Linux Kernel Documentation :: filesystems : nilfs2.txt

1	NILFS2
2	------
3	
4	NILFS2 is a log-structured file system (LFS) supporting continuous
5	snapshotting.  In addition to versioning capability of the entire file
6	system, users can even restore files mistakenly overwritten or
7	destroyed just a few seconds ago.  Since NILFS2 can keep consistency
8	like conventional LFS, it achieves quick recovery after system
9	crashes.
10	
11	NILFS2 creates a number of checkpoints every few seconds or per
12	synchronous write basis (unless there is no change).  Users can select
13	significant versions among continuously created checkpoints, and can
14	change them into snapshots which will be preserved until they are
15	changed back to checkpoints.
16	
17	There is no limit on the number of snapshots until the volume gets
18	full.  Each snapshot is mountable as a read-only file system
19	concurrently with its writable mount, and this feature is convenient
20	for online backup.
21	
22	The userland tools are included in nilfs-utils package, which is
23	available from the following download page.  At least "mkfs.nilfs2",
24	"mount.nilfs2", "umount.nilfs2", and "nilfs_cleanerd" (so called
25	cleaner or garbage collector) are required.  Details on the tools are
26	described in the man pages included in the package.
27	
28	Project web page:    http://www.nilfs.org/en/
29	Download page:       http://www.nilfs.org/en/download.html
30	Git tree web page:   http://www.nilfs.org/git/
31	NILFS mailing lists: http://www.nilfs.org/mailman/listinfo/users
32	
33	Caveats
34	=======
35	
36	Features which NILFS2 does not support yet:
37	
38		- atime
39		- extended attributes
40		- POSIX ACLs
41		- quotas
42		- fsck
43		- resize
44		- defragmentation
45	
46	Mount options
47	=============
48	
49	NILFS2 supports the following mount options:
50	(*) == default
51	
52	barrier=on(*)		This enables/disables barriers. barrier=off disables
53				it, barrier=on enables it.
54	errors=continue(*)	Keep going on a filesystem error.
55	errors=remount-ro	Remount the filesystem read-only on an error.
56	errors=panic		Panic and halt the machine if an error occurs.
57	cp=n			Specify the checkpoint-number of the snapshot to be
58				mounted.  Checkpoints and snapshots are listed by lscp
59				user command.  Only the checkpoints marked as snapshot
60				are mountable with this option.  Snapshot is read-only,
61				so a read-only mount option must be specified together.
62	order=relaxed(*)	Apply relaxed order semantics that allows modified data
63				blocks to be written to disk without making a
64				checkpoint if no metadata update is going.  This mode
65				is equivalent to the ordered data mode of the ext3
66				filesystem except for the updates on data blocks still
67				conserve atomicity.  This will improve synchronous
68				write performance for overwriting.
69	order=strict		Apply strict in-order semantics that preserves sequence
70				of all file operations including overwriting of data
71				blocks.  That means, it is guaranteed that no
72				overtaking of events occurs in the recovered file
73				system after a crash.
74	
75	NILFS2 usage
76	============
77	
78	To use nilfs2 as a local file system, simply:
79	
80	 # mkfs -t nilfs2 /dev/block_device
81	 # mount -t nilfs2 /dev/block_device /dir
82	
83	This will also invoke the cleaner through the mount helper program
84	(mount.nilfs2).
85	
86	Checkpoints and snapshots are managed by the following commands.
87	Their manpages are included in the nilfs-utils package above.
88	
89	  lscp     list checkpoints or snapshots.
90	  mkcp     make a checkpoint or a snapshot.
91	  chcp     change an existing checkpoint to a snapshot or vice versa.
92	  rmcp     invalidate specified checkpoint(s).
93	
94	To mount a snapshot,
95	
96	 # mount -t nilfs2 -r -o cp=<cno> /dev/block_device /snap_dir
97	
98	where <cno> is the checkpoint number of the snapshot.
99	
100	To unmount the NILFS2 mount point or snapshot, simply:
101	
102	 # umount /dir
103	
104	Then, the cleaner daemon is automatically shut down by the umount
105	helper program (umount.nilfs2).
