--- /dev/null
+.. 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.
+++ /dev/null
-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.
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