Documentation/filesystems/btrfs.txt

   1
   2 BTRFS
   3 =====
   4
   5 Btrfs is a copy on write filesystem for Linux aimed at
   6 implementing advanced features while focusing on fault tolerance,
   7 repair and easy administration. Initially developed by Oracle, Btrfs
   8 is licensed under the GPL and open for contribution from anyone.
   9
  10 Linux has a wealth of filesystems to choose from, but we are facing a
  11 number of challenges with scaling to the large storage subsystems that
  12 are becoming common in today's data centers. Filesystems need to scale
  13 in their ability to address and manage large storage, and also in
  14 their ability to detect, repair and tolerate errors in the data stored
  15 on disk.  Btrfs is under heavy development, and is not suitable for
  16 any uses other than benchmarking and review. The Btrfs disk format is
  17 not yet finalized.
  18
  19 The main Btrfs features include:
  20
  21     * Extent based file storage (2^64 max file size)
  22     * Space efficient packing of small files
  23     * Space efficient indexed directories
  24     * Dynamic inode allocation
  25     * Writable snapshots
  26     * Subvolumes (separate internal filesystem roots)
  27     * Object level mirroring and striping
  28     * Checksums on data and metadata (multiple algorithms available)
  29     * Compression
  30     * Integrated multiple device support, with several raid algorithms
  31     * Online filesystem check (not yet implemented)
  32     * Very fast offline filesystem check
  33     * Efficient incremental backup and FS mirroring (not yet implemented)
  34     * Online filesystem defragmentation
  35
  36
  37 Mount Options
  38 =============
  39
  40 When mounting a btrfs filesystem, the following option are accepted.
  41 Options with (*) are default options and will not show in the mount options.
  42
  43   alloc_start=<bytes>
  44         Debugging option to force all block allocations above a certain
  45         byte threshold on each block device.  The value is specified in
  46         bytes, optionally with a K, M, or G suffix, case insensitive.
  47         Default is 1MB.
  48
  49   noautodefrag(*)
  50   autodefrag
  51         Disable/enable auto defragmentation.
  52         Auto defragmentation detects small random writes into files and queue
  53         them up for the defrag process.  Works best for small files;
  54         Not well suited for large database workloads.
  55
  56   check_int
  57   check_int_data
  58   check_int_print_mask=<value>
  59         These debugging options control the behavior of the integrity checking
  60         module (the BTRFS_FS_CHECK_INTEGRITY config option required).
  61
  62         check_int enables the integrity checker module, which examines all
  63         block write requests to ensure on-disk consistency, at a large
  64         memory and CPU cost.
  65
  66         check_int_data includes extent data in the integrity checks, and
  67         implies the check_int option.
  68
  69         check_int_print_mask takes a bitmask of BTRFSIC_PRINT_MASK_* values
  70         as defined in fs/btrfs/check-integrity.c, to control the integrity
  71         checker module behavior.
  72
  73         See comments at the top of fs/btrfs/check-integrity.c for more info.
  74
  75   commit=<seconds>
  76         Set the interval of periodic commit, 30 seconds by default. Higher
  77         values defer data being synced to permanent storage with obvious
  78         consequences when the system crashes. The upper bound is not forced,
  79         but a warning is printed if it's more than 300 seconds (5 minutes).
  80
  81   compress
  82   compress=<type>
  83   compress-force
  84   compress-force=<type>
  85         Control BTRFS file data compression.  Type may be specified as "zlib"
  86         "lzo" or "no" (for no compression, used for remounting).  If no type
  87         is specified, zlib is used.  If compress-force is specified,
  88         all files will be compressed, whether or not they compress well.
  89         If compression is enabled, nodatacow and nodatasum are disabled.
  90
  91   degraded
  92         Allow mounts to continue with missing devices.  A read-write mount may
  93         fail with too many devices missing, for example if a stripe member
  94         is completely missing.
  95
  96   device=<devicepath>
  97         Specify a device during mount so that ioctls on the control device
  98         can be avoided.  Especially useful when trying to mount a multi-device
  99         setup as root.  May be specified multiple times for multiple devices.
 100
 101   discard
 102         Issue frequent commands to let the block device reclaim space freed by
 103         the filesystem.  This is useful for SSD devices, thinly provisioned
 104         LUNs and virtual machine images, but may have a significant
 105         performance impact.  (The fstrim command is also available to
 106         initiate batch trims from userspace).
 107
 108   enospc_debug
 109         Debugging option to be more verbose in some ENOSPC conditions.
 110
 111   fatal_errors=<action>
 112         Action to take when encountering a fatal error:
 113           "bug" - BUG() on a fatal error.  This is the default.
 114           "panic" - panic() on a fatal error.
 115
 116   flushoncommit
 117         The 'flushoncommit' mount option forces any data dirtied by a write in a
 118         prior transaction to commit as part of the current commit.  This makes
 119         the committed state a fully consistent view of the file system from the
 120         application's perspective (i.e., it includes all completed file system
 121         operations).  This was previously the behavior only when a snapshot is
 122         created.
 123
 124   inode_cache
 125         Enable free inode number caching.   Defaults to off due to an overflow
 126         problem when the free space crcs don't fit inside a single page.