106	
107	Disk format
108	===========
109	
110	A nilfs2 volume is equally divided into a number of segments except
111	for the super block (SB) and segment #0.  A segment is the container
112	of logs.  Each log is composed of summary information blocks, payload
113	blocks, and an optional super root block (SR):
114	
115	   ______________________________________________________
116	  | |SB| | Segment | Segment | Segment | ... | Segment | |
117	  |_|__|_|____0____|____1____|____2____|_____|____N____|_|
118	  0 +1K +4K       +8M       +16M      +24M  +(8MB x N)
119	       .             .            (Typical offsets for 4KB-block)
120	    .                  .
121	  .______________________.
122	  | log | log |... | log |
123	  |__1__|__2__|____|__m__|
124	        .       .
125	      .               .
126	    .                       .
127	  .______________________________.
128	  | Summary | Payload blocks  |SR|
129	  |_blocks__|_________________|__|
130	
131	The payload blocks are organized per file, and each file consists of
132	data blocks and B-tree node blocks:
133	
134	    |<---       File-A        --->|<---       File-B        --->|
135	   _______________________________________________________________
136	    | Data blocks | B-tree blocks | Data blocks | B-tree blocks | ...
137	   _|_____________|_______________|_____________|_______________|_
138	
139	
140	Since only the modified blocks are written in the log, it may have
141	files without data blocks or B-tree node blocks.
142	
143	The organization of the blocks is recorded in the summary information
144	blocks, which contains a header structure (nilfs_segment_summary), per
145	file structures (nilfs_finfo), and per block structures (nilfs_binfo):
146	
147	  _________________________________________________________________________
148	 | Summary | finfo | binfo | ... | binfo | finfo | binfo | ... | binfo |...
149	 |_blocks__|___A___|_(A,1)_|_____|(A,Na)_|___B___|_(B,1)_|_____|(B,Nb)_|___
150	
151	
152	The logs include regular files, directory files, symbolic link files
153	and several meta data files.  The mata data files are the files used
154	to maintain file system meta data.  The current version of NILFS2 uses
155	the following meta data files:
156	
157	 1) Inode file (ifile)             -- Stores on-disk inodes
158	 2) Checkpoint file (cpfile)       -- Stores checkpoints
159	 3) Segment usage file (sufile)    -- Stores allocation state of segments
160	 4) Data address translation file  -- Maps virtual block numbers to usual
161	    (DAT)                             block numbers.  This file serves to
162	                                      make on-disk blocks relocatable.
163	
164	The following figure shows a typical organization of the logs:
165	
166	  _________________________________________________________________________
167	 | Summary | regular file | file  | ... | ifile | cpfile | sufile | DAT |SR|
168	 |_blocks__|_or_directory_|_______|_____|_______|________|________|_____|__|
169	
170	
171	To stride over segment boundaries, this sequence of files may be split
172	into multiple logs.  The sequence of logs that should be treated as
173	logically one log, is delimited with flags marked in the segment
174	summary.  The recovery code of nilfs2 looks this boundary information
175	to ensure atomicity of updates.
176	
177	The super root block is inserted for every checkpoints.  It includes
178	three special inodes, inodes for the DAT, cpfile, and sufile.  Inodes
179	of regular files, directories, symlinks and other special files, are
180	included in the ifile.  The inode of ifile itself is included in the
181	corresponding checkpoint entry in the cpfile.  Thus, the hierarchy
182	among NILFS2 files can be depicted as follows:
183	
184	  Super block (SB)
185	       |
186	       v
187	  Super root block (the latest cno=xx)
188	       |-- DAT
189	       |-- sufile
190	       `-- cpfile
191	              |-- ifile (cno=c1)
192	              |-- ifile (cno=c2) ---- file (ino=i1)
193	              :        :          |-- file (ino=i2)
194	              `-- ifile (cno=xx)  |-- file (ino=i3)
195	                                  :        :
196	                                  `-- file (ino=yy)
197	                                    ( regular file, directory, or symlink )
198	
199	For detail on the format of each file, please see include/linux/nilfs2_fs.h.

Documentation / filesystems / nilfs2.txt