258 lines
9.0 KiB
Plaintext
258 lines
9.0 KiB
Plaintext
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BTRFS
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=====
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Btrfs is a copy on write filesystem for Linux aimed at
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implementing advanced features while focusing on fault tolerance,
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repair and easy administration. Initially developed by Oracle, Btrfs
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is licensed under the GPL and open for contribution from anyone.
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Linux has a wealth of filesystems to choose from, but we are facing a
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number of challenges with scaling to the large storage subsystems that
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are becoming common in today's data centers. Filesystems need to scale
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in their ability to address and manage large storage, and also in
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their ability to detect, repair and tolerate errors in the data stored
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on disk. Btrfs is under heavy development, and is not suitable for
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any uses other than benchmarking and review. The Btrfs disk format is
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not yet finalized.
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The main Btrfs features include:
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* Extent based file storage (2^64 max file size)
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* Space efficient packing of small files
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* Space efficient indexed directories
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* Dynamic inode allocation
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* Writable snapshots
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* Subvolumes (separate internal filesystem roots)
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* Object level mirroring and striping
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* Checksums on data and metadata (multiple algorithms available)
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* Compression
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* Integrated multiple device support, with several raid algorithms
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* Online filesystem check (not yet implemented)
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* Very fast offline filesystem check
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* Efficient incremental backup and FS mirroring (not yet implemented)
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* Online filesystem defragmentation
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Mount Options
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=============
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When mounting a btrfs filesystem, the following option are accepted.
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Unless otherwise specified, all options default to off.
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alloc_start=<bytes>
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Debugging option to force all block allocations above a certain
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byte threshold on each block device. The value is specified in
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bytes, optionally with a K, M, or G suffix, case insensitive.
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Default is 1MB.
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autodefrag
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Detect small random writes into files and queue them up for the
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defrag process. Works best for small files; Not well suited for
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large database workloads.
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check_int
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check_int_data
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check_int_print_mask=<value>
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These debugging options control the behavior of the integrity checking
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module (the BTRFS_FS_CHECK_INTEGRITY config option required).
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check_int enables the integrity checker module, which examines all
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block write requests to ensure on-disk consistency, at a large
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memory and CPU cost.
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check_int_data includes extent data in the integrity checks, and
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implies the check_int option.
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check_int_print_mask takes a bitmask of BTRFSIC_PRINT_MASK_* values
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as defined in fs/btrfs/check-integrity.c, to control the integrity
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checker module behavior.
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See comments at the top of fs/btrfs/check-integrity.c for more info.
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compress
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compress=<type>
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compress-force
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compress-force=<type>
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Control BTRFS file data compression. Type may be specified as "zlib"
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"lzo" or "no" (for no compression, used for remounting). If no type
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is specified, zlib is used. If compress-force is specified,
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all files will be compressed, whether or not they compress well.
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If compression is enabled, nodatacow and nodatasum are disabled.
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degraded
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Allow mounts to continue with missing devices. A read-write mount may
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fail with too many devices missing, for example if a stripe member
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is completely missing.
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device=<devicepath>
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Specify a device during mount so that ioctls on the control device
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can be avoided. Especially useful when trying to mount a multi-device
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setup as root. May be specified multiple times for multiple devices.
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discard
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Issue frequent commands to let the block device reclaim space freed by
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the filesystem. This is useful for SSD devices, thinly provisioned
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LUNs and virtual machine images, but may have a significant
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performance impact. (The fstrim command is also available to
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initiate batch trims from userspace).
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enospc_debug
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Debugging option to be more verbose in some ENOSPC conditions.
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fatal_errors=<action>
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Action to take when encountering a fatal error:
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"bug" - BUG() on a fatal error. This is the default.
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"panic" - panic() on a fatal error.
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flushoncommit
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The 'flushoncommit' mount option forces any data dirtied by a write in a
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prior transaction to commit as part of the current commit. This makes
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the committed state a fully consistent view of the file system from the
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application's perspective (i.e., it includes all completed file system
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operations). This was previously the behavior only when a snapshot is
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created.
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inode_cache
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Enable free inode number caching. Defaults to off due to an overflow
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problem when the free space crcs don't fit inside a single page.
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max_inline=<bytes>
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Specify the maximum amount of space, in bytes, that can be inlined in
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a metadata B-tree leaf. The value is specified in bytes, optionally
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with a K, M, or G suffix, case insensitive. In practice, this value
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is limited by the root sector size, with some space unavailable due
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to leaf headers. For a 4k sectorsize, max inline data is ~3900 bytes.
