nilfs2: add document

This adds a document describing the features, mount options, userland
tools, usage, disk format, and related URLs for the nilfs2 file system.
Signed-off-by: NRyusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Documentation/filesystems/00-INDEX

@@ -68,6 +68,8 @@ ncpfs.txt
 	- info on Novell Netware(tm) filesystem using NCP protocol.
 nfsroot.txt
 	- short guide on setting up a diskless box with NFS root filesystem.
+nilfs2.txt
+	- info and mount options for the NILFS2 filesystem.
 ntfs.txt
 	- info and mount options for the NTFS filesystem (Windows NT).
 ocfs2.txt

Documentation/filesystems/nilfs2.txt

+NILFS2
+------
+
+NILFS2 is a log-structured file system (LFS) supporting continuous
+snapshotting.  In addition to versioning capability of the entire file
+system, users can even restore files mistakenly overwritten or
+destroyed just a few seconds ago.  Since NILFS2 can keep consistency
+like conventional LFS, it achieves quick recovery after system
+crashes.
+
+NILFS2 creates a number of checkpoints every few seconds or per
+synchronous write basis (unless there is no change).  Users can select
+significant versions among continuously created checkpoints, and can
+change them into snapshots which will be preserved until they are
+changed back to checkpoints.
+
+There is no limit on the number of snapshots until the volume gets
+full.  Each snapshot is mountable as a read-only file system
+concurrently with its writable mount, and this feature is convenient
+for online backup.
+
+The userland tools are included in nilfs-utils package, which is
+available from the following download page.  At least "mkfs.nilfs2",
+"mount.nilfs2", "umount.nilfs2", and "nilfs_cleanerd" (so called
+cleaner or garbage collector) are required.  Details on the tools are
+described in the man pages included in the package.
+
+Project web page:    http://www.nilfs.org/en/
+Download page:       http://www.nilfs.org/en/download.html
+Git tree web page:   http://www.nilfs.org/git/
+NILFS mailing lists: http://www.nilfs.org/mailman/listinfo/users
+
+Caveats
+=======
+
+Features which NILFS2 does not support yet:
+
+	- atime
+	- extended attributes
+	- POSIX ACLs
+	- quotas
+	- writable snapshots
+	- remote backup (CDP)
+	- data integrity
+	- defragmentation
+
+Mount options
+=============
+
+NILFS2 supports the following mount options:
+(*) == default
+
+barrier=on(*)		This enables/disables barriers. barrier=off disables
+			it, barrier=on enables it.
+errors=continue(*)	Keep going on a filesystem error.
+errors=remount-ro	Remount the filesystem read-only on an error.
+errors=panic		Panic and halt the machine if an error occurs.
+cp=n			Specify the checkpoint-number of the snapshot to be
+			mounted.  Checkpoints and snapshots are listed by lscp
+			user command.  Only the checkpoints marked as snapshot
+			are mountable with this option.  Snapshot is read-only,
+			so a read-only mount option must be specified together.
+order=relaxed(*)	Apply relaxed order semantics that allows modified data
+			blocks to be written to disk without making a
+			checkpoint if no metadata update is going.  This mode
+			is equivalent to the ordered data mode of the ext3
+			filesystem except for the updates on data blocks still
+			conserve atomicity.  This will improve synchronous
+			write performance for overwriting.
+order=strict		Apply strict in-order semantics that preserves sequence
+			of all file operations including overwriting of data
+			blocks.  That means, it is guaranteed that no
+			overtaking of events occurs in the recovered file
+			system after a crash.
+
+NILFS2 usage
+============
+
+To use nilfs2 as a local file system, simply:
+
+ # mkfs -t nilfs2 /dev/block_device
+ # mount -t nilfs2 /dev/block_device /dir
+
+This will also invoke the cleaner through the mount helper program
+(mount.nilfs2).
+
+Checkpoints and snapshots are managed by the following commands.
+Their manpages are included in the nilfs-utils package above.
+
+  lscp     list checkpoints or snapshots.
+  mkcp     make a checkpoint or a snapshot.
+  chcp     change an existing checkpoint to a snapshot or vice versa.
+  rmcp     invalidate specified checkpoint(s).
+
+To mount a snapshot,
+
+ # mount -t nilfs2 -r -o cp=<cno> /dev/block_device /snap_dir
+
+where <cno> is the checkpoint number of the snapshot.
+
+To unmount the NILFS2 mount point or snapshot, simply:
