of single/data is not
satisfied and the filesystem cannot be mounted.
device=devicepath
Specify a path to a device that will be scanned for BTRFS
filesystem during mount. This is usually done automatically by a
device manager (like udev) or using the btrfs device scan command
(eg. run from the initial ramdisk). In cases where this is not
possible the device mount option can help.
Note
booting eg. a RAID1 system may fail even if all filesystem's
device paths are provided as the actual device nodes may not be
discovered by the system at that point.
discard, discard=sync, discard=async, nodiscard
(default: off, async support since: 5.6)
Enable discarding of freed file blocks. This is useful for SSD
devices, thinly provisioned LUNs, or virtual machine images;
however, every storage layer must support discard for it to work.
In the synchronous mode (sync or without option value), lack of
asynchronous queued TRIM on the backing device TRIM can severely
degrade performance, because a synchronous TRIM operation will be
attempted instead. Queued TRIM requires newer than SATA revision
3.1 chipsets and devices.
The asynchronous mode (async) gathers extents in larger chunks
before sending them to the devices for TRIM. The overhead and
performance impact should be negligible compared to the previous
mode and it's supposed to be the preferred mode if needed.
If it is not necessary to immediately discard freed blocks, then
the fstrim tool can be used to discard all free blocks in a batch.
Scheduling a TRIM during a period of low system activity will
prevent latent interference with the performance of other
operations. Also, a device may ignore the TRIM command if the range
is too small, so running a batch discard has a greater probability
of actually discarding the blocks.
enospc_debug, noenospc_debug
(default: off)
Enable verbose output for some ENOSPC conditions. It's safe to use
but can be noisy if the system reaches near-full state.
fatal_errors=action
(since: 3.4, default: bug)
Action to take when encountering a fatal error.
bug
BUG() on a fatal error, the system will stay in the crashed
state and may be still partially usable, but reboot is required
for full operation
panic
panic() on a fatal error, depending on other system
configuration, this may be followed by a reboot. Please refer
to the documentation of kernel boot parameters, eg. panic,
oops or crashkernel.
flushoncommit, noflushoncommit
(default: off)
This option forces any data dirtied by a write in a prior
transaction to commit as part of the current commit, effectively a
full filesystem sync.
This makes the committed state a fully consistent view of the file
system from the application's perspective (i.e. it includes all
completed file system operations). This was previously the behavior
only when a snapshot was created.
When off, the filesystem is consistent but buffered writes may last
more than one transaction commit.
fragment=type
(depends on compile-time option BTRFS_DEBUG, since: 4.4, default:
off)
A debugging helper to intentionally fragment given type of block
groups. The type can be data, metadata or all. This mount option
should not be used outside of debugging environments and is not
recognized if the kernel config option BTRFS_DEBUG is not enabled.
inode_cache, noinode_cache
(since: 3.0, default: off)
Enable free inode number caching. Not recommended to use unless
files on your filesystem get assigned inode numbers that are
approaching 264. Normally, new files in each subvolume get assigned
incrementally (plus one from the last time) and are not reused. The
mount option turns on caching of the existing inode numbers and
reuse of inode numbers of deleted files.
This option may slow down your system at first run, or after
mounting without the option.
Note
Defaults to off due to a potential overflow problem when the
free space checksums don't fit inside a single page.
Don't use this option unless you really need it. The inode number
limit on 64bit system is 264, which is practically enough for the
whole filesystem lifetime. Due to implementation of linux VFS
layer, the inode numbers on 32bit systems are only 32 bits wide.
This lowers the limit significantly and makes it possible to reach
it. In such case, this mount option will help. Alternatively, files
with high inode numbers can be copied to a new subvolume which will
effectively start the inode numbers from the beginning again.
nologreplay
(default: off, even read-only)
The tree-log contains pending updates to the filesystem until the
full commit. The log is replayed on next mount, this can be
disabled by this option. See also treelog. Note that nologreplay is
the same as norecovery.
Warning
currently, the tree log is replayed even with a read-only
mount! To disable that behaviour, mount also with nologreplay.
max_inline=bytes
(default: min(2048, page size) )
Specify the maximum amount of space, that can be inlined in a
metadata B-tree leaf. The value is specified in bytes, optionally
with a K suffix (case insensitive). In practice, this value is
limited by the filesystem block size (named sectorsize at mkfs
time), and memory page size of the system. In case of sectorsize
limit, there's some space unavailable due to leaf headers. For
example, a 4k sectorsize, maximum size of inline data is about 3900
bytes.
