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 - defragmentation 44 45Mount options 46============= 47 48NILFS2 supports the following mount options: 49(*) == default 50 51barrier(*) This enables/disables the use of write barriers. This 52nobarrier requires an IO stack which can support barriers, and 53 if nilfs gets an error on a barrier write, it will 54 disable again with a warning. 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. 75norecovery Disable recovery of the filesystem on mount. 76 This disables every write access on the device for 77 read-only mounts or snapshots. This option will fail 78 for r/w mounts on an unclean volume. 79discard This enables/disables the use of discard/TRIM commands. 80nodiscard(*) The discard/TRIM commands are sent to the underlying 81 block device when blocks are freed. This is useful 82 for SSD devices and sparse/thinly-provisioned LUNs. 83 84NILFS2 usage 85============ 86 87To use nilfs2 as a local file system, simply: 88 89 # mkfs -t nilfs2 /dev/block_device 90 # mount -t nilfs2 /dev/block_device /dir 91 92This will also invoke the cleaner through the mount helper program 93(mount.nilfs2). 94 95Checkpoints and snapshots are managed by the following commands. 96Their manpages are included in the nilfs-utils package above. 97 98 lscp list checkpoints or snapshots. 99 mkcp make a checkpoint or a snapshot. 100 chcp change an existing checkpoint to a snapshot or vice versa. 101 rmcp invalidate specified checkpoint(s). 102 103To mount a snapshot, 104 105 # mount -t nilfs2 -r -o cp=<cno> /dev/block_device /snap_dir 106 107where <cno> is the checkpoint number of the snapshot. 108 109To unmount the NILFS2 mount point or snapshot, simply: 110 111 # umount /dir 112 113Then, the cleaner daemon is automatically shut down by the umount 114helper program (umount.nilfs2). 115 116Disk format 117=========== 118 119A nilfs2 volume is equally divided into a number of segments except 120for the super block (SB) and segment #0. A segment is the container 121of logs. Each log is composed of summary information blocks, payload 122blocks, and an optional super root block (SR): 123 124 ______________________________________________________ 125 | |SB| | Segment | Segment | Segment | ... | Segment | | 126 |_|__|_|____0____|____1____|____2____|_____|____N____|_| 127 0 +1K +4K +8M +16M +24M +(8MB x N) 128 . . (Typical offsets for 4KB-block) 129 . . 130 .______________________. 131 | log | log |... | log | 132 |__1__|__2__|____|__m__| 133 . . 134 . . 135 . . 136 .______________________________. 137 | Summary | Payload blocks |SR| 138 |_blocks__|_________________|__| 139 140The payload blocks are organized per file, and each file consists of 141data blocks and B-tree node blocks: 142 143 |<--- File-A --->|<--- File-B --->| 144 _______________________________________________________________ 145 | Data blocks | B-tree blocks | Data blocks | B-tree blocks | ... 146 _|_____________|_______________|_____________|_______________|_ 147 148 149Since only the modified blocks are written in the log, it may have 150files without data blocks or B-tree node blocks. 151 152The organization of the blocks is recorded in the summary information 153blocks, which contains a header structure (nilfs_segment_summary), per 154file structures (nilfs_finfo), and per block structures (nilfs_binfo): 155 156 _________________________________________________________________________ 157 | Summary | finfo | binfo | ... | binfo | finfo | binfo | ... | binfo |... 158 |_blocks__|___A___|_(A,1)_|_____|(A,Na)_|___B___|_(B,1)_|_____|(B,Nb)_|___ 159 160 161The logs include regular files, directory files, symbolic link files 162and several meta data files. The mata data files are the files used 163to maintain file system meta data. The current version of NILFS2 uses 164the following meta data files: 165 166 1) Inode file (ifile) -- Stores on-disk inodes 167 2) Checkpoint file (cpfile) -- Stores checkpoints 168 3) Segment usage file (sufile) -- Stores allocation state of segments 169 4) Data address translation file -- Maps virtual block numbers to usual 170 (DAT) block numbers. This file serves to 171 make on-disk blocks relocatable. 172 173The following figure shows a typical organization of the logs: 174 175 _________________________________________________________________________ 176 | Summary | regular file | file | ... | ifile | cpfile | sufile | DAT |SR| 177 |_blocks__|_or_directory_|_______|_____|_______|________|________|_____|__| 178 179 180To stride over segment boundaries, this sequence of files may be split 181into multiple logs. The sequence of logs that should be treated as 182logically one log, is delimited with flags marked in the segment 183summary. The recovery code of nilfs2 looks this boundary information 184to ensure atomicity of updates. 185 186The super root block is inserted for every checkpoints. It includes 187three special inodes, inodes for the DAT, cpfile, and sufile. Inodes 188of regular files, directories, symlinks and other special files, are 189included in the ifile. The inode of ifile itself is included in the 190corresponding checkpoint entry in the cpfile. Thus, the hierarchy 191among NILFS2 files can be depicted as follows: 192 193 Super block (SB) 194 | 195 v 196 Super root block (the latest cno=xx) 197 |-- DAT 198 |-- sufile 199 `-- cpfile 200 |-- ifile (cno=c1) 201 |-- ifile (cno=c2) ---- file (ino=i1) 202 : : |-- file (ino=i2) 203 `-- ifile (cno=xx) |-- file (ino=i3) 204 : : 205 `-- file (ino=yy) 206 ( regular file, directory, or symlink ) 207 208For detail on the format of each file, please see include/linux/nilfs2_fs.h. 209