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authorRyusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>2009-04-06 22:01:20 -0400
committerLinus Torvalds <torvalds@linux-foundation.org>2009-04-07 11:31:12 -0400
commit962281a7ab3aeb97eed004b8f0bdb0979cf35347 (patch)
tree600628383553d807f4151d93a21d5cfd70389de3 /Documentation/filesystems/nilfs2.txt
parent2c5510d4e84988ea95f86488d1e23244284bc1ed (diff)
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: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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1NILFS2
2------
3
4NILFS2 is a log-structured file system (LFS) supporting continuous
5snapshotting. In addition to versioning capability of the entire file
6system, users can even restore files mistakenly overwritten or
7destroyed just a few seconds ago. Since NILFS2 can keep consistency
8like conventional LFS, it achieves quick recovery after system
9crashes.
10
11NILFS2 creates a number of checkpoints every few seconds or per
12synchronous write basis (unless there is no change). Users can select
13significant versions among continuously created checkpoints, and can
14change them into snapshots which will be preserved until they are
15changed back to checkpoints.
16
17There is no limit on the number of snapshots until the volume gets
18full. Each snapshot is mountable as a read-only file system
19concurrently with its writable mount, and this feature is convenient
20for online backup.
21
22The userland tools are included in nilfs-utils package, which is
23available from the following download page. At least "mkfs.nilfs2",
24"mount.nilfs2", "umount.nilfs2", and "nilfs_cleanerd" (so called
25cleaner or garbage collector) are required. Details on the tools are
26described in the man pages included in the package.
27
28Project web page: http://www.nilfs.org/en/
29Download page: http://www.nilfs.org/en/download.html
30Git tree web page: http://www.nilfs.org/git/
31NILFS mailing lists: http://www.nilfs.org/mailman/listinfo/users
32
33Caveats
34=======
35
36Features which NILFS2 does not support yet:
37
38 - atime
39 - extended attributes
40 - POSIX ACLs
41 - quotas
42 - writable snapshots
43 - remote backup (CDP)
44 - data integrity
45 - defragmentation
46
47Mount options
48=============
49
50NILFS2 supports the following mount options:
51(*) == default
52
53barrier=on(*) This enables/disables barriers. barrier=off disables
54 it, barrier=on enables it.
55errors=continue(*) Keep going on a filesystem error.
56errors=remount-ro Remount the filesystem read-only on an error.
57errors=panic Panic and halt the machine if an error occurs.
58cp=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.
63order=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.
70order=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
76NILFS2 usage
77============
78
79To use nilfs2 as a local file system, simply:
80
81 # mkfs -t nilfs2 /dev/block_device
82 # mount -t nilfs2 /dev/block_device /dir
83
84This will also invoke the cleaner through the mount helper program
85(mount.nilfs2).
86
87Checkpoints and snapshots are managed by the following commands.
88Their manpages are included in the nilfs-utils package above.
89
90 lscp list checkpoints or snapshots.
91 mkcp make a checkpoint or a snapshot.
92 chcp change an existing checkpoint to a snapshot or vice versa.
93 rmcp invalidate specified checkpoint(s).
94
95To mount a snapshot,
96
97 # mount -t nilfs2 -r -o cp=<cno> /dev/block_device /snap_dir
98
99where <cno> is the checkpoint number of the snapshot.
100
101To unmount the NILFS2 mount point or snapshot, simply:
102
103 # umount /dir
104
105Then, the cleaner daemon is automatically shut down by the umount
106helper program (umount.nilfs2).
107
108Disk format
109===========
110
111A nilfs2 volume is equally divided into a number of segments except
112for the super block (SB) and segment #0. A segment is the container
113of logs. Each log is composed of summary information blocks, payload
114blocks, and an optional super root block (SR):
115
116 ______________________________________________________
117 | |SB| | Segment | Segment | Segment | ... | Segment | |
118 |_|__|_|____0____|____1____|____2____|_____|____N____|_|
119 0 +1K +4K +8M +16M +24M +(8MB x N)
120 . . (Typical offsets for 4KB-block)
121 . .
122 .______________________.
123 | log | log |... | log |
124 |__1__|__2__|____|__m__|
125 . .
126 . .
127 . .
128 .______________________________.
129 | Summary | Payload blocks |SR|
130 |_blocks__|_________________|__|
131
132The payload blocks are organized per file, and each file consists of
133data blocks and B-tree node blocks:
134
135 |<--- File-A --->|<--- File-B --->|
136 _______________________________________________________________
137 | Data blocks | B-tree blocks | Data blocks | B-tree blocks | ...
138 _|_____________|_______________|_____________|_______________|_
139
140
141Since only the modified blocks are written in the log, it may have
142files without data blocks or B-tree node blocks.
143
144The organization of the blocks is recorded in the summary information
145blocks, which contains a header structure (nilfs_segment_summary), per
146file structures (nilfs_finfo), and per block structures (nilfs_binfo):
147
148 _________________________________________________________________________
149 | Summary | finfo | binfo | ... | binfo | finfo | binfo | ... | binfo |...
150 |_blocks__|___A___|_(A,1)_|_____|(A,Na)_|___B___|_(B,1)_|_____|(B,Nb)_|___
151
152
153The logs include regular files, directory files, symbolic link files
154and several meta data files. The mata data files are the files used
155to maintain file system meta data. The current version of NILFS2 uses
156the following meta data files:
157
158 1) Inode file (ifile) -- Stores on-disk inodes
159 2) Checkpoint file (cpfile) -- Stores checkpoints
160 3) Segment usage file (sufile) -- Stores allocation state of segments
161 4) Data address translation file -- Maps virtual block numbers to usual
162 (DAT) block numbers. This file serves to
163 make on-disk blocks relocatable.
164 5) Sketch file (sketch) -- Keeps read-only data which can be
165 associated with checkpoints (optional)
166
167The following figure shows a typical organization of the logs:
168
169 _________________________________________________________________________
170 | Summary | regular file | file | ... | ifile | cpfile | sufile | DAT |SR|
171 |_blocks__|_or_directory_|_______|_____|_______|________|________|_____|__|
172
173
174To stride over segment boundaries, this sequence of files may be split
175into multiple logs. The sequence of logs that should be treated as
176logically one log, is delimited with flags marked in the segment
177summary. The recovery code of nilfs2 looks this boundary information
178to ensure atomicity of updates.
179
180The super root block is inserted for every checkpoints. It includes
181three special inodes, inodes for the DAT, cpfile, and sufile. Inodes
182of regular files, directories, symlinks and other special files, are
183included in the ifile. The inode of ifile itself is included in the
184corresponding checkpoint entry in the cpfile. Thus, the hierarchy
185among NILFS2 files can be depicted as follows:
186
187 Super block (SB)
188 |
189 v
190 Super root block (the latest cno=xx)
191 |-- DAT
192 |-- sufile
193 `-- cpfile
194 |-- ifile (cno=c1)
195 |-- ifile (cno=c2) ---- file (ino=i1)
196 : : |-- file (ino=i2)
197 `-- ifile (cno=xx) |-- file (ino=i3)
198 : :
199 `-- file (ino=yy)
200 ( regular file, directory, or symlink )
201
202For detail on the format of each file, please see include/linux/nilfs2_fs.h.