Documentation/filesystems/nilfs2.txt

   1 NILFS2
   2 ------
   3
   4 NILFS2 is a log-structured file system (LFS) supporting continuous
   5 snapshotting.  In addition to versioning capability of the entire file
   6 system, users can even restore files mistakenly overwritten or
   7 destroyed just a few seconds ago.  Since NILFS2 can keep consistency
   8 like conventional LFS, it achieves quick recovery after system
   9 crashes.
  10
  11 NILFS2 creates a number of checkpoints every few seconds or per
  12 synchronous write basis (unless there is no change).  Users can select
  13 significant versions among continuously created checkpoints, and can
  14 change them into snapshots which will be preserved until they are
  15 changed back to checkpoints.
  16
  17 There is no limit on the number of snapshots until the volume gets
  18 full.  Each snapshot is mountable as a read-only file system
  19 concurrently with its writable mount, and this feature is convenient
  20 for online backup.
  21
  22 The userland tools are included in nilfs-utils package, which is
  23 available from the following download page.  At least "mkfs.nilfs2",
  24 "mount.nilfs2", "umount.nilfs2", and "nilfs_cleanerd" (so called
  25 cleaner or garbage collector) are required.  Details on the tools are
  26 described in the man pages included in the package.
  27
  28 Project web page:    http://www.nilfs.org/en/
  29 Download page:       http://www.nilfs.org/en/download.html
  30 Git tree web page:   http://www.nilfs.org/git/
  31 List info:           http://vger.kernel.org/vger-lists.html#linux-nilfs
  32
  33 Caveats
  34 =======
  35
  36 Features which NILFS2 does not support yet:
  37
  38         - atime
  39         - extended attributes
  40         - POSIX ACLs
  41         - quotas
  42         - fsck
  43         - defragmentation
  44
  45 Mount options
  46 =============
  47
  48 NILFS2 supports the following mount options:
  49 (*) == default
  50
  51 barrier(*)              This enables/disables the use of write barriers.  This
  52 nobarrier               requires an IO stack which can support barriers, and
  53                         if nilfs gets an error on a barrier write, it will
  54                         disable again with a warning.
  55 errors=continue         Keep going on a filesystem error.
  56 errors=remount-ro(*)    Remount the filesystem read-only on an error.
  57 errors=panic            Panic and halt the machine if an error occurs.
  58 cp=n                    Specify the checkpoint-number of the snapshot to be
  59                         mounted.  Checkpoints and snapshots are listed by lscp
  60                         user command.  Only the checkpoints marked as snapshot
  61                         are mountable with this option.  Snapshot is read-only,
  62                         so a read-only mount option must be specified together.
  63 order=relaxed(*)        Apply relaxed order semantics that allows modified data
  64                         blocks to be written to disk without making a
  65                         checkpoint if no metadata update is going.  This mode
  66                         is equivalent to the ordered data mode of the ext3
  67                         filesystem except for the updates on data blocks still
  68                         conserve atomicity.  This will improve synchronous
  69                         write performance for overwriting.
  70 order=strict            Apply strict in-order semantics that preserves sequence
  71                         of all file operations including overwriting of data
  72                         blocks.  That means, it is guaranteed that no
  73                         overtaking of events occurs in the recovered file
  74                         system after a crash.
  75 norecovery              Disable recovery of the filesystem on mount.
  76                         This disables every write access on the device for
  77                         read-only mounts or snapshots.  This option will fail
  78                         for r/w mounts on an unclean volume.
  79 discard                 This enables/disables the use of discard/TRIM commands.
  80 nodiscard(*)            The discard/TRIM commands are sent to the underlying
  81                         block device when blocks are freed.  This is useful
  82                         for SSD devices and sparse/thinly-provisioned LUNs.
  83
  84 Ioctls
  85 ======
  86
  87 There is some NILFS2 specific functionality which can be accessed by applications
  88 through the system call interfaces. The list of all NILFS2 specific ioctls are
  89 shown in the table below.