Inlining can be completely turned off by specifying 0. This will
increase data block slack if file sizes are much smaller than block
size but will reduce metadata consumption in return.
Note
the default value has changed to 2048 in kernel 4.6.
metadata_ratio=value
(default: 0, internal logic)
Specifies that 1 metadata chunk should be allocated after every
value data chunks. Default behaviour depends on internal logic,
some percent of unused metadata space is attempted to be maintained
but is not always possible if there's not enough space left for
chunk allocation. The option could be useful to override the
internal logic in favor of the metadata allocation if the expected
workload is supposed to be metadata intense (snapshots, reflinks,
xattrs, inlined files).
norecovery
(since: 4.5, default: off)
Do not attempt any data recovery at mount time. This will disable
logreplay and avoids other write operations. Note that this option
is the same as nologreplay.
Note
The opposite option recovery used to have different meaning but
was changed for consistency with other filesystems, where
norecovery is used for skipping log replay. BTRFS does the same
and in general will try to avoid any write operations.
rescan_uuid_tree
(since: 3.12, default: off)
Force check and rebuild procedure of the UUID tree. This should not
normally be needed.
skip_balance
(since: 3.3, default: off)
Skip automatic resume of an interrupted balance operation. The
operation can later be resumed with btrfs balance resume, or the
paused state can be removed with btrfs balance cancel. The default
behaviour is to resume an interrupted balance immediately after a
volume is mounted.
space_cache, space_cache=version, nospace_cache
(nospace_cache since: 3.2, space_cache=v1 and space_cache=v2 since
4.5, default: space_cache=v1)
Options to control the free space cache. The free space cache
greatly improves performance when reading block group free space
into memory. However, managing the space cache consumes some
resources, including a small amount of disk space.
There are two implementations of the free space cache. The original
one, referred to as v1, is the safe default. The v1 space cache can
be disabled at mount time with nospace_cache without clearing.
On very large filesystems (many terabytes) and certain workloads,
the performance of the v1 space cache may degrade drastically. The
v2 implementation, which adds a new B-tree called the free space
tree, addresses this issue. Once enabled, the v2 space cache will
always be used and cannot be disabled unless it is cleared. Use
clear_cache,space_cache=v1 or clear_cache,nospace_cache to do so.
If v2 is enabled, kernels without v2 support will only be able to
mount the filesystem in read-only mode. The btrfs(8) command
currently only has read-only support for v2. A read-write command
may be run on a v2 filesystem by clearing the cache, running the
command, and then remounting with space_cache=v2.
If a version is not explicitly specified, the default
implementation will be chosen, which is v1.
ssd, ssd_spread, nossd, nossd_spread
(default: SSD autodetected)
Options to control SSD allocation schemes. By default, BTRFS will
enable or disable SSD optimizations depending on status of a device
with respect to rotational or non-rotational type. This is
determined by the contents of /sys/block/DEV/queue/rotational). If
it is 0, the ssd option is turned on. The option nossd will disable
the autodetection.
The optimizations make use of the absence of the seek penalty
that's inherent for the rotational devices. The blocks can be
typically written faster and are not offloaded to separate threads.
Note
Since 4.14, the block layout optimizations have been dropped.
This used to help with first generations of SSD devices. Their
FTL (flash translation layer) was not effective and the
optimization was supposed to improve the wear by better
aligning blocks. This is no longer true with modern SSD devices
and the optimization had no real benefit. Furthermore it caused
increased fragmentation. The layout tuning has been kept intact
for the option ssd_spread.
The ssd_spread mount option attempts to allocate into bigger and
aligned chunks of unused space, and may perform better on low-end
SSDs. ssd_spread implies ssd, enabling all other SSD heuristics as
well. The option nossd will disable all SSD options while
nossd_spread only disables ssd_spread.
subvol=path
Mount subvolume from path rather than the toplevel subvolume. The
path is always treated as relative to the toplevel subvolume. This
mount option overrides the default subvolume set for the given
filesystem.
subvolid=subvolid
Mount subvolume specified by a subvolid number rather than the
toplevel subvolume. You can use btrfs subvolume list of btrfs
subvolume show to see subvolume ID numbers. This mount option
overrides the default subvolume set for the given filesystem.