+
+ # umount /dir
+
+Then, the cleaner daemon is automatically shut down by the umount
+helper program (umount.nilfs2).
+
+Disk format
+===========
+
+A nilfs2 volume is equally divided into a number of segments except
+for the super block (SB) and segment #0.  A segment is the container
+of logs.  Each log is composed of summary information blocks, payload
+blocks, and an optional super root block (SR):
+
+   ______________________________________________________
+  | |SB| | Segment | Segment | Segment | ... | Segment | |
+  |_|__|_|____0____|____1____|____2____|_____|____N____|_|
+  0 +1K +4K       +8M       +16M      +24M  +(8MB x N)
+       .             .            (Typical offsets for 4KB-block)
+    .                  .
+  .______________________.
+  | log | log |... | log |
+  |__1__|__2__|____|__m__|
+        .       .
+      .               .
+    .                       .
+  .______________________________.
+  | Summary | Payload blocks  |SR|
+  |_blocks__|_________________|__|
+
+The payload blocks are organized per file, and each file consists of
+data blocks and B-tree node blocks:
+
+    |<---       File-A        --->|<---       File-B        --->|
+   _______________________________________________________________
+    | Data blocks | B-tree blocks | Data blocks | B-tree blocks | ...
+   _|_____________|_______________|_____________|_______________|_
+
+
+Since only the modified blocks are written in the log, it may have
+files without data blocks or B-tree node blocks.
+
+The organization of the blocks is recorded in the summary information
+blocks, which contains a header structure (nilfs_segment_summary), per
+file structures (nilfs_finfo), and per block structures (nilfs_binfo):
+
+  _________________________________________________________________________
+ | Summary | finfo | binfo | ... | binfo | finfo | binfo | ... | binfo |...
+ |_blocks__|___A___|_(A,1)_|_____|(A,Na)_|___B___|_(B,1)_|_____|(B,Nb)_|___
+
+
+The logs include regular files, directory files, symbolic link files
+and several meta data files.  The mata data files are the files used
+to maintain file system meta data.  The current version of NILFS2 uses
+the following meta data files:
+
+ 1) Inode file (ifile)             -- Stores on-disk inodes
+ 2) Checkpoint file (cpfile)       -- Stores checkpoints
+ 3) Segment usage file (sufile)    -- Stores allocation state of segments
+ 4) Data address translation file  -- Maps virtual block numbers to usual
+    (DAT)                             block numbers.  This file serves to
+                                      make on-disk blocks relocatable.
+ 5) Sketch file (sketch)           -- Keeps read-only data which can be
+                                      associated with checkpoints (optional)
+
+The following figure shows a typical organization of the logs:
+
+  _________________________________________________________________________
+ | Summary | regular file | file  | ... | ifile | cpfile | sufile | DAT |SR|
+ |_blocks__|_or_directory_|_______|_____|_______|________|________|_____|__|
+
+
+To stride over segment boundaries, this sequence of files may be split
+into multiple logs.  The sequence of logs that should be treated as
+logically one log, is delimited with flags marked in the segment
+summary.  The recovery code of nilfs2 looks this boundary information
+to ensure atomicity of updates.
+
+The super root block is inserted for every checkpoints.  It includes
+three special inodes, inodes for the DAT, cpfile, and sufile.  Inodes
+of regular files, directories, symlinks and other special files, are
+included in the ifile.  The inode of ifile itself is included in the
+corresponding checkpoint entry in the cpfile.  Thus, the hierarchy
+among NILFS2 files can be depicted as follows:
+
+  Super block (SB)
+       |
+       v
+  Super root block (the latest cno=xx)
+       |-- DAT
+       |-- sufile
+       `-- cpfile
+              |-- ifile (cno=c1)
+              |-- ifile (cno=c2) ---- file (ino=i1)
+              :        :          |-- file (ino=i2)
+              `-- ifile (cno=xx)  |-- file (ino=i3)
+                                  :        :
+                                  `-- file (ino=yy)
+                                    ( regular file, directory, or symlink )
+
+For detail on the format of each file, please see include/linux/nilfs2_fs.h.