  90
  91 Table of NILFS2 specific ioctls
  92 ..............................................................................
  93  Ioctl                          Description
  94  NILFS_IOCTL_CHANGE_CPMODE      Change mode of given checkpoint between
  95                                 checkpoint and snapshot state. This ioctl is
  96                                 used in chcp and mkcp utilities.
  97
  98  NILFS_IOCTL_DELETE_CHECKPOINT  Remove checkpoint from NILFS2 file system.
  99                                 This ioctl is used in rmcp utility.
 100
 101  NILFS_IOCTL_GET_CPINFO         Return info about requested checkpoints. This
 102                                 ioctl is used in lscp utility and by
 103                                 nilfs_cleanerd daemon.
 104
 105  NILFS_IOCTL_GET_CPSTAT         Return checkpoints statistics. This ioctl is
 106                                 used by lscp, rmcp utilities and by
 107                                 nilfs_cleanerd daemon.
 108
 109  NILFS_IOCTL_GET_SUINFO         Return segment usage info about requested
 110                                 segments. This ioctl is used in lssu,
 111                                 nilfs_resize utilities and by nilfs_cleanerd
 112                                 daemon.
 113
 114  NILFS_IOCTL_GET_SUSTAT         Return segment usage statistics. This ioctl
 115                                 is used in lssu, nilfs_resize utilities and
 116                                 by nilfs_cleanerd daemon.
 117
 118  NILFS_IOCTL_GET_VINFO          Return information on virtual block addresses.
 119                                 This ioctl is used by nilfs_cleanerd daemon.
 120
 121  NILFS_IOCTL_GET_BDESCS         Return information about descriptors of disk
 122                                 block numbers. This ioctl is used by
 123                                 nilfs_cleanerd daemon.
 124
 125  NILFS_IOCTL_CLEAN_SEGMENTS     Do garbage collection operation in the
 126                                 environment of requested parameters from
 127                                 userspace. This ioctl is used by
 128                                 nilfs_cleanerd daemon.
 129
 130  NILFS_IOCTL_SYNC               Make a checkpoint. This ioctl is used in
 131                                 mkcp utility.
 132
 133  NILFS_IOCTL_RESIZE             Resize NILFS2 volume. This ioctl is used
 134                                 by nilfs_resize utility.
 135
 136  NILFS_IOCTL_SET_ALLOC_RANGE    Define lower limit of segments in bytes and
 137                                 upper limit of segments in bytes. This ioctl
 138                                 is used by nilfs_resize utility.
 139
 140 NILFS2 usage
 141 ============
 142
 143 To use nilfs2 as a local file system, simply:
 144
 145  # mkfs -t nilfs2 /dev/block_device
 146  # mount -t nilfs2 /dev/block_device /dir
 147
 148 This will also invoke the cleaner through the mount helper program
 149 (mount.nilfs2).
 150
 151 Checkpoints and snapshots are managed by the following commands.
 152 Their manpages are included in the nilfs-utils package above.
 153
 154   lscp     list checkpoints or snapshots.
 155   mkcp     make a checkpoint or a snapshot.
 156   chcp     change an existing checkpoint to a snapshot or vice versa.
 157   rmcp     invalidate specified checkpoint(s).
 158
 159 To mount a snapshot,
 160
 161  # mount -t nilfs2 -r -o cp=<cno> /dev/block_device /snap_dir
 162
 163 where <cno> is the checkpoint number of the snapshot.
 164
 165 To unmount the NILFS2 mount point or snapshot, simply:
 166
 167  # umount /dir
 168
 169 Then, the cleaner daemon is automatically shut down by the umount
 170 helper program (umount.nilfs2).
