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diff --git a/Documentation/filesystems/btrfs.txt b/Documentation/filesystems/btrfs.txt new file mode 100644 index 00000000..b349d57b --- /dev/null +++ b/Documentation/filesystems/btrfs.txt @@ -0,0 +1,257 @@ + +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. +Unless otherwise specified, all options default to off. + + 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. + + autodefrag + Detect 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. + + 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. Especialy useful when trying to mount a multi-device + setup as root. May be specified multiple times for multiple devices. + + discard + Issue 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). + + enospc_debug + 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. + + 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. + + noacl + Disable support for Posix Access Control Lists (ACLs). See the + acl(5) manual page for more information about ACLs. + + nobarrier + Disables 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 used on a device with a volatile (non-battery-backed) + write-back cache, this option will lead to filesystem corruption on a + system crash or power loss. + + nodatacow + Disable data copy-on-write for newly created files. Implies nodatasum, + and disables all compression. + + nodatasum + Disable data checksumming for newly created files. + + notreelog + 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. + + 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 + +btrfsctl: control program to create snapshots and subvolumes: + + mount /dev/sda2 /mnt + btrfsctl -s new_subvol_name /mnt + btrfsctl -s snapshot_of_default /mnt/default + btrfsctl -s snapshot_of_new_subvol /mnt/new_subvol_name + btrfsctl -s snapshot_of_a_snapshot /mnt/snapshot_of_new_subvol + ls /mnt + default snapshot_of_a_snapshot snapshot_of_new_subvol + new_subvol_name snapshot_of_default + + Snapshots and subvolumes cannot be deleted right now, but you can + rm -rf all the files and directories inside them. + +btrfsck: do a limited check of the FS extent trees. + +btrfs-debug-tree: print all of the FS metadata in text form. Example: + + btrfs-debug-tree /dev/sda2 >& big_output_file |