1.. SPDX-License-Identifier: GPL-2.0 2 3====== 4NILFS2 5====== 6 7NILFS2 is a log-structured file system (LFS) supporting continuous 8snapshotting. In addition to versioning capability of the entire file 9system, users can even restore files mistakenly overwritten or 10destroyed just a few seconds ago. Since NILFS2 can keep consistency 11like conventional LFS, it achieves quick recovery after system 12crashes. 13 14NILFS2 creates a number of checkpoints every few seconds or per 15synchronous write basis (unless there is no change). Users can select 16significant versions among continuously created checkpoints, and can 17change them into snapshots which will be preserved until they are 18changed back to checkpoints. 19 20There is no limit on the number of snapshots until the volume gets 21full. Each snapshot is mountable as a read-only file system 22concurrently with its writable mount, and this feature is convenient 23for online backup. 24 25The userland tools are included in nilfs-utils package, which is 26available from the following download page. At least "mkfs.nilfs2", 27"mount.nilfs2", "umount.nilfs2", and "nilfs_cleanerd" (so called 28cleaner or garbage collector) are required. Details on the tools are 29described in the man pages included in the package. 30 31:Project web page: https://nilfs.sourceforge.io/ 32:Download page: https://nilfs.sourceforge.io/en/download.html 33:List info: http://vger.kernel.org/vger-lists.html#linux-nilfs 34 35Caveats 36======= 37 38Features which NILFS2 does not support yet: 39 40 - atime 41 - extended attributes 42 - POSIX ACLs 43 - quotas 44 - fsck 45 - defragmentation 46 47Mount options 48============= 49 50NILFS2 supports the following mount options: 51(*) == default 52 53======================= ======================================================= 54barrier(*) This enables/disables the use of write barriers. This 55nobarrier requires an IO stack which can support barriers, and 56 if nilfs gets an error on a barrier write, it will 57 disable again with a warning. 58errors=continue Keep going on a filesystem error. 59errors=remount-ro(*) Remount the filesystem read-only on an error. 60errors=panic Panic and halt the machine if an error occurs. 61cp=n Specify the checkpoint-number of the snapshot to be 62 mounted. Checkpoints and snapshots are listed by lscp 63 user command. Only the checkpoints marked as snapshot 64 are mountable with this option. Snapshot is read-only, 65 so a read-only mount option must be specified together. 66order=relaxed(*) Apply relaxed order semantics that allows modified data 67 blocks to be written to disk without making a 68 checkpoint if no metadata update is going. This mode 69 is equivalent to the ordered data mode of the ext3 70 filesystem except for the updates on data blocks still 71 conserve atomicity. This will improve synchronous 72 write performance for overwriting. 73order=strict Apply strict in-order semantics that preserves sequence 74 of all file operations including overwriting of data 75 blocks. That means, it is guaranteed that no 76 overtaking of events occurs in the recovered file 77 system after a crash. 78norecovery Disable recovery of the filesystem on mount. 79 This disables every write access on the device for 80 read-only mounts or snapshots. This option will fail 81 for r/w mounts on an unclean volume. 82discard This enables/disables the use of discard/TRIM commands. 83nodiscard(*) The discard/TRIM commands are sent to the underlying 84 block device when blocks are freed. This is useful 85 for SSD devices and sparse/thinly-provisioned LUNs. 86======================= ======================================================= 87 88Ioctls 89====== 90 91There is some NILFS2 specific functionality which can be accessed by applications 92through the system call interfaces. The list of all NILFS2 specific ioctls are 93shown in the table below. 94 95Table of NILFS2 specific ioctls: 96 97 ============================== =============================================== 98 Ioctl Description 99 ============================== =============================================== 100 NILFS_IOCTL_CHANGE_CPMODE Change mode of given checkpoint between 101 checkpoint and snapshot state. This ioctl is 102 used in chcp and mkcp utilities. 103 104 NILFS_IOCTL_DELETE_CHECKPOINT Remove checkpoint from NILFS2 file system. 105 This ioctl is used in rmcp utility. 106 107 NILFS_IOCTL_GET_CPINFO Return info about requested checkpoints. This 108 ioctl is used in lscp utility and by 109 nilfs_cleanerd daemon. 110 111 NILFS_IOCTL_GET_CPSTAT Return checkpoints statistics. This ioctl is 112 used by lscp, rmcp utilities and by 113 nilfs_cleanerd daemon. 114 115 NILFS_IOCTL_GET_SUINFO Return segment usage info about requested 116 segments. This ioctl is used in lssu, 117 nilfs_resize utilities and by nilfs_cleanerd 118 daemon. 119 120 NILFS_IOCTL_SET_SUINFO Modify segment usage info of requested 121 segments. This ioctl is used by 122 nilfs_cleanerd daemon to skip unnecessary 123 cleaning operation of segments and reduce 124 performance penalty or wear of flash device 125 due to redundant move of in-use blocks. 126 127 NILFS_IOCTL_GET_SUSTAT Return segment usage statistics. This ioctl 128 is used in lssu, nilfs_resize utilities and 129 by nilfs_cleanerd daemon. 130 131 NILFS_IOCTL_GET_VINFO Return information on virtual block addresses. 132 This ioctl is used by nilfs_cleanerd daemon. 133 134 NILFS_IOCTL_GET_BDESCS Return information about descriptors of disk 135 block numbers. This ioctl is used by 136 nilfs_cleanerd daemon. 137 138 NILFS_IOCTL_CLEAN_SEGMENTS Do garbage collection operation in the 139 environment of requested parameters from 140 userspace. This ioctl is used by 141 nilfs_cleanerd daemon. 