Based on kernel version 2.6.33. Page generated on 2010-02-24 15:36 EST.
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 - resize 44 - defragmentation 45 46 Mount options 47 ============= 48 49 NILFS2 supports the following mount options: 50 (*) == default 51 52 nobarrier Disables barriers. 53 errors=continue(*) Keep going on a filesystem error. 54 errors=remount-ro Remount the filesystem read-only on an error. 55 errors=panic Panic and halt the machine if an error occurs. 56 cp=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. 61 order=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. 68 order=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. 73 norecovery 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. 77 78 NILFS2 usage 79 ============ 80 81 To 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 86 This will also invoke the cleaner through the mount helper program 87 (mount.nilfs2). 88 89 Checkpoints and snapshots are managed by the following commands. 90 Their 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 97 To mount a snapshot, 98 99 # mount -t nilfs2 -r -o cp=<cno> /dev/block_device /snap_dir 100 101 where <cno> is the checkpoint number of the snapshot. 102 103 To unmount the NILFS2 mount point or snapshot, simply: 104 105 # umount /dir 106 107 Then, the cleaner daemon is automatically shut down by the umount 108 helper program (umount.nilfs2). 109 110 Disk format 111 =========== 112 113 A nilfs2 volume is equally divided into a number of segments except 114 for the super block (SB) and segment #0. A segment is the container 115 of logs. Each log is composed of summary information blocks, payload 116 blocks, 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__|_________________|__| 133 134 The payload blocks are organized per file, and each file consists of 135 data 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 _|_____________|_______________|_____________|_______________|_ 141 142 143 Since only the modified blocks are written in the log, it may have 144 files without data blocks or B-tree node blocks. 145 146 The organization of the blocks is recorded in the summary information 147 blocks, which contains a header structure (nilfs_segment_summary), per 148 file 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)_|___ 153 154 155 The logs include regular files, directory files, symbolic link files 156 and several meta data files. The mata data files are the files used 157 to maintain file system meta data. The current version of NILFS2 uses 158 the 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. 166 167 The 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_|_______|_____|_______|________|________|_____|__| 172 173 174 To stride over segment boundaries, this sequence of files may be split 175 into multiple logs. The sequence of logs that should be treated as 176 logically one log, is delimited with flags marked in the segment 177 summary. The recovery code of nilfs2 looks this boundary information 178 to ensure atomicity of updates. 179 180 The super root block is inserted for every checkpoints. It includes 181 three special inodes, inodes for the DAT, cpfile, and sufile. Inodes 182 of regular files, directories, symlinks and other special files, are 183 included in the ifile. The inode of ifile itself is included in the 184 corresponding checkpoint entry in the cpfile. Thus, the hierarchy 185 among 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 ) 201 202 For detail on the format of each file, please see include/linux/nilfs2_fs.h.