Note
if both subvolid and subvol are specified, they must point at
the same subvolume, otherwise the mount will fail.
thread_pool=number
(default: min(NRCPUS + 2, 8) )
The number of worker threads to start. NRCPUS is number of on-line
CPUs detected at the time of mount. Small number leads to less
parallelism in processing data and metadata, higher numbers could
lead to a performance hit due to increased locking contention,
process scheduling, cache-line bouncing or costly data transfers
between local CPU memories.
treelog, notreelog
(default: on)
Enable the tree logging used for fsync and O_SYNC writes. The tree
log stores changes without the need of a full filesystem sync. The
log operations are flushed at sync and transaction commit. If the
system crashes between two such syncs, the pending tree log
operations are replayed during mount.
Warning
currently, the tree log is replayed even with a read-only
mount! To disable that behaviour, also mount with nologreplay.
The tree log could contain new files/directories, these would not
exist on a mounted filesystem if the log is not replayed.
usebackuproot, nousebackuproot
(since: 4.6, default: off)
Enable autorecovery attempts if a bad tree root is found at mount
time. Currently this scans a backup list of several previous tree
roots and tries to use the first readable. This can be used with
read-only mounts as well.
Note
This option has replaced recovery.
user_subvol_rm_allowed
(default: off)
Allow subvolumes to be deleted by their respective owner.
Otherwise, only the root user can do that.
Note
historically, any user could create a snapshot even if he was
not owner of the source subvolume, the subvolume deletion has
been restricted for that reason. The subvolume creation has
been restricted but this mount option is still required. This
is a usability issue. Since 4.18, the rmdir(2) syscall can
delete an empty subvolume just like an ordinary directory.
Whether this is possible can be detected at runtime, see
rmdir_subvol feature in FILESYSTEM FEATURES.
DEPRECATED MOUNT OPTIONS
List of mount options that have been removed, kept for backward
compatibility.
alloc_start=bytes
(default: 1M, minimum: 1M, deprecated since: 4.13)
Debugging option to force all block allocations above a certain
byte threshold on each block device. The value is specified in
bytes, optionally with a K, M, or G suffix (case insensitive).
recovery
(since: 3.2, default: off, deprecated since: 4.5)
Note
this option has been replaced by usebackuproot and should not
be used but will work on 4.5+ kernels.
subvolrootid=objectid
(irrelevant since: 3.2, formally deprecated since: 3.10)
A workaround option from times (pre 3.2) when it was not possible
to mount a subvolume that did not reside directly under the
toplevel subvolume.
NOTES ON GENERIC MOUNT OPTIONS
Some of the general mount options from mount(8) that affect BTRFS and
are worth mentioning.
noatime
under read intensive work-loads, specifying noatime significantly
improves performance because no new access time information needs
to be written. Without this option, the default is relatime, which
only reduces the number of inode atime updates in comparison to the
traditional strictatime. The worst case for atime updates under
relatime occurs when many files are read whose atime is older than
24 h and which are freshly snapshotted. In that case the atime is
updated and COW happens - for each file - in bulk. See also
https://lwn.net/Articles/499293/ - Atime and btrfs: a bad
combination? (LWN, 2012-05-31).
Note that noatime may break applications that rely on atime uptimes
like the venerable Mutt (unless you use maildir mailboxes).
FILESYSTEM FEATURES
The basic set of filesystem features gets extended over time. The
backward compatibility is maintained and the features are optional,
need to be explicitly asked for so accidental use will not create
incompatibilities.
There are several classes and the respective tools to manage the
features:
at mkfs time only
This is namely for core structures, like the b-tree nodesize or
checksum algorithm, see mkfs.btrfs(8) for more details.
after mkfs, on an unmounted filesystem
Features that may optimize internal structures or add new
structures to support new functionality, see btrfstune(8). The
command btrfs inspect-internal dump-super device will dump a
superblock, you can map the value of incompat_flags to the features
listed below
after mkfs, on a mounted filesystem
The features of a filesystem (with a given UUID) are listed in
/sys/fs/btrfs/UUID/features/, one file per feature. The status is
stored inside the file. The value 1 is for enabled and active,
while 0 means the feature was enabled at mount time but turned off
afterwards.
Whether a particular feature can be turned on a mounted filesystem
can be found in the directory /sys/fs/btrfs/features/, one file per
feature. The value 1 means the feature can be enabled.
List of features (see also mkfs.btrfs(8) section FILESYSTEM FEATURES):
big_metadata
(since: 3.4)
the filesystem uses nodesize for metadata blocks, this can be
bigger than the page size
compress_lzo
(since: 2.6.38)
the lzo compression has been used on the filesystem, either as a
mount option or via btrfs filesystem defrag.
compress_zstd
(since: 4.14)
the zstd compression has been used on the filesystem, either as a
mount option or via btrfs filesystem defrag.
default_subvol
(since: 2.6.34)
the default subvolume has been set on the filesystem
extended_iref
(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
free_space_tree
(since: 4.5)
free space representation using a dedicated b-tree, successor of v1
space cache
metadata_uuid
(since: 5.0)
the main filesystem UUID is the metadata_uuid, which stores the new
UUID only in the superblock while all metadata blocks still have
the UUID set at mkfs time, see btrfstune(8) for more
mixed_backref
(since: 2.6.31)
the last major disk format change, improved backreferences, now
default
mixed_groups
(since: 2.6.37)
mixed data and metadata block groups, ie. the data and metadata are
not separated and occupy the same block groups, this mode is
suitable for small volumes as there are no constraints how the
remaining space should be used (compared to the split mode, where
empty metadata space cannot be used for data and vice versa)
on the other hand, the final layout is quite unpredictable and
possibly highly fragmented, which means worse performance
no_holes
(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
raid1c34
(since: 5.5)
extended RAID1 mode with copies on 3 or 4 devices respectively
raid56
(since: 3.9)
the filesystem contains or contained a raid56 profile of block
groups
rmdir_subvol
(since: 4.18)
indicate that rmdir(2) syscall can delete an empty subvolume just
like an ordinary directory. Note that this feature only depends on
the kernel version.
skinny_metadata
(since: 3.10)
reduced-size metadata for extent references, saves a few percent of
metadata
supported_checksums
(since: 5.5)
list of checksum algorithms supported by the kernel module, the
respective modules or built-in implementing the algorithms need to
be present to mount the filesystem
SWAPFILE SUPPORT
The swapfile is supported since kernel 5.0. Use swapon(8) to activate
the swapfile. There are some limitations of the implementation in btrfs
and linux swap subsystem:
o filesystem - must be only single device
o swapfile - the containing subvolume cannot be snapshotted
o swapfile - must be preallocated
o swapfile - must be nodatacow (ie. also nodatasum)
o swapfile - must not be compressed
The limitations come namely from the COW-based design and mapping layer
of blocks that allows the advanced features like relocation and
multi-device filesystems. However, the swap subsystem expects simpler
mapping and no background changes of the file blocks once they've been
attached to swap.
With active swapfiles, the following whole-filesystem operations will
skip swapfile extents or may fail:
o balance - block groups with swapfile extents are skipped and
reported, the rest will be processed normally
o resize grow - unaffected
o resize shrink - works as long as the extents are outside of the
shrunk range
o device add - a new device does not interfere with existing
swapfile and this operation will work, though no new swapfile can
be activated afterwards
o device delete - if the device has been added as above, it can be
also deleted
o device replace - ditto
When there are no active swapfiles and a whole-filesystem exclusive
operation is running (ie. balance, device delete, shrink), the
swapfiles cannot be temporarily activated. The operation must finish
first.
# truncate -s 0 swapfile
# chattr +C swapfile
# fallocate -l 2G swapfile
# chmod 0600 swapfile
# mkswap swapfile
# swapon swapfile
CHECKSUM ALGORITHMS
There are several checksum algorithms supported. The default and
backward compatible is crc32c. Since kernel 5.5 there are three more
with different characteristics and trade-offs regarding speed and
strength. The following list may help you to decide which one to
select.
CRC32C (32bit digest)
default, best backward compatibility, very fast, modern CPUs have
instruction-level support, not collision-resistant but still good
error detection capabilities
XXHASH (64bit digest)
can be used as CRC32C successor, very fast, optimized for modern
CPUs utilizing instruction pipelining, good collision resistance
and error detection
SHA256 (256bit digest)
a cryptographic-strength hash, relatively slow but with possible
CPU instruction acceleration or specialized hardware cards, FIPS
certified and in wide use
BLAKE2b (256bit digest)
a cryptographic-strength hash, relatively fast with possible CPU
acceleration using SIMD extensions, not standardized but based on
BLAKE which was a SHA3 finalist, in wide use, the algorithm used is
BLAKE2b-256 that's optimized for 64bit platforms
The digest size affects overall size of data block checksums stored in
the filesystem. The metadata blocks have a fixed area up to 256bits (32
bytes), so there's no increase. Each data block has a separate checksum
stored, with additional overhead of the b-tree leaves.
Approximate relative performance of the algorithms, measured against
CRC32C using reference software implementations on a 3.5GHz intel CPU:
[ cols="^,>,>",width="50%" ]
+--------+-------------+-------+
| | | |
|Digest | Cycles/4KiB | Ratio |
+--------+-------------+-------+
| | | |
|CRC32C | 1700 | 1.00 |
+--------+-------------+-------+
| | | |
|XXHASH | 2500 | 1.44 |
+--------+-------------+-------+
| | | |
|SHA256 | 105000 | 61 |
+--------+-------------+-------+
| | | |
|BLAKE2b | 22000 | 13 |
+--------+-------------+-------+
FILESYSTEM LIMITS
maximum file name length
255
maximum symlink target length
depends on the nodesize value, for 4k it's 3949 bytes, for larger
nodesize it's 4095 due to the system limit PATH_MAX
The symlink target may not be a valid path, ie. the path name
components can exceed the limits (NAME_MAX), there's no content
validation at symlink(3) creation.
maximum number of inodes
264 but depends on the available metadata space as the inodes are
created dynamically
inode numbers
minimum number: 256 (for subvolumes), regular files and
directories: 257
maximum file length
inherent limit of btrfs is 264 (16 EiB) but the linux VFS limit is
263 (8 EiB)
maximum number of subvolumes
the subvolume ids can go up to 264 but the number of actual
subvolumes depends on the available metadata space, the space
consumed by all subvolume metadata includes bookkeeping of shared
extents can be large (MiB, GiB)
maximum number of hardlinks of a file in a directory
65536 when the extref feature is turned on during mkfs (default),
roughly 100 otherwise
BOOTLOADER SUPPORT
GRUB2 (https://www.gnu.org/software/grub) has the most advanced support
of booting from BTRFS with respect to features.
U-boot (https://www.denx.de/wiki/U-Boot/) has decent support for
booting but not all BTRFS features are implemented, check the
documentation.
EXTLINUX (from the https://syslinux.org project) can boot but does not
support all features. Please check the upstream documentation before
you use it.
The first 1MiB on each device is unused with the exception of primary
superblock that is on the offset 64KiB and spans 4KiB.
FILE ATTRIBUTES
The btrfs filesystem supports setting file attributes or flags. Note
there are old and new interfaces, with confusing names. The following
list should clarify that:
o attributes: chattr(1) or lsattr(1) utilities (the ioctls are
FS_IOC_GETFLAGS and FS_IOC_SETFLAGS), due to the ioctl names the
attributes are also called flags
o xflags: to distinguish from the previous, it's extended flags,
with tunable bits similar to the attributes but extensible and new
bits will be added in the future (the ioctls are FS_IOC_FSGETXATTR
and FS_IOC_FSSETXATTR but they are not related to extended
attributes that are also called xattrs), there's no standard tool
to change the bits, there's support in xfs_io(8) as command xfs_io
-c chattr
ATTRIBUTES
a
append only, new writes are always written at the end of the file
A
no atime updates
c
compress data, all data written after this attribute is set will be
compressed. Please note that compression is also affected by the
mount options or the parent directory attributes.
When set on a directory, all newly created files will inherit this
attribute.
C
no copy-on-write, file data modifications are done in-place
When set on a directory, all newly created files will inherit this
attribute.
Note
due to implementation limitations, this flag can be set/unset
only on empty files.
d
no dump, makes sense with 3rd party tools like dump(8), on BTRFS
the attribute can be set/unset but no other special handling is
done
D
synchronous directory updates, for more details search open(2) for
O_SYNC and O_DSYNC
i
immutable, no file data and metadata changes allowed even to the
root user as long as this attribute is set (obviously the exception
is unsetting the attribute)
S
synchronous updates, for more details search open(2) for O_SYNC and
O_DSYNC
X
no compression, permanently turn off compression on the given file.
Any compression mount options will not affect this file.
When set on a directory, all newly created files will inherit this
attribute.
No other attributes are supported. For the complete list please refer
to the chattr(1) manual page.
XFLAGS
There's overlap of letters assigned to the bits with the attributes,
this list refers to what xfs_io(8) provides:
i
immutable, same as the attribute
a
append only, same as the attribute
s
synchronous updates, same as the atribute S
A
no atime updates, same as the attribute
d
no dump, same as the attribute
CONTROL DEVICE
There's a character special device /dev/btrfs-control with major and
minor numbers 10 and 234 (the device can be found under the misc
category).
$ ls -l /dev/btrfs-control
crw------- 1 root root 10, 234 Jan 1 12:00 /dev/btrfs-control
The device accepts some ioctl calls that can perform following actions
on the filesystem module:
o scan devices for btrfs filesystem (ie. to let multi-device
filesystems mount automatically) and register them with the kernel
module
o similar to scan, but also wait until the device scanning process
is finished for a given filesystem
o get the supported features (can be also found under
/sys/fs/btrfs/features)
The device is usually created by a system device node manager (eg.
udev), but can be created manually:
# mknod --mode=600 c 10 234 /dev/btrfs-control
The control device is not strictly required but the device scanning
will not work and a workaround would need to be used to mount a
multi-device filesystem. The mount option device can trigger the device
scanning during mount.
FILESYSTEM WITH MULTIPLE PROFILES
It is possible that a btrfs filesystem contains multiple block group
profiles of the same type. This could happen when a profile conversion
using balance filters is interrupted (see btrfs-balance(8)). Some btrfs
commands perform a test to detect this kind of condition and print a
warning like this:
WARNING: Multiple block group profiles detected, see 'man btrfs(5)'.
WARNING: Data: single, raid1
WARNING: Metadata: single, raid1
The corresponding output of btrfs filesystem df might look like:
WARNING: Multiple block group profiles detected, see 'man btrfs(5)'.
WARNING: Data: single, raid1
WARNING: Metadata: single, raid1
Data, RAID1: total=832.00MiB, used=0.00B
Data, single: total=1.63GiB, used=0.00B
System, single: total=4.00MiB, used=16.00KiB
Metadata, single: total=8.00MiB, used=112.00KiB
Metadata, RAID1: total=64.00MiB, used=32.00KiB
GlobalReserve, single: total=16.25MiB, used=0.00B
There's more than one line for type Data and Metadata, while the
profiles are single and RAID1.
This state of the filesystem OK but most likely needs the
user/administrator to take an action and finish the interrupted tasks.
This cannot be easily done automatically, also the user knows the
expected final profiles.
In the example above, the filesystem started as a single device and
single block group profile. Then another device was added, followed by
balance with convert=raid1 but for some reason hasn't finished.
Restarting the balance with convert=raid1 will continue and end up with
filesystem with all block group profiles RAID1.
Note
If you're familiar with balance filters, you can use
convert=raid1,profiles=single,soft, which will take only the
unconverted single profiles and convert them to raid1. This may
speed up the conversion as it would not try to rewrite the already
convert raid1 profiles.
Having just one profile is desired as this also clearly defines the
profile of newly allocated block groups, otherwise this depends on
internal allocation policy. When there are multiple profiles present,
the order of selection is RAID6, RAID5, RAID10, RAID1, RAID0 as long as
the device number constraints are satisfied.
Commands that print the warning were chosen so they're brought to user
attention when the filesystem state is being changed in that regard.
This is: device add, device delete, balance cancel, balance pause.
Commands that report space usage: filesystem df, device usage. The
command filesystem usage provides a line in the overall summary:
Multiple profiles: yes (data, metadata)
SEE ALSO
acl(5), btrfs(8), chattr(1), fstrim(8), ioctl(2), mkfs.btrfs(8),
mount(8), swapon(8)
2020-07-05 BTRFS-MAN5(5)