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://nilfs.sourceforge.net/
  29 Download page:       http://nilfs.sourceforge.net/en/download.html
  30 List info:           http://vger.kernel.org/vger-lists.html#linux-nilfs
  31
  32 Caveats
  33 =======
  34
  35 Features which NILFS2 does not support yet:
  36
  37         - atime
  38         - extended attributes
  39         - POSIX ACLs
  40         - quotas
  41         - fsck
  42         - defragmentation
  43
  44 Mount options
  45 =============
  46
  47 NILFS2 supports the following mount options:
  48 (*) == default
  49
  50 barrier(*)              This enables/disables the use of write barriers.  This
  51 nobarrier               requires an IO stack which can support barriers, and
  52                         if nilfs gets an error on a barrier write, it will
  53                         disable again with a warning.
  54 errors=continue         Keep going on a filesystem error.
  55 errors=remount-ro(*)    Remount the filesystem read-only on an error.
  56 errors=panic            Panic and halt the machine if an error occurs.
  57 cp=n                    Specify the checkpoint-number of the snapshot to be
  58                         mounted.  Checkpoints and snapshots are listed by lscp
  59                         user command.  Only the checkpoints marked as snapshot
  60                         are mountable with this option.  Snapshot is read-only,
  61                         so a read-only mount option must be specified together.
  62 order=relaxed(*)        Apply relaxed order semantics that allows modified data
  63                         blocks to be written to disk without making a
  64                         checkpoint if no metadata update is going.  This mode
  65                         is equivalent to the ordered data mode of the ext3
  66                         filesystem except for the updates on data blocks still
  67                         conserve atomicity.  This will improve synchronous
  68                         write performance for overwriting.
  69 order=strict            Apply strict in-order semantics that preserves sequence
  70                         of all file operations including overwriting of data
  71                         blocks.  That means, it is guaranteed that no
  72                         overtaking of events occurs in the recovered file
  73                         system after a crash.
  74 norecovery              Disable recovery of the filesystem on mount.
  75                         This disables every write access on the device for
  76                         read-only mounts or snapshots.  This option will fail
  77                         for r/w mounts on an unclean volume.
  78 discard                 This enables/disables the use of discard/TRIM commands.
  79 nodiscard(*)            The discard/TRIM commands are sent to the underlying
  80                         block device when blocks are freed.  This is useful
  81                         for SSD devices and sparse/thinly-provisioned LUNs.
  82
  83 Ioctls
  84 ======
  85
  86 There is some NILFS2 specific functionality which can be accessed by applications
  87 through the system call interfaces. The list of all NILFS2 specific ioctls are
  88 shown in the table below.
  89
  90 Table of NILFS2 specific ioctls
  91 ..............................................................................
  92  Ioctl                          Description
  93  NILFS_IOCTL_CHANGE_CPMODE      Change mode of given checkpoint between
  94                                 checkpoint and snapshot state. This ioctl is
  95                                 used in chcp and mkcp utilities.
  96
  97  NILFS_IOCTL_DELETE_CHECKPOINT  Remove checkpoint from NILFS2 file system.
  98                                 This ioctl is used in rmcp utility.
  99
 100  NILFS_IOCTL_GET_CPINFO         Return info about requested checkpoints. This
 101                                 ioctl is used in lscp utility and by
 102                                 nilfs_cleanerd daemon.
 103
 104  NILFS_IOCTL_GET_CPSTAT         Return checkpoints statistics. This ioctl is
 105                                 used by lscp, rmcp utilities and by
 106                                 nilfs_cleanerd daemon.
 107
 108  NILFS_IOCTL_GET_SUINFO         Return segment usage info about requested
 109                                 segments. This ioctl is used in lssu,
 110                                 nilfs_resize utilities and by nilfs_cleanerd
 111                                 daemon.
 112
 113  NILFS_IOCTL_SET_SUINFO         Modify segment usage info of requested
 114                                 segments. This ioctl is used by
 115                                 nilfs_cleanerd daemon to skip unnecessary
 116                                 cleaning operation of segments and reduce
 117                                 performance penalty or wear of flash device
 118                                 due to redundant move of in-use blocks.
 119
 120  NILFS_IOCTL_GET_SUSTAT         Return segment usage statistics. This ioctl
 121                                 is used in lssu, nilfs_resize utilities and
 122                                 by nilfs_cleanerd daemon.
 123
 124  NILFS_IOCTL_GET_VINFO          Return information on virtual block addresses.
 125                                 This ioctl is used by nilfs_cleanerd daemon.
 126
 127  NILFS_IOCTL_GET_BDESCS         Return information about descriptors of disk
 128                                 block numbers. This ioctl is used by
 129                                 nilfs_cleanerd daemon.
 130
 131  NILFS_IOCTL_CLEAN_SEGMENTS     Do garbage collection operation in the
 132                                 environment of requested parameters from
 133                                 userspace. This ioctl is used by
 134                                 nilfs_cleanerd daemon.
 135
 136  NILFS_IOCTL_SYNC               Make a checkpoint. This ioctl is used in
 137                                 mkcp utility.
 138
 139  NILFS_IOCTL_RESIZE             Resize NILFS2 volume. This ioctl is used
 140                                 by nilfs_resize utility.
 141
 142  NILFS_IOCTL_SET_ALLOC_RANGE    Define lower limit of segments in bytes and
 143                                 upper limit of segments in bytes. This ioctl
 144                                 is used by nilfs_resize utility.