 127
 128   max_inline=<bytes>
 129         Specify the maximum amount of space, in bytes, that can be inlined in
 130         a metadata B-tree leaf.  The value is specified in bytes, optionally
 131         with a K, M, or G suffix, case insensitive.  In practice, this value
 132         is limited by the root sector size, with some space unavailable due
 133         to leaf headers.  For a 4k sectorsize, max inline data is ~3900 bytes.
 134
 135   metadata_ratio=<value>
 136         Specify that 1 metadata chunk should be allocated after every <value>
 137         data chunks.  Off by default.
 138
 139   noacl
 140         Disable support for Posix Access Control Lists (ACLs).  See the
 141         acl(5) manual page for more information about ACLs.
 142
 143   barrier(*)
 144   nobarrier
 145         Enable/disable the use of block layer write barriers.  Write barriers
 146         ensure that certain IOs make it through the device cache and are on
 147         persistent storage. If disabled on a device with a volatile
 148         (non-battery-backed) write-back cache, nobarrier option will lead to
 149         filesystem corruption on a system crash or power loss.
 150
 151   nodatacow
 152         Disable data copy-on-write for newly created files.  Implies nodatasum,
 153         and disables all compression.
 154
 155   nodatasum
 156         Disable data checksumming for newly created files.
 157
 158   notreelog
 159         Disable the tree logging used for fsync and O_SYNC writes.
 160
 161   recovery
 162         Enable autorecovery attempts if a bad tree root is found at mount time.
 163         Currently this scans a list of several previous tree roots and tries to
 164         use the first readable.
 165
 166   rescan_uuid_tree
 167         Force check and rebuild procedure of the UUID tree. This should not
 168         normally be needed.
 169
 170   skip_balance
 171         Skip automatic resume of interrupted balance operation after mount.
 172         May be resumed with "btrfs balance resume."
 173
 174   space_cache (*)
 175         Enable the on-disk freespace cache.
 176   nospace_cache
 177         Disable freespace cache loading without clearing the cache.
 178   clear_cache
 179         Force clearing and rebuilding of the disk space cache if something
 180         has gone wrong.
 181
 182   ssd
 183   nossd
 184   ssd_spread
 185         Options to control ssd allocation schemes.  By default, BTRFS will
 186         enable or disable ssd allocation heuristics depending on whether a
 187         rotational or nonrotational disk is in use.  The ssd and nossd options
 188         can override this autodetection.
 189
 190         The ssd_spread mount option attempts to allocate into big chunks
 191         of unused space, and may perform better on low-end ssds.  ssd_spread
 192         implies ssd, enabling all other ssd heuristics as well.
 193
 194   subvol=<path>
 195         Mount subvolume at <path> rather than the root subvolume.  <path> is
 196         relative to the top level subvolume.
 197
 198   subvolid=<ID>
 199         Mount subvolume specified by an ID number rather than the root subvolume.
 200         This allows mounting of subvolumes which are not in the root of the mounted
 201         filesystem.
 202         You can use "btrfs subvolume list" to see subvolume ID numbers.
 203
 204   subvolrootid=<objectid> (deprecated)
 205         Mount subvolume specified by <objectid> rather than the root subvolume.
 206         This allows mounting of subvolumes which are not in the root of the mounted
 207         filesystem.
 208         You can use "btrfs subvolume show " to see the object ID for a subvolume.
 209
 210   thread_pool=<number>
 211         The number of worker threads to allocate.  The default number is equal
 212         to the number of CPUs + 2, or 8, whichever is smaller.
 213
 214   user_subvol_rm_allowed
 215         Allow subvolumes to be deleted by a non-root user. Use with caution.
 216
 217 MAILING LIST
 218 ============
 219
 220 There is a Btrfs mailing list hosted on vger.kernel.org. You can
 221 find details on how to subscribe here:
 222
 223 http://vger.kernel.org/vger-lists.html#linux-btrfs
 224
 225 Mailing list archives are available from gmane:
 226
 227 http://dir.gmane.org/gmane.comp.file-systems.btrfs
 228
 229
 230
 231 IRC
 232 ===
 233
 234 Discussion of Btrfs also occurs on the #btrfs channel of the Freenode
 235 IRC network.
 236
 237
 238
 239         UTILITIES
 240         =========
 241
 242 Userspace tools for creating and manipulating Btrfs file systems are
 243 available from the git repository at the following location:
 244
 245  http://git.kernel.org/?p=linux/kernel/git/mason/btrfs-progs.git
 246  git://git.kernel.org/pub/scm/linux/kernel/git/mason/btrfs-progs.git
 247
 248 These include the following tools:
 249
 250 * mkfs.btrfs: create a filesystem
 251
 252 * btrfs: a single tool to manage the filesystems, refer to the manpage for more details
 253
 254 * 'btrfsck' or 'btrfs check': do a consistency check of the filesystem
 255
 256 Other tools for specific tasks:
 257
 258 * btrfs-convert: in-place conversion from ext2/3/4 filesystems
 259
 260 * btrfs-image: dump filesystem metadata for debugging