| |
| 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=<bytes> |
| 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=<value> |
| 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=<seconds> |
| 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=<type> |
| compress-force |
| compress-force=<type> |
| 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=<devicepath> |
| Specify a device during mount so that ioctls on the control device |
| can be avoided. Especially 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> |
| 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=<bytes> |
| 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=<value> |
| Specify that 1 metadata chunk should be allocated after every <value> |
| 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. |
| |
| recovery |
| Enable autorecovery attempts if a bad tree root is found at mount time. |
| Currently this scans a list of several previous tree roots and tries to |
| use the first readable. |
| |
| 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=<path> |
| Mount subvolume at <path> rather than the root subvolume. <path> is |
| relative to the top level subvolume. |
| |
| subvolid=<ID> |
| 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=<objectid> (deprecated) |
| Mount subvolume specified by <objectid> 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 show " to see the object ID for a subvolume. |
| |
| thread_pool=<number> |
| 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 |
