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/ 31List info: http://vger.kernel.org/vger-lists.html#linux-nilfs 32 33Caveats 34======= 35 36Features which NILFS2 does not support yet: 37 38 - atime 39 - extended attributes 40 - POSIX ACLs 41 - quotas 42 - fsck 43 - resize 44 - defragmentation 45 46Mount options 47============= 48 49NILFS2 supports the following mount options: 50(*) == default 51 52nobarrier Disables barriers. 53errors=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. 73norecovery 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. 77discard Issue discard/TRIM commands to the underlying block 78 device when blocks are freed. This is useful for SSD 79 devices and sparse/thinly-provisioned LUNs. 80 81NILFS2 usage 82============ 83 84To use nilfs2 as a local file system, simply: 85 86 # mkfs -t nilfs2 /dev/block_device 87 # mount -t nilfs2 /dev/block_device /dir 88 89This will also invoke the cleaner through the mount helper program 90(mount.nilfs2). 91 92Checkpoints and snapshots are managed by the following commands. 93Their manpages are included in the nilfs-utils package above. 94 95 lscp list checkpoints or snapshots. 96 mkcp make a checkpoint or a snapshot. 97 chcp change an existing checkpoint to a snapshot or vice versa. 98 rmcp invalidate specified checkpoint(s). 99 100To mount a snapshot, 101 102 # mount -t nilfs2 -r -o cp=<cno> /dev/block_device /snap_dir 103 104where <cno> is the checkpoint number of the snapshot. 105 106To unmount the NILFS2 mount point or snapshot, simply: 107 108 # umount /dir 109 110Then, the cleaner daemon is automatically shut down by the umount 111helper program (umount.nilfs2). 112 113Disk format 114=========== 115 116A nilfs2 volume is equally divided into a number of segments except 117for the super block (SB) and segment #0. A segment is the container 118of logs. Each log is composed of summary information blocks, payload 119blocks, and an optional super root block (SR): 120 121 ______________________________________________________ 122 | |SB| | Segment | Segment | Segment | ... | Segment | | 123 |_|__|_|____0____|____1____|____2____|_____|____N____|_| 124 0 +1K +4K +8M +16M +24M +(8MB x N) 125 . . (Typical offsets for 4KB-block) 126 . . 127 .______________________. 128 | log | log |... | log | 129 |__1__|__2__|____|__m__| 130 . . 131 . . 132 . . 133 .______________________________. 134 | Summary | Payload blocks |SR| 135 |_blocks__|_________________|__| 136 137The payload blocks are organized per file, and each file consists of 138data blocks and B-tree node blocks: 139 140 |<--- File-A --->|<--- File-B --->| 141 _______________________________________________________________ 142 | Data blocks | B-tree blocks | Data blocks | B-tree blocks | ... 143 _|_____________|_______________|_____________|_______________|_ 144 145 146Since only the modified blocks are written in the log, it may have 147files without data blocks or B-tree node blocks. 148 149The organization of the blocks is recorded in the summary information 150blocks, which contains a header structure (nilfs_segment_summary), per 151file structures (nilfs_finfo), and per block structures (nilfs_binfo): 152 153 _________________________________________________________________________ 154 | Summary | finfo | binfo | ... | binfo | finfo | binfo | ... | binfo |... 155 |_blocks__|___A___|_(A,1)_|_____|(A,Na)_|___B___|_(B,1)_|_____|(B,Nb)_|___ 156 157 158The logs include regular files, directory files, symbolic link files 159and several meta data files. The mata data files are the files used 160to maintain file system meta data. The current version of NILFS2 uses 161the following meta data files: 162 163 1) Inode file (ifile) -- Stores on-disk inodes 164 2) Checkpoint file (cpfile) -- Stores checkpoints 165 3) Segment usage file (sufile) -- Stores allocation state of segments 166 4) Data address translation file -- Maps virtual block numbers to usual 167 (DAT) block numbers. This file serves to 168 make on-disk blocks relocatable. 169 170The following figure shows a typical organization of the logs: 171 172 _________________________________________________________________________ 173 | Summary | regular file | file | ... | ifile | cpfile | sufile | DAT |SR| 174 |_blocks__|_or_directory_|_______|_____|_______|________|________|_____|__| 175 176 177To stride over segment boundaries, this sequence of files may be split 178into multiple logs. The sequence of logs that should be treated as 179logically one log, is delimited with flags marked in the segment 180summary. The recovery code of nilfs2 looks this boundary information 181to ensure atomicity of updates. 182 183The super root block is inserted for every checkpoints. It includes 184three special inodes, inodes for the DAT, cpfile, and sufile. Inodes 185of regular files, directories, symlinks and other special files, are 186included in the ifile. The inode of ifile itself is included in the 187corresponding checkpoint entry in the cpfile. Thus, the hierarchy 188among NILFS2 files can be depicted as follows: 189 190 Super block (SB) 191 | 192 v 193 Super root block (the latest cno=xx) 194 |-- DAT 195 |-- sufile 196 `-- cpfile 197 |-- ifile (cno=c1) 198 |-- ifile (cno=c2) ---- file (ino=i1) 199 : : |-- file (ino=i2) 200 `-- ifile (cno=xx) |-- file (ino=i3) 201 : : 202 `-- file (ino=yy) 203 ( regular file, directory, or symlink ) 204 205For detail on the format of each file, please see include/linux/nilfs2_fs.h. 206