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-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: http://www.nilfs.org/en/
-Download page: http://www.nilfs.org/en/download.html
-Git tree web page: http://www.nilfs.org/git/
-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.
-
-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 include/linux/nilfs2_fs.h.