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+NILFS2
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+------
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+
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+NILFS2 is a log-structured file system (LFS) supporting continuous
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+snapshotting. In addition to versioning capability of the entire file
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+system, users can even restore files mistakenly overwritten or
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+destroyed just a few seconds ago. Since NILFS2 can keep consistency
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+like conventional LFS, it achieves quick recovery after system
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+crashes.
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+
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+NILFS2 creates a number of checkpoints every few seconds or per
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+synchronous write basis (unless there is no change). Users can select
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+significant versions among continuously created checkpoints, and can
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+change them into snapshots which will be preserved until they are
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+changed back to checkpoints.
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+
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+There is no limit on the number of snapshots until the volume gets
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+full. Each snapshot is mountable as a read-only file system
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+concurrently with its writable mount, and this feature is convenient
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+for online backup.
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+
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+The userland tools are included in nilfs-utils package, which is
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+available from the following download page. At least "mkfs.nilfs2",
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+"mount.nilfs2", "umount.nilfs2", and "nilfs_cleanerd" (so called
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+cleaner or garbage collector) are required. Details on the tools are
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+described in the man pages included in the package.
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+
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+Project web page: http://www.nilfs.org/en/
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+Download page: http://www.nilfs.org/en/download.html
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+Git tree web page: http://www.nilfs.org/git/
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+NILFS mailing lists: http://www.nilfs.org/mailman/listinfo/users
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+
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+Caveats
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+=======
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+
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+Features which NILFS2 does not support yet:
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+
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+ - atime
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+ - extended attributes
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+ - POSIX ACLs
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+ - quotas
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+ - writable snapshots
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+ - remote backup (CDP)
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+ - data integrity
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+ - defragmentation
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+
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+Mount options
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+=============
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+
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+NILFS2 supports the following mount options:
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+(*) == default
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+
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+barrier=on(*) This enables/disables barriers. barrier=off disables
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+ it, barrier=on enables it.
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+errors=continue(*) Keep going on a filesystem error.
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+errors=remount-ro Remount the filesystem read-only on an error.
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+errors=panic Panic and halt the machine if an error occurs.
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+cp=n Specify the checkpoint-number of the snapshot to be
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+ mounted. Checkpoints and snapshots are listed by lscp
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+ user command. Only the checkpoints marked as snapshot
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+ are mountable with this option. Snapshot is read-only,
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+ so a read-only mount option must be specified together.
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+order=relaxed(*) Apply relaxed order semantics that allows modified data
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+ blocks to be written to disk without making a
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+ checkpoint if no metadata update is going. This mode
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+ is equivalent to the ordered data mode of the ext3
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+ filesystem except for the updates on data blocks still
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+ conserve atomicity. This will improve synchronous
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+ write performance for overwriting.
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+order=strict Apply strict in-order semantics that preserves sequence
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+ of all file operations including overwriting of data
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+ blocks. That means, it is guaranteed that no
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+ overtaking of events occurs in the recovered file
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+ system after a crash.
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+
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+NILFS2 usage
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+============
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+
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+To use nilfs2 as a local file system, simply:
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+
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+ # mkfs -t nilfs2 /dev/block_device
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+ # mount -t nilfs2 /dev/block_device /dir
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+
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+This will also invoke the cleaner through the mount helper program
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+(mount.nilfs2).
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+
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+Checkpoints and snapshots are managed by the following commands.
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+Their manpages are included in the nilfs-utils package above.
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+
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+ lscp list checkpoints or snapshots.
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+ mkcp make a checkpoint or a snapshot.
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+ chcp change an existing checkpoint to a snapshot or vice versa.
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+ rmcp invalidate specified checkpoint(s).
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+
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+To mount a snapshot,
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+
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+ # mount -t nilfs2 -r -o cp=<cno> /dev/block_device /snap_dir
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+
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+where <cno> is the checkpoint number of the snapshot.
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+
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+To unmount the NILFS2 mount point or snapshot, simply:
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+
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+ # umount /dir
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+
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+Then, the cleaner daemon is automatically shut down by the umount
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+helper program (umount.nilfs2).
