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Ryusuke Konishi962281a2009-04-06 19:01:20 -07001NILFS2
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
Ryusuke Konishifb6e7112009-05-30 11:27:17 +090042 - fsck
43 - resize
Ryusuke Konishi962281a2009-04-06 19:01:20 -070044 - defragmentation
45
46Mount options
47=============
48
49NILFS2 supports the following mount options:
50(*) == default
51
52barrier=on(*) This enables/disables barriers. barrier=off disables
53 it, barrier=on enables it.
54errors=continue(*) Keep going on a filesystem error.
55errors=remount-ro Remount the filesystem read-only on an error.
56errors=panic Panic and halt the machine if an error occurs.
57cp=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.
62order=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.
69order=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
75NILFS2 usage
76============
77
78To use nilfs2 as a local file system, simply:
79
80 # mkfs -t nilfs2 /dev/block_device
81 # mount -t nilfs2 /dev/block_device /dir
82
83This will also invoke the cleaner through the mount helper program
84(mount.nilfs2).
85
86Checkpoints and snapshots are managed by the following commands.
87Their manpages are included in the nilfs-utils package above.
88
89 lscp list checkpoints or snapshots.
90 mkcp make a checkpoint or a snapshot.
91 chcp change an existing checkpoint to a snapshot or vice versa.
92 rmcp invalidate specified checkpoint(s).
93
94To mount a snapshot,
95
96 # mount -t nilfs2 -r -o cp=<cno> /dev/block_device /snap_dir
97
98where <cno> is the checkpoint number of the snapshot.
99
100To unmount the NILFS2 mount point or snapshot, simply:
101
102 # umount /dir
103
104Then, the cleaner daemon is automatically shut down by the umount
105helper program (umount.nilfs2).
106
107Disk format
108===========
109
110A nilfs2 volume is equally divided into a number of segments except
111for the super block (SB) and segment #0. A segment is the container
112of logs. Each log is composed of summary information blocks, payload
113blocks, and an optional super root block (SR):
114
115 ______________________________________________________
116 | |SB| | Segment | Segment | Segment | ... | Segment | |
117 |_|__|_|____0____|____1____|____2____|_____|____N____|_|
118 0 +1K +4K +8M +16M +24M +(8MB x N)
119 . . (Typical offsets for 4KB-block)
120 . .
121 .______________________.
122 | log | log |... | log |
123 |__1__|__2__|____|__m__|
124 . .
125 . .
126 . .
127 .______________________________.
128 | Summary | Payload blocks |SR|
129 |_blocks__|_________________|__|
130
131The payload blocks are organized per file, and each file consists of
132data blocks and B-tree node blocks:
133
134 |<--- File-A --->|<--- File-B --->|
135 _______________________________________________________________
136 | Data blocks | B-tree blocks | Data blocks | B-tree blocks | ...
137 _|_____________|_______________|_____________|_______________|_
138
139
140Since only the modified blocks are written in the log, it may have
141files without data blocks or B-tree node blocks.
142
143The organization of the blocks is recorded in the summary information
144blocks, which contains a header structure (nilfs_segment_summary), per
145file structures (nilfs_finfo), and per block structures (nilfs_binfo):
146
147 _________________________________________________________________________
148 | Summary | finfo | binfo | ... | binfo | finfo | binfo | ... | binfo |...
149 |_blocks__|___A___|_(A,1)_|_____|(A,Na)_|___B___|_(B,1)_|_____|(B,Nb)_|___
150
151
152The logs include regular files, directory files, symbolic link files
153and several meta data files. The mata data files are the files used
154to maintain file system meta data. The current version of NILFS2 uses
155the following meta data files:
156
157 1) Inode file (ifile) -- Stores on-disk inodes
158 2) Checkpoint file (cpfile) -- Stores checkpoints
159 3) Segment usage file (sufile) -- Stores allocation state of segments
160 4) Data address translation file -- Maps virtual block numbers to usual
161 (DAT) block numbers. This file serves to
162 make on-disk blocks relocatable.
Ryusuke Konishi962281a2009-04-06 19:01:20 -0700163
164The following figure shows a typical organization of the logs:
165
166 _________________________________________________________________________
167 | Summary | regular file | file | ... | ifile | cpfile | sufile | DAT |SR|
168 |_blocks__|_or_directory_|_______|_____|_______|________|________|_____|__|
169
170
171To stride over segment boundaries, this sequence of files may be split
172into multiple logs. The sequence of logs that should be treated as
173logically one log, is delimited with flags marked in the segment
174summary. The recovery code of nilfs2 looks this boundary information
175to ensure atomicity of updates.
176
177The super root block is inserted for every checkpoints. It includes
178three special inodes, inodes for the DAT, cpfile, and sufile. Inodes
179of regular files, directories, symlinks and other special files, are
180included in the ifile. The inode of ifile itself is included in the
181corresponding checkpoint entry in the cpfile. Thus, the hierarchy
182among NILFS2 files can be depicted as follows:
183
184 Super block (SB)
185 |
186 v
187 Super root block (the latest cno=xx)
188 |-- DAT
189 |-- sufile
190 `-- cpfile
191 |-- ifile (cno=c1)
192 |-- ifile (cno=c2) ---- file (ino=i1)
193 : : |-- file (ino=i2)
194 `-- ifile (cno=xx) |-- file (ino=i3)
195 : :
196 `-- file (ino=yy)
197 ( regular file, directory, or symlink )
198
199For detail on the format of each file, please see include/linux/nilfs2_fs.h.