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://nilfs.sourceforge.net/
  Download page:       http://nilfs.sourceforge.net/en/download.html
  List info:           http://vger.kernel.org/vger-lists.html#linux-nilfs
  
  Caveats
  =======
  
  Features which NILFS2 does not support yet:
  
  	- atime
  	- extended attributes
  	- POSIX ACLs
  	- quotas
  	- fsck
  	- defragmentation
  
  Mount options
  =============
  
  NILFS2 supports the following mount options:
  (*) == default
  
  barrier(*)		This enables/disables the use of write barriers.  This
  nobarrier		requires an IO stack which can support barriers, and
  			if nilfs gets an error on a barrier write, it will
  			disable again with a warning.
  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.
  norecovery		Disable recovery of the filesystem on mount.
  			This disables every write access on the device for
  			read-only mounts or snapshots.  This option will fail
  			for r/w mounts on an unclean volume.
  discard			This enables/disables the use of discard/TRIM commands.
  nodiscard(*)		The discard/TRIM commands are sent to the underlying
  			block device when blocks are freed.  This is useful
  			for SSD devices and sparse/thinly-provisioned LUNs.
  
  Ioctls
  ======
  
  There is some NILFS2 specific functionality which can be accessed by applications
  through the system call interfaces. The list of all NILFS2 specific ioctls are
  shown in the table below.
  
  Table of NILFS2 specific ioctls
  ..............................................................................
   Ioctl			        Description
   NILFS_IOCTL_CHANGE_CPMODE      Change mode of given checkpoint between
  			        checkpoint and snapshot state. This ioctl is
  			        used in chcp and mkcp utilities.
  
   NILFS_IOCTL_DELETE_CHECKPOINT  Remove checkpoint from NILFS2 file system.
  			        This ioctl is used in rmcp utility.
  
   NILFS_IOCTL_GET_CPINFO         Return info about requested checkpoints. This
  			        ioctl is used in lscp utility and by
  			        nilfs_cleanerd daemon.
  
   NILFS_IOCTL_GET_CPSTAT         Return checkpoints statistics. This ioctl is
  			        used by lscp, rmcp utilities and by
  			        nilfs_cleanerd daemon.
  
   NILFS_IOCTL_GET_SUINFO         Return segment usage info about requested
  			        segments. This ioctl is used in lssu,
  			        nilfs_resize utilities and by nilfs_cleanerd
  			        daemon.
  
   NILFS_IOCTL_SET_SUINFO         Modify segment usage info of requested
  				segments. This ioctl is used by
  				nilfs_cleanerd daemon to skip unnecessary
  				cleaning operation of segments and reduce
  				performance penalty or wear of flash device
  				due to redundant move of in-use blocks.
  
   NILFS_IOCTL_GET_SUSTAT         Return segment usage statistics. This ioctl
  			        is used in lssu, nilfs_resize utilities and
  			        by nilfs_cleanerd daemon.
  
   NILFS_IOCTL_GET_VINFO          Return information on virtual block addresses.
  			        This ioctl is used by nilfs_cleanerd daemon.
  
   NILFS_IOCTL_GET_BDESCS         Return information about descriptors of disk
  			        block numbers. This ioctl is used by
  			        nilfs_cleanerd daemon.
  
   NILFS_IOCTL_CLEAN_SEGMENTS     Do garbage collection operation in the
  			        environment of requested parameters from
  			        userspace. This ioctl is used by
  			        nilfs_cleanerd daemon.
  
   NILFS_IOCTL_SYNC               Make a checkpoint. This ioctl is used in
  			        mkcp utility.
  
   NILFS_IOCTL_RESIZE             Resize NILFS2 volume. This ioctl is used
  			        by nilfs_resize utility.
  
   NILFS_IOCTL_SET_ALLOC_RANGE    Define lower limit of segments in bytes and
  			        upper limit of segments in bytes. This ioctl
  			        is used by nilfs_resize utility.
  
  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.
  
  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 nilfs2_ondisk.h
  located at include/uapi/linux directory.
  
  There are no patents or other intellectual property that we protect
  with regard to the design of NILFS2.  It is allowed to replicate the
  design in hopes that other operating systems could share (mount, read,
  write, etc.) data stored in this format.