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+
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+Disk format
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+===========
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+
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+A nilfs2 volume is equally divided into a number of segments except
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+for the super block (SB) and segment #0. A segment is the container
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+of logs. Each log is composed of summary information blocks, payload
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+blocks, and an optional super root block (SR):
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+
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+ ______________________________________________________
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+ | |SB| | Segment | Segment | Segment | ... | Segment | |
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+ |_|__|_|____0____|____1____|____2____|_____|____N____|_|
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+ 0 +1K +4K +8M +16M +24M +(8MB x N)
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+ . . (Typical offsets for 4KB-block)
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+ . .
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+ .______________________.
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+ | log | log |... | log |
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+ |__1__|__2__|____|__m__|
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+ . .
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+ . .
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+ . .
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+ .______________________________.
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+ | Summary | Payload blocks |SR|
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+ |_blocks__|_________________|__|
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+
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+The payload blocks are organized per file, and each file consists of
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+data blocks and B-tree node blocks:
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+
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+ |<--- File-A --->|<--- File-B --->|
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+ _______________________________________________________________
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+ | Data blocks | B-tree blocks | Data blocks | B-tree blocks | ...
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+ _|_____________|_______________|_____________|_______________|_
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+
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+
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+Since only the modified blocks are written in the log, it may have
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+files without data blocks or B-tree node blocks.
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+
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+The organization of the blocks is recorded in the summary information
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+blocks, which contains a header structure (nilfs_segment_summary), per
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+file structures (nilfs_finfo), and per block structures (nilfs_binfo):
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+
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+ _________________________________________________________________________
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+ | Summary | finfo | binfo | ... | binfo | finfo | binfo | ... | binfo |...
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+ |_blocks__|___A___|_(A,1)_|_____|(A,Na)_|___B___|_(B,1)_|_____|(B,Nb)_|___
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+
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+
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+The logs include regular files, directory files, symbolic link files
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+and several meta data files. The mata data files are the files used
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+to maintain file system meta data. The current version of NILFS2 uses
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+the following meta data files:
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+
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+ 1) Inode file (ifile) -- Stores on-disk inodes
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+ 2) Checkpoint file (cpfile) -- Stores checkpoints
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+ 3) Segment usage file (sufile) -- Stores allocation state of segments
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+ 4) Data address translation file -- Maps virtual block numbers to usual
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+ (DAT) block numbers. This file serves to
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+ make on-disk blocks relocatable.
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+
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+The following figure shows a typical organization of the logs:
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+
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+ _________________________________________________________________________
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+ | Summary | regular file | file | ... | ifile | cpfile | sufile | DAT |SR|
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+ |_blocks__|_or_directory_|_______|_____|_______|________|________|_____|__|
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+
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+
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+To stride over segment boundaries, this sequence of files may be split
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+into multiple logs. The sequence of logs that should be treated as
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+logically one log, is delimited with flags marked in the segment
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+summary. The recovery code of nilfs2 looks this boundary information
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+to ensure atomicity of updates.
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+
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+The super root block is inserted for every checkpoints. It includes
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+three special inodes, inodes for the DAT, cpfile, and sufile. Inodes
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+of regular files, directories, symlinks and other special files, are
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+included in the ifile. The inode of ifile itself is included in the
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+corresponding checkpoint entry in the cpfile. Thus, the hierarchy
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+among NILFS2 files can be depicted as follows:
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+
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+ Super block (SB)
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+ |
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+ v
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+ Super root block (the latest cno=xx)
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+ |-- DAT
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+ |-- sufile
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+ `-- cpfile
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+ |-- ifile (cno=c1)
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+ |-- ifile (cno=c2) ---- file (ino=i1)
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+ : : |-- file (ino=i2)
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+ `-- ifile (cno=xx) |-- file (ino=i3)
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+ : :
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+ `-- file (ino=yy)
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+ ( regular file, directory, or symlink )
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+
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+For detail on the format of each file, please see include/linux/nilfs2_fs.h.
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