142 143 NILFS_IOCTL_SYNC Make a checkpoint. This ioctl is used in 144 mkcp utility. 145 146 NILFS_IOCTL_RESIZE Resize NILFS2 volume. This ioctl is used 147 by nilfs_resize utility. 148 149 NILFS_IOCTL_SET_ALLOC_RANGE Define lower limit of segments in bytes and 150 upper limit of segments in bytes. This ioctl 151 is used by nilfs_resize utility. 152 ============================== =============================================== 153 154NILFS2 usage 155============ 156 157To use nilfs2 as a local file system, simply:: 158 159 # mkfs -t nilfs2 /dev/block_device 160 # mount -t nilfs2 /dev/block_device /dir 161 162This will also invoke the cleaner through the mount helper program 163(mount.nilfs2). 164 165Checkpoints and snapshots are managed by the following commands. 166Their manpages are included in the nilfs-utils package above. 167 168 ==== =========================================================== 169 lscp list checkpoints or snapshots. 170 mkcp make a checkpoint or a snapshot. 171 chcp change an existing checkpoint to a snapshot or vice versa. 172 rmcp invalidate specified checkpoint(s). 173 ==== =========================================================== 174 175To mount a snapshot:: 176 177 # mount -t nilfs2 -r -o cp=<cno> /dev/block_device /snap_dir 178 179where <cno> is the checkpoint number of the snapshot. 180 181To unmount the NILFS2 mount point or snapshot, simply:: 182 183 # umount /dir 184 185Then, the cleaner daemon is automatically shut down by the umount 186helper program (umount.nilfs2). 187 188Disk format 189=========== 190 191A nilfs2 volume is equally divided into a number of segments except 192for the super block (SB) and segment #0. A segment is the container 193of logs. Each log is composed of summary information blocks, payload 194blocks, and an optional super root block (SR):: 195 196 ______________________________________________________ 197 | |SB| | Segment | Segment | Segment | ... | Segment | | 198 |_|__|_|____0____|____1____|____2____|_____|____N____|_| 199 0 +1K +4K +8M +16M +24M +(8MB x N) 200 . . (Typical offsets for 4KB-block) 201 . . 202 .______________________. 203 | log | log |... | log | 204 |__1__|__2__|____|__m__| 205 . . 206 . . 207 . . 208 .______________________________. 209 | Summary | Payload blocks |SR| 210 |_blocks__|_________________|__| 211 212The payload blocks are organized per file, and each file consists of 213data blocks and B-tree node blocks:: 214 215 |<--- File-A --->|<--- File-B --->| 216 _______________________________________________________________ 217 | Data blocks | B-tree blocks | Data blocks | B-tree blocks | ... 218 _|_____________|_______________|_____________|_______________|_ 219 220 221Since only the modified blocks are written in the log, it may have 222files without data blocks or B-tree node blocks. 223 224The organization of the blocks is recorded in the summary information 225blocks, which contains a header structure (nilfs_segment_summary), per 226file structures (nilfs_finfo), and per block structures (nilfs_binfo):: 227 228 _________________________________________________________________________ 229 | Summary | finfo | binfo | ... | binfo | finfo | binfo | ... | binfo |... 230 |_blocks__|___A___|_(A,1)_|_____|(A,Na)_|___B___|_(B,1)_|_____|(B,Nb)_|___ 231 232 233The logs include regular files, directory files, symbolic link files 234and several meta data files. The meta data files are the files used 235to maintain file system meta data. The current version of NILFS2 uses 236the following meta data files:: 237 238 1) Inode file (ifile) -- Stores on-disk inodes 239 2) Checkpoint file (cpfile) -- Stores checkpoints 240 3) Segment usage file (sufile) -- Stores allocation state of segments 241 4) Data address translation file -- Maps virtual block numbers to usual 242 (DAT) block numbers. This file serves to 243 make on-disk blocks relocatable. 244 245The following figure shows a typical organization of the logs:: 246 247 _________________________________________________________________________ 248 | Summary | regular file | file | ... | ifile | cpfile | sufile | DAT |SR| 249 |_blocks__|_or_directory_|_______|_____|_______|________|________|_____|__| 250 251 252To stride over segment boundaries, this sequence of files may be split 253into multiple logs. The sequence of logs that should be treated as 254logically one log, is delimited with flags marked in the segment 255summary. The recovery code of nilfs2 looks this boundary information 256to ensure atomicity of updates. 257 258The super root block is inserted for every checkpoints. It includes 259three special inodes, inodes for the DAT, cpfile, and sufile. Inodes 260of regular files, directories, symlinks and other special files, are 261included in the ifile. The inode of ifile itself is included in the 262corresponding checkpoint entry in the cpfile. Thus, the hierarchy 263among NILFS2 files can be depicted as follows:: 264 265 Super block (SB) 266 | 267 v 268 Super root block (the latest cno=xx) 269 |-- DAT 270 |-- sufile 271 `-- cpfile 272 |-- ifile (cno=c1) 273 |-- ifile (cno=c2) ---- file (ino=i1) 274 : : |-- file (ino=i2) 275 `-- ifile (cno=xx) |-- file (ino=i3) 276 : : 277 `-- file (ino=yy) 278 ( regular file, directory, or symlink ) 279 280For detail on the format of each file, please see nilfs2_ondisk.h 281located at include/uapi/linux directory. 282 283There are no patents or other intellectual property that we protect 284with regard to the design of NILFS2. It is allowed to replicate the 285design in hopes that other operating systems could share (mount, read, 286write, etc.) data stored in this format. 287
