Documentation: btrfs: remove usage specific information

The document in the kernel sources is yet another palce where the
documentation would need to be updated, while it is not the primary
source. We actively maintain the wiki pages.

Signed-off-by: David Sterba <dsterba@suse.com>

Documentation/filesystems/btrfs.txt

@@ -1,20 +1,10 @@
-
 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.
+Btrfs is a copy on write filesystem for Linux aimed at implementing advanced
+features while focusing on fault tolerance, repair and easy administration.
+Jointly developed by several companies, licensed under the GPL and open for
+contribution from anyone.
 
 The main Btrfs features include:
 
@@ -28,256 +18,14 @@ The main Btrfs features include:
     * 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)
+    * Offline filesystem check
+    * Efficient incremental backup and FS mirroring
     * Online filesystem defragmentation
 
+For more information please refer to the wiki
 
-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 sector size, 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.
-
-  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 non-rotational 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
+  https://btrfs.wiki.kernel.org
 
-* btrfs-image: dump filesystem metadata for debugging
+that maintains information about administration tasks, frequently asked
+questions, use cases, mount options, comprehensible changelogs, features,
+manual pages, source code repositories, contacts etc.