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metadata_ratio=<value>
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Specify that 1 metadata chunk should be allocated after every <value>
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data chunks. Off by default.
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noacl
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Disable support for Posix Access Control Lists (ACLs). See the
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acl(5) manual page for more information about ACLs.
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nobarrier
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Disables the use of block layer write barriers. Write barriers ensure
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that certain IOs make it through the device cache and are on persistent
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storage. If used on a device with a volatile (non-battery-backed)
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write-back cache, this option will lead to filesystem corruption on a
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system crash or power loss.
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nodatacow
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Disable data copy-on-write for newly created files. Implies nodatasum,
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and disables all compression.
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nodatasum
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Disable data checksumming for newly created files.
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notreelog
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Disable the tree logging used for fsync and O_SYNC writes.
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recovery
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Enable autorecovery attempts if a bad tree root is found at mount time.
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Currently this scans a list of several previous tree roots and tries to
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use the first readable.
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skip_balance
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Skip automatic resume of interrupted balance operation after mount.
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May be resumed with "btrfs balance resume."
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space_cache (*)
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Enable the on-disk freespace cache.
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nospace_cache
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Disable freespace cache loading without clearing the cache.
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clear_cache
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Force clearing and rebuilding of the disk space cache if something
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has gone wrong.
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ssd
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nossd
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ssd_spread
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Options to control ssd allocation schemes. By default, BTRFS will
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enable or disable ssd allocation heuristics depending on whether a
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rotational or nonrotational disk is in use. The ssd and nossd options
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can override this autodetection.
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The ssd_spread mount option attempts to allocate into big chunks
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of unused space, and may perform better on low-end ssds. ssd_spread
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implies ssd, enabling all other ssd heuristics as well.
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subvol=<path>
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Mount subvolume at <path> rather than the root subvolume. <path> is
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relative to the top level subvolume.
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subvolid=<ID>
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Mount subvolume specified by an ID number rather than the root subvolume.
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This allows mounting of subvolumes which are not in the root of the mounted
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filesystem.
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You can use "btrfs subvolume list" to see subvolume ID numbers.
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subvolrootid=<objectid> (deprecated)
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Mount subvolume specified by <objectid> rather than the root subvolume.
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This allows mounting of subvolumes which are not in the root of the mounted
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filesystem.
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You can use "btrfs subvolume show " to see the object ID for a subvolume.
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thread_pool=<number>
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The number of worker threads to allocate. The default number is equal
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to the number of CPUs + 2, or 8, whichever is smaller.
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user_subvol_rm_allowed
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Allow subvolumes to be deleted by a non-root user. Use with caution.
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MAILING LIST
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============
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There is a Btrfs mailing list hosted on vger.kernel.org. You can
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find details on how to subscribe here:
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http://vger.kernel.org/vger-lists.html#linux-btrfs
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Mailing list archives are available from gmane:
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http://dir.gmane.org/gmane.comp.file-systems.btrfs
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IRC
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===
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Discussion of Btrfs also occurs on the #btrfs channel of the Freenode
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IRC network.
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UTILITIES
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=========
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Userspace tools for creating and manipulating Btrfs file systems are
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available from the git repository at the following location:
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http://git.kernel.org/?p=linux/kernel/git/mason/btrfs-progs.git
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git://git.kernel.org/pub/scm/linux/kernel/git/mason/btrfs-progs.git
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These include the following tools:
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mkfs.btrfs: create a filesystem
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btrfsctl: control program to create snapshots and subvolumes:
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mount /dev/sda2 /mnt
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btrfsctl -s new_subvol_name /mnt
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btrfsctl -s snapshot_of_default /mnt/default
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btrfsctl -s snapshot_of_new_subvol /mnt/new_subvol_name
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btrfsctl -s snapshot_of_a_snapshot /mnt/snapshot_of_new_subvol
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ls /mnt
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default snapshot_of_a_snapshot snapshot_of_new_subvol
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new_subvol_name snapshot_of_default
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Snapshots and subvolumes cannot be deleted right now, but you can
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rm -rf all the files and directories inside them.
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btrfsck: do a limited check of the FS extent trees.
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btrfs-debug-tree: print all of the FS metadata in text form. Example:
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btrfs-debug-tree /dev/sda2 >& big_output_file
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