 171
 172 Disk format
 173 ===========
 174
 175 A nilfs2 volume is equally divided into a number of segments except
 176 for the super block (SB) and segment #0.  A segment is the container
 177 of logs.  Each log is composed of summary information blocks, payload
 178 blocks, and an optional super root block (SR):
 179
 180    ______________________________________________________
 181   | |SB| | Segment | Segment | Segment | ... | Segment | |
 182   |_|__|_|____0____|____1____|____2____|_____|____N____|_|
 183   0 +1K +4K       +8M       +16M      +24M  +(8MB x N)
 184        .             .            (Typical offsets for 4KB-block)
 185     .                  .
 186   .______________________.
 187   | log | log |... | log |
 188   |__1__|__2__|____|__m__|
 189         .       .
 190       .               .
 191     .                       .
 192   .______________________________.
 193   | Summary | Payload blocks  |SR|
 194   |_blocks__|_________________|__|
 195
 196 The payload blocks are organized per file, and each file consists of
 197 data blocks and B-tree node blocks:
 198
 199     |<---       File-A        --->|<---       File-B        --->|
 200    _______________________________________________________________
 201     | Data blocks | B-tree blocks | Data blocks | B-tree blocks | ...
 202    _|_____________|_______________|_____________|_______________|_
 203
 204
 205 Since only the modified blocks are written in the log, it may have
 206 files without data blocks or B-tree node blocks.
 207
 208 The organization of the blocks is recorded in the summary information
 209 blocks, which contains a header structure (nilfs_segment_summary), per
 210 file structures (nilfs_finfo), and per block structures (nilfs_binfo):
 211
 212   _________________________________________________________________________
 213  | Summary | finfo | binfo | ... | binfo | finfo | binfo | ... | binfo |...
 214  |_blocks__|___A___|_(A,1)_|_____|(A,Na)_|___B___|_(B,1)_|_____|(B,Nb)_|___
 215
 216
 217 The logs include regular files, directory files, symbolic link files
 218 and several meta data files.  The mata data files are the files used
 219 to maintain file system meta data.  The current version of NILFS2 uses
 220 the following meta data files:
 221
 222  1) Inode file (ifile)             -- Stores on-disk inodes
 223  2) Checkpoint file (cpfile)       -- Stores checkpoints
 224  3) Segment usage file (sufile)    -- Stores allocation state of segments
 225  4) Data address translation file  -- Maps virtual block numbers to usual
 226     (DAT)                             block numbers.  This file serves to
 227                                       make on-disk blocks relocatable.
 228
 229 The following figure shows a typical organization of the logs:
 230
 231   _________________________________________________________________________
 232  | Summary | regular file | file  | ... | ifile | cpfile | sufile | DAT |SR|
 233  |_blocks__|_or_directory_|_______|_____|_______|________|________|_____|__|
 234
 235
 236 To stride over segment boundaries, this sequence of files may be split
 237 into multiple logs.  The sequence of logs that should be treated as
 238 logically one log, is delimited with flags marked in the segment
 239 summary.  The recovery code of nilfs2 looks this boundary information
 240 to ensure atomicity of updates.
 241
 242 The super root block is inserted for every checkpoints.  It includes
 243 three special inodes, inodes for the DAT, cpfile, and sufile.  Inodes
 244 of regular files, directories, symlinks and other special files, are
 245 included in the ifile.  The inode of ifile itself is included in the
 246 corresponding checkpoint entry in the cpfile.  Thus, the hierarchy
 247 among NILFS2 files can be depicted as follows:
 248
 249   Super block (SB)
 250        |
 251        v
 252   Super root block (the latest cno=xx)
 253        |-- DAT
 254        |-- sufile
 255        `-- cpfile
 256               |-- ifile (cno=c1)
 257               |-- ifile (cno=c2) ---- file (ino=i1)
 258               :        :          |-- file (ino=i2)
 259               `-- ifile (cno=xx)  |-- file (ino=i3)
 260                                   :        :
 261                                   `-- file (ino=yy)
 262                                     ( regular file, directory, or symlink )
 263
 264 For detail on the format of each file, please see include/linux/nilfs2_fs.h.