 145
 146 NILFS2 usage
 147 ============
 148
 149 To use nilfs2 as a local file system, simply:
 150
 151  # mkfs -t nilfs2 /dev/block_device
 152  # mount -t nilfs2 /dev/block_device /dir
 153
 154 This will also invoke the cleaner through the mount helper program
 155 (mount.nilfs2).
 156
 157 Checkpoints and snapshots are managed by the following commands.
 158 Their manpages are included in the nilfs-utils package above.
 159
 160   lscp     list checkpoints or snapshots.
 161   mkcp     make a checkpoint or a snapshot.
 162   chcp     change an existing checkpoint to a snapshot or vice versa.
 163   rmcp     invalidate specified checkpoint(s).
 164
 165 To mount a snapshot,
 166
 167  # mount -t nilfs2 -r -o cp=<cno> /dev/block_device /snap_dir
 168
 169 where <cno> is the checkpoint number of the snapshot.
 170
 171 To unmount the NILFS2 mount point or snapshot, simply:
 172
 173  # umount /dir
 174
 175 Then, the cleaner daemon is automatically shut down by the umount
 176 helper program (umount.nilfs2).
 177
 178 Disk format
 179 ===========
 180
 181 A nilfs2 volume is equally divided into a number of segments except
 182 for the super block (SB) and segment #0.  A segment is the container
 183 of logs.  Each log is composed of summary information blocks, payload
 184 blocks, and an optional super root block (SR):
 185
 186    ______________________________________________________
 187   | |SB| | Segment | Segment | Segment | ... | Segment | |
 188   |_|__|_|____0____|____1____|____2____|_____|____N____|_|
 189   0 +1K +4K       +8M       +16M      +24M  +(8MB x N)
 190        .             .            (Typical offsets for 4KB-block)
 191     .                  .
 192   .______________________.
 193   | log | log |... | log |
 194   |__1__|__2__|____|__m__|
 195         .       .
 196       .               .
 197     .                       .
 198   .______________________________.
 199   | Summary | Payload blocks  |SR|
 200   |_blocks__|_________________|__|
 201
 202 The payload blocks are organized per file, and each file consists of
 203 data blocks and B-tree node blocks:
 204
 205     |<---       File-A        --->|<---       File-B        --->|
 206    _______________________________________________________________
 207     | Data blocks | B-tree blocks | Data blocks | B-tree blocks | ...
 208    _|_____________|_______________|_____________|_______________|_
 209
 210
 211 Since only the modified blocks are written in the log, it may have
 212 files without data blocks or B-tree node blocks.
 213
 214 The organization of the blocks is recorded in the summary information
 215 blocks, which contains a header structure (nilfs_segment_summary), per
 216 file structures (nilfs_finfo), and per block structures (nilfs_binfo):
 217
 218   _________________________________________________________________________
 219  | Summary | finfo | binfo | ... | binfo | finfo | binfo | ... | binfo |...
 220  |_blocks__|___A___|_(A,1)_|_____|(A,Na)_|___B___|_(B,1)_|_____|(B,Nb)_|___
 221
 222
 223 The logs include regular files, directory files, symbolic link files
 224 and several meta data files.  The mata data files are the files used
 225 to maintain file system meta data.  The current version of NILFS2 uses
 226 the following meta data files:
 227
 228  1) Inode file (ifile)             -- Stores on-disk inodes
 229  2) Checkpoint file (cpfile)       -- Stores checkpoints
 230  3) Segment usage file (sufile)    -- Stores allocation state of segments
 231  4) Data address translation file  -- Maps virtual block numbers to usual
 232     (DAT)                             block numbers.  This file serves to
 233                                       make on-disk blocks relocatable.
 234
 235 The following figure shows a typical organization of the logs:
 236
 237   _________________________________________________________________________
 238  | Summary | regular file | file  | ... | ifile | cpfile | sufile | DAT |SR|
 239  |_blocks__|_or_directory_|_______|_____|_______|________|________|_____|__|
 240
 241
 242 To stride over segment boundaries, this sequence of files may be split
 243 into multiple logs.  The sequence of logs that should be treated as
 244 logically one log, is delimited with flags marked in the segment
 245 summary.  The recovery code of nilfs2 looks this boundary information
 246 to ensure atomicity of updates.
 247
 248 The super root block is inserted for every checkpoints.  It includes
 249 three special inodes, inodes for the DAT, cpfile, and sufile.  Inodes
 250 of regular files, directories, symlinks and other special files, are
 251 included in the ifile.  The inode of ifile itself is included in the
 252 corresponding checkpoint entry in the cpfile.  Thus, the hierarchy
 253 among NILFS2 files can be depicted as follows:
 254
 255   Super block (SB)
 256        |
 257        v
 258   Super root block (the latest cno=xx)
 259        |-- DAT
 260        |-- sufile
 261        `-- cpfile
 262               |-- ifile (cno=c1)
 263               |-- ifile (cno=c2) ---- file (ino=i1)
 264               :        :          |-- file (ino=i2)
 265               `-- ifile (cno=xx)  |-- file (ino=i3)
 266                                   :        :
 267                                   `-- file (ino=yy)
 268                                     ( regular file, directory, or symlink )
 269
 270 For detail on the format of each file, please see nilfs2_ondisk.h
 271 located at include/uapi/linux directory.
 272
 273 There are no patents or other intellectual property that we protect
 274 with regard to the design of NILFS2.  It is allowed to replicate the
 275 design in hopes that other operating systems could share (mount, read,
 276 write, etc.) data stored in this format.