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diff --git a/Documentation/filesystems/nilfs2.rst b/Documentation/filesystems/nilfs2.rst new file mode 100644 index 000000000..6c49f04e9 --- /dev/null +++ b/Documentation/filesystems/nilfs2.rst @@ -0,0 +1,286 @@ +.. SPDX-License-Identifier: GPL-2.0 + +====== +NILFS2 +====== + +NILFS2 is a log-structured file system (LFS) supporting continuous +snapshotting. In addition to versioning capability of the entire file +system, users can even restore files mistakenly overwritten or +destroyed just a few seconds ago. Since NILFS2 can keep consistency +like conventional LFS, it achieves quick recovery after system +crashes. + +NILFS2 creates a number of checkpoints every few seconds or per +synchronous write basis (unless there is no change). Users can select +significant versions among continuously created checkpoints, and can +change them into snapshots which will be preserved until they are +changed back to checkpoints. + +There is no limit on the number of snapshots until the volume gets +full. Each snapshot is mountable as a read-only file system +concurrently with its writable mount, and this feature is convenient +for online backup. + +The userland tools are included in nilfs-utils package, which is +available from the following download page. At least "mkfs.nilfs2", +"mount.nilfs2", "umount.nilfs2", and "nilfs_cleanerd" (so called +cleaner or garbage collector) are required. Details on the tools are +described in the man pages included in the package. + +:Project web page: https://nilfs.sourceforge.io/ +:Download page: https://nilfs.sourceforge.io/en/download.html +:List info: http://vger.kernel.org/vger-lists.html#linux-nilfs + +Caveats +======= + +Features which NILFS2 does not support yet: + + - atime + - extended attributes + - POSIX ACLs + - quotas + - fsck + - defragmentation + +Mount options +============= + +NILFS2 supports the following mount options: +(*) == default + +======================= ======================================================= +barrier(*) This enables/disables the use of write barriers. This +nobarrier requires an IO stack which can support barriers, and + if nilfs gets an error on a barrier write, it will + disable again with a warning. +errors=continue Keep going on a filesystem error. +errors=remount-ro(*) Remount the filesystem read-only on an error. +errors=panic Panic and halt the machine if an error occurs. +cp=n Specify the checkpoint-number of the snapshot to be + mounted. Checkpoints and snapshots are listed by lscp + user command. Only the checkpoints marked as snapshot + are mountable with this option. Snapshot is read-only, + so a read-only mount option must be specified together. +order=relaxed(*) Apply relaxed order semantics that allows modified data + blocks to be written to disk without making a + checkpoint if no metadata update is going. This mode + is equivalent to the ordered data mode of the ext3 + filesystem except for the updates on data blocks still + conserve atomicity. This will improve synchronous + write performance for overwriting. +order=strict Apply strict in-order semantics that preserves sequence + of all file operations including overwriting of data + blocks. That means, it is guaranteed that no + overtaking of events occurs in the recovered file + system after a crash. +norecovery Disable recovery of the filesystem on mount. + This disables every write access on the device for + read-only mounts or snapshots. This option will fail + for r/w mounts on an unclean volume. +discard This enables/disables the use of discard/TRIM commands. +nodiscard(*) The discard/TRIM commands are sent to the underlying + block device when blocks are freed. This is useful + for SSD devices and sparse/thinly-provisioned LUNs. +======================= ======================================================= + +Ioctls +====== + +There is some NILFS2 specific functionality which can be accessed by applications +through the system call interfaces. The list of all NILFS2 specific ioctls are +shown in the table below. + +Table of NILFS2 specific ioctls: + + ============================== =============================================== + Ioctl Description + ============================== =============================================== + NILFS_IOCTL_CHANGE_CPMODE Change mode of given checkpoint between + checkpoint and snapshot state. This ioctl is + used in chcp and mkcp utilities. + + NILFS_IOCTL_DELETE_CHECKPOINT Remove checkpoint from NILFS2 file system. + This ioctl is used in rmcp utility. + + NILFS_IOCTL_GET_CPINFO Return info about requested checkpoints. This + ioctl is used in lscp utility and by + nilfs_cleanerd daemon. + + NILFS_IOCTL_GET_CPSTAT Return checkpoints statistics. This ioctl is + used by lscp, rmcp utilities and by + nilfs_cleanerd daemon. + + NILFS_IOCTL_GET_SUINFO Return segment usage info about requested + segments. This ioctl is used in lssu, + nilfs_resize utilities and by nilfs_cleanerd + daemon. + + NILFS_IOCTL_SET_SUINFO Modify segment usage info of requested + segments. This ioctl is used by + nilfs_cleanerd daemon to skip unnecessary + cleaning operation of segments and reduce + performance penalty or wear of flash device + due to redundant move of in-use blocks. + + NILFS_IOCTL_GET_SUSTAT Return segment usage statistics. This ioctl + is used in lssu, nilfs_resize utilities and + by nilfs_cleanerd daemon. + + NILFS_IOCTL_GET_VINFO Return information on virtual block addresses. + This ioctl is used by nilfs_cleanerd daemon. + + NILFS_IOCTL_GET_BDESCS Return information about descriptors of disk + block numbers. This ioctl is used by + nilfs_cleanerd daemon. + + NILFS_IOCTL_CLEAN_SEGMENTS Do garbage collection operation in the + environment of requested parameters from + userspace. This ioctl is used by + nilfs_cleanerd daemon. + + NILFS_IOCTL_SYNC Make a checkpoint. This ioctl is used in + mkcp utility. + + NILFS_IOCTL_RESIZE Resize NILFS2 volume. This ioctl is used + by nilfs_resize utility. + + NILFS_IOCTL_SET_ALLOC_RANGE Define lower limit of segments in bytes and + upper limit of segments in bytes. This ioctl + is used by nilfs_resize utility. + ============================== =============================================== + +NILFS2 usage +============ + +To use nilfs2 as a local file system, simply:: + + # mkfs -t nilfs2 /dev/block_device + # mount -t nilfs2 /dev/block_device /dir + +This will also invoke the cleaner through the mount helper program +(mount.nilfs2). + +Checkpoints and snapshots are managed by the following commands. +Their manpages are included in the nilfs-utils package above. + + ==== =========================================================== + lscp list checkpoints or snapshots. + mkcp make a checkpoint or a snapshot. + chcp change an existing checkpoint to a snapshot or vice versa. + rmcp invalidate specified checkpoint(s). + ==== =========================================================== + +To mount a snapshot:: + + # mount -t nilfs2 -r -o cp=<cno> /dev/block_device /snap_dir + +where <cno> is the checkpoint number of the snapshot. + +To unmount the NILFS2 mount point or snapshot, simply:: + + # umount /dir + +Then, the cleaner daemon is automatically shut down by the umount +helper program (umount.nilfs2). + +Disk format +=========== + +A nilfs2 volume is equally divided into a number of segments except +for the super block (SB) and segment #0. A segment is the container +of logs. Each log is composed of summary information blocks, payload +blocks, and an optional super root block (SR):: + + ______________________________________________________ + | |SB| | Segment | Segment | Segment | ... | Segment | | + |_|__|_|____0____|____1____|____2____|_____|____N____|_| + 0 +1K +4K +8M +16M +24M +(8MB x N) + . . (Typical offsets for 4KB-block) + . . + .______________________. + | log | log |... | log | + |__1__|__2__|____|__m__| + . . + . . + . . + .______________________________. + | Summary | Payload blocks |SR| + |_blocks__|_________________|__| + +The payload blocks are organized per file, and each file consists of +data blocks and B-tree node blocks:: + + |<--- File-A --->|<--- File-B --->| + _______________________________________________________________ + | Data blocks | B-tree blocks | Data blocks | B-tree blocks | ... + _|_____________|_______________|_____________|_______________|_ + + +Since only the modified blocks are written in the log, it may have +files without data blocks or B-tree node blocks. + +The organization of the blocks is recorded in the summary information +blocks, which contains a header structure (nilfs_segment_summary), per +file structures (nilfs_finfo), and per block structures (nilfs_binfo):: + + _________________________________________________________________________ + | Summary | finfo | binfo | ... | binfo | finfo | binfo | ... | binfo |... + |_blocks__|___A___|_(A,1)_|_____|(A,Na)_|___B___|_(B,1)_|_____|(B,Nb)_|___ + + +The logs include regular files, directory files, symbolic link files +and several meta data files. The mata data files are the files used +to maintain file system meta data. The current version of NILFS2 uses +the following meta data files:: + + 1) Inode file (ifile) -- Stores on-disk inodes + 2) Checkpoint file (cpfile) -- Stores checkpoints + 3) Segment usage file (sufile) -- Stores allocation state of segments + 4) Data address translation file -- Maps virtual block numbers to usual + (DAT) block numbers. This file serves to + make on-disk blocks relocatable. + +The following figure shows a typical organization of the logs:: + + _________________________________________________________________________ + | Summary | regular file | file | ... | ifile | cpfile | sufile | DAT |SR| + |_blocks__|_or_directory_|_______|_____|_______|________|________|_____|__| + + +To stride over segment boundaries, this sequence of files may be split +into multiple logs. The sequence of logs that should be treated as +logically one log, is delimited with flags marked in the segment +summary. The recovery code of nilfs2 looks this boundary information +to ensure atomicity of updates. + +The super root block is inserted for every checkpoints. It includes +three special inodes, inodes for the DAT, cpfile, and sufile. Inodes +of regular files, directories, symlinks and other special files, are +included in the ifile. The inode of ifile itself is included in the +corresponding checkpoint entry in the cpfile. Thus, the hierarchy +among NILFS2 files can be depicted as follows:: + + Super block (SB) + | + v + Super root block (the latest cno=xx) + |-- DAT + |-- sufile + `-- cpfile + |-- ifile (cno=c1) + |-- ifile (cno=c2) ---- file (ino=i1) + : : |-- file (ino=i2) + `-- ifile (cno=xx) |-- file (ino=i3) + : : + `-- file (ino=yy) + ( regular file, directory, or symlink ) + +For detail on the format of each file, please see nilfs2_ondisk.h +located at include/uapi/linux directory. + +There are no patents or other intellectual property that we protect +with regard to the design of NILFS2. It is allowed to replicate the +design in hopes that other operating systems could share (mount, read, +write, etc.) data stored in this format. |