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Documentation / filesystems / nilfs2.txt




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Based on kernel version 2.6.33. Page generated on 2010-02-24 15:36 EST.

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	List info:           http://vger.kernel.org/vger-lists.html#linux-nilfs
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	nobarrier		Disables barriers.
53	errors=continue(*)	Keep going on a filesystem error.
54	errors=remount-ro	Remount the filesystem read-only on an error.
55	errors=panic		Panic and halt the machine if an error occurs.
56	cp=n			Specify the checkpoint-number of the snapshot to be
57				mounted.  Checkpoints and snapshots are listed by lscp
58				user command.  Only the checkpoints marked as snapshot
59				are mountable with this option.  Snapshot is read-only,
60				so a read-only mount option must be specified together.
61	order=relaxed(*)	Apply relaxed order semantics that allows modified data
62				blocks to be written to disk without making a
63				checkpoint if no metadata update is going.  This mode
64				is equivalent to the ordered data mode of the ext3
65				filesystem except for the updates on data blocks still
66				conserve atomicity.  This will improve synchronous
67				write performance for overwriting.
68	order=strict		Apply strict in-order semantics that preserves sequence
69				of all file operations including overwriting of data
70				blocks.  That means, it is guaranteed that no
71				overtaking of events occurs in the recovered file
72				system after a crash.
73	norecovery		Disable recovery of the filesystem on mount.
74				This disables every write access on the device for
75				read-only mounts or snapshots.  This option will fail
76				for r/w mounts on an unclean volume.
77	
78	NILFS2 usage
79	============
80	
81	To use nilfs2 as a local file system, simply:
82	
83	 # mkfs -t nilfs2 /dev/block_device
84	 # mount -t nilfs2 /dev/block_device /dir
85	
86	This will also invoke the cleaner through the mount helper program
87	(mount.nilfs2).
88	
89	Checkpoints and snapshots are managed by the following commands.
90	Their manpages are included in the nilfs-utils package above.
91	
92	  lscp     list checkpoints or snapshots.
93	  mkcp     make a checkpoint or a snapshot.
94	  chcp     change an existing checkpoint to a snapshot or vice versa.
95	  rmcp     invalidate specified checkpoint(s).
96	
97	To mount a snapshot,
98	
99	 # mount -t nilfs2 -r -o cp=<cno> /dev/block_device /snap_dir
100	
101	where <cno> is the checkpoint number of the snapshot.
102	
103	To unmount the NILFS2 mount point or snapshot, simply:
104	
105	 # umount /dir
106	
107	Then, the cleaner daemon is automatically shut down by the umount
108	helper program (umount.nilfs2).
109	
110	Disk format
111	===========
112	
113	A nilfs2 volume is equally divided into a number of segments except
114	for the super block (SB) and segment #0.  A segment is the container
115	of logs.  Each log is composed of summary information blocks, payload
116	blocks, