BTRFS
Btrfs is a copy on write filesystem for Linux aimed at
implementing advanced features while focusing on fault tolerance,
repair and easy administration. Initially developed by Oracle, Btrfs
is licensed under the GPL and open for contribution from anyone.
Linux has a wealth of filesystems to choose from, but we are facing a
number of challenges with scaling to the large storage subsystems that
are becoming common in today’s data centers. Filesystems need to scale
in their ability to address and manage large storage, and also in
their ability to detect, repair and tolerate errors in the data stored
on disk. Btrfs is under heavy development, and is not suitable for
any uses other than benchmarking and review. The Btrfs disk format is
not yet finalized.
The main Btrfs features include:
* Extent based file storage (2^64 max file size)
* Space efficient packing of small files
* Space efficient indexed directories
* Dynamic inode allocation
* Writable snapshots
* Subvolumes (separate internal filesystem roots)
* Object level mirroring and striping
* Checksums on data and metadata (multiple algorithms available)
* Compression
* Integrated multiple device support, with several raid algorithms
* Online filesystem check (not yet implemented)
* Very fast offline filesystem check
* Efficient incremental backup and FS mirroring (not yet implemented)
* Online filesystem defragmentation
Mount Options
When mounting a btrfs filesystem, the following option are accepted.
Options with (*) are default options and will not show in the mount options.
alloc_start=
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.
Default is 1MB.
noautodefrag(*)
autodefrag
Disable/enable auto defragmentation.
Auto defragmentation detects small random writes into files and queue
them up for the defrag process. Works best for small files;
Not well suited for large database workloads.
check_int
check_int_data
check_int_print_mask=
These debugging options control the behavior of the integrity checking
module (the BTRFS_FS_CHECK_INTEGRITY config option required).
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 info.
commit=
Set the interval of periodic commit, 30 seconds by default. Higher
values defer data being synced to permanent storage with 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).
compress
compress=
compress-force
compress-force=
Control BTRFS file data compression. Type may be specified as “zlib”
“lzo” or “no” (for no compression, used for remounting). If no type
is specified, zlib is used. If compress-force is specified,
all files will be compressed, whether or not they compress well.
If compression is enabled, nodatacow and nodatasum are disabled.
degraded
Allow mounts to continue with missing devices. A read-write mount may
fail with too many devices missing, for example if a stripe member
is completely missing.
device=
Specify a device during mount so that ioctls on the control device
can be avoided. Especialy useful when trying to mount a multi-device
setup as root. May be specified multiple times for multiple devices.
nodiscard(*)
discard
Disable/enable discard mount option.
Discard issues frequent commands to let the block device reclaim space
freed by the filesystem.
This is useful for SSD devices, thinly provisioned
LUNs and virtual machine images, but may have a significant
performance impact. (The fstrim command is also available to
initiate batch trims from userspace).
noenospc_debug(*)
enospc_debug
Disable/enable debugging option to be more verbose in some ENOSPC conditions.
fatal_errors=
Action to take when encountering a fatal error:
“bug” - BUG() on a fatal error. This is the default.
“panic” - panic() on a fatal error.
noflushoncommit(*)
flushoncommit
The ‘flushoncommit’ mount option forces any data dirtied by a write in a
prior transaction to commit as part of the current commit. 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 is
created.
inode_cache
Enable free inode number caching. Defaults to off due to an overflow
problem when the free space crcs don’t fit inside a single page.
max_inline=
Specify the maximum amount of space, in bytes, that can be inlined in
a metadata B-tree leaf. The value is specified in bytes, optionally
with a K, M, or G suffix, case insensitive. In practice, this value
is limited by the root sector size, with some space unavailable due
to leaf headers. For a 4k sectorsize, max inline data is ~3900 bytes.
metadata_ratio=
Specify that 1 metadata chunk should be allocated after every
data chunks. Off by default.
acl(*)
noacl
Enable/disable support for Posix Access Control Lists (ACLs). See the
acl(5) manual page for more information about ACLs.
barrier(*)
nobarrier
Enable/disable the use of block layer write barriers. Write barriers
ensure that certain IOs make it through the device cache and are on
persistent storage. If disabled on a device with a volatile
(non-battery-backed) write-back cache, nobarrier option will lead to
filesystem corruption on a system crash or power loss.
datacow(*)
nodatacow
Enable/disable data copy-on-write for newly created files.
Nodatacow implies nodatasum, and disables all compression.
datasum(*)
nodatasum
Enable/disable data checksumming for newly created files.
Datasum implies datacow.
treelog(*)
notreelog
Enable/disable the tree logging used for fsync and O_SYNC writes.
nologreplay
Disable the log tree replay at mount time to prevent filesystem
from getting modified.
Must be used with ‘ro’ mount option.
A filesystem mounted with this option cannot transition to a
read-write mount via remount,rw - the filesystem must be unmounted
and mounted back again if read-write access is desired.
usebackuproot
Enable attempts to use backup tree roots if a bad tree root is found at
mount time.
Currently this scans a list of 4 previous tree roots and tries to
use the first readable.
And since the mount option doesn’t affect any behavior after mount,
it won’t be shown in mount info.
Prior to 4.6, this was done by ‘recovery’ option that has been
deprecated, but will work.
rescan_uuid_tree
Force check and rebuild procedure of the UUID tree. This should not
normally be needed.
skip_balance
Skip automatic resume of interrupted balance operation after mount.
May be resumed with “btrfs balance resume.”
space_cache (*)
Enable the on-disk freespace cache.
nospace_cache
Disable freespace cache loading without clearing the cache.
clear_cache
Force clearing and rebuilding of the disk space cache if something
has gone wrong.
ssd
nossd
ssd_spread
Options to control ssd allocation schemes. By default, BTRFS will
enable or disable ssd allocation heuristics depending on whether a
rotational or nonrotational disk is in use. The ssd and nossd options
can override this autodetection.
The ssd_spread mount option attempts to allocate into big chunks
of unused space, and may perform better on low-end ssds. ssd_spread
implies ssd, enabling all other ssd heuristics as well.
subvol=
Mount subvolume at
relative to the top level subvolume.
subvolid=
Mount subvolume specified by an ID number rather than the root subvolume.
This allows mounting of subvolumes which are not in the root of the mounted
filesystem.
You can use “btrfs subvolume list” to see subvolume ID numbers.
subvolrootid=
Mount subvolume specified by
This allows mounting of subvolumes which are not in the root of the mounted
filesystem.
You can use “btrfs subvolume show “ to see the object ID for a subvolume.
thread_pool=
The number of worker threads to allocate. The default number is equal
to the number of CPUs + 2, or 8, whichever is smaller.
user_subvol_rm_allowed
Allow subvolumes to be deleted by a non-root user. Use with caution.
MAILING LIST
There is a Btrfs mailing list hosted on vger.kernel.org. You can
find details on how to subscribe here:
http://vger.kernel.org/vger-lists.html#linux-btrfs
Mailing list archives are available from gmane:
http://dir.gmane.org/gmane.comp.file-systems.btrfs
IRC
Discussion of Btrfs also occurs on the #btrfs channel of the Freenode
IRC network.
UTILITIES
=========
Userspace tools for creating and manipulating Btrfs file systems are
available from the git repository at the following location:
http://git.kernel.org/?p=linux/kernel/git/mason/btrfs-progs.git
git://git.kernel.org/pub/scm/linux/kernel/git/mason/btrfs-progs.git
These include the following tools:
mkfs.btrfs: create a filesystem
btrfs: a single tool to manage the filesystems, refer to the manpage for more details
‘btrfsck’ or ‘btrfs check’: do a consistency check of the filesystem
Other tools for specific tasks:
btrfs-convert: in-place conversion from ext2/3/4 filesystems
btrfs-image: dump filesystem metadata for debugging
