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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/
Ryusuke Konishi6aff43f2010-01-02 21:41:53 +090031List info: http://vger.kernel.org/vger-lists.html#linux-nilfs
Ryusuke Konishi962281a2009-04-06 19:01:20 -070032
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
Jiro SEKIBA91f19532009-11-12 14:07:26 +090052nobarrier Disables barriers.
Ryusuke Konishi962281a2009-04-06 19:01:20 -070053errors=continue(*) Keep going on a filesystem error.
54errors=remount-ro Remount the filesystem read-only on an error.
55errors=panic Panic and halt the machine if an error occurs.
56cp=n Specify the checkpoint-number of the snapshot to be
57 mounted. Checkpoints and snapshots are listed by lscp
58 user command. Only the checkpoints marked as snapshot
59 are mountable with this option. Snapshot is read-only,
60 so a read-only mount option must be specified together.
61order=relaxed(*) Apply relaxed order semantics that allows modified data
62 blocks to be written to disk without making a
63 checkpoint if no metadata update is going. This mode
64 is equivalent to the ordered data mode of the ext3
65 filesystem except for the updates on data blocks still
66 conserve atomicity. This will improve synchronous
67 write performance for overwriting.
68order=strict Apply strict in-order semantics that preserves sequence
69 of all file operations including overwriting of data
70 blocks. That means, it is guaranteed that no
71 overtaking of events occurs in the recovered file
72 system after a crash.
Ryusuke Konishi02345762009-11-20 03:28:01 +090073norecovery Disable recovery of the filesystem on mount.
74 This disables every write access on the device for
75 read-only mounts or snapshots. This option will fail
76 for r/w mounts on an unclean volume.
Ryusuke Konishi962281a2009-04-06 19:01:20 -070077
78NILFS2 usage
79============
80
81To use nilfs2 as a local file system, simply:
82
83 # mkfs -t nilfs2 /dev/block_device
84 # mount -t nilfs2 /dev/block_device /dir
85
86This will also invoke the cleaner through the mount helper program
87(mount.nilfs2).
88
89Checkpoints and snapshots are managed by the following commands.
90Their manpages are included in the nilfs-utils package above.
91
92 lscp list checkpoints or snapshots.
93 mkcp make a checkpoint or a snapshot.
94 chcp change an existing checkpoint to a snapshot or vice versa.
95 rmcp invalidate specified checkpoint(s).
96
97To mount a snapshot,
98
99 # mount -t nilfs2 -r -o cp=<cno> /dev/block_device /snap_dir
100
101where <cno> is the checkpoint number of the snapshot.
102
103To unmount the NILFS2 mount point or snapshot, simply:
104
105 # umount /dir
106
107Then, the cleaner daemon is automatically shut down by the umount
108helper program (umount.nilfs2).
109
110Disk format
111===========
112
113A nilfs2 volume is equally divided into a number of segments except
114for the super block (SB) and segment #0. A segment is the container
115of logs. Each log is composed of summary information blocks, payload
116blocks, and an optional super root block (SR):
117
118 ______________________________________________________
119 | |SB| | Segment | Segment | Segment | ... | Segment | |
120 |_|__|_|____0____|____1____|____2____|_____|____N____|_|
121 0 +1K +4K +8M +16M +24M +(8MB x N)
122 . . (Typical offsets for 4KB-block)
123 . .
124 .______________________.
125 | log | log |... | log |
126 |__1__|__2__|____|__m__|
127 . .
128 . .
129 . .
130 .______________________________.
131 | Summary | Payload blocks |SR|
132 |_blocks__|_________________|__|
133
134The payload blocks are organized per file, and each file consists of
135data blocks and B-tree node blocks:
136
137 |<--- File-A --->|<--- File-B --->|
138 _______________________________________________________________
139 | Data blocks | B-tree blocks | Data blocks | B-tree blocks | ...
140 _|_____________|_______________|_____________|_______________|_
141
142
143Since only the modified blocks are written in the log, it may have
144files without data blocks or B-tree node blocks.
145
146The organization of the blocks is recorded in the summary information
147blocks, which contains a header structure (nilfs_segment_summary), per
148file structures (nilfs_finfo), and per block structures (nilfs_binfo):
149
150 _________________________________________________________________________
151 | Summary | finfo | binfo | ... | binfo | finfo | binfo | ... | binfo |...
152 |_blocks__|___A___|_(A,1)_|_____|(A,Na)_|___B___|_(B,1)_|_____|(B,Nb)_|___
153
154
155The logs include regular files, directory files, symbolic link files
156and several meta data files. The mata data files are the files used
157to maintain file system meta data. The current version of NILFS2 uses
158the following meta data files:
159
160 1) Inode file (ifile) -- Stores on-disk inodes
161 2) Checkpoint file (cpfile) -- Stores checkpoints
162 3) Segment usage file (sufile) -- Stores allocation state of segments
163 4) Data address translation file -- Maps virtual block numbers to usual
164 (DAT) block numbers. This file serves to
165 make on-disk blocks relocatable.
Ryusuke Konishi962281a2009-04-06 19:01:20 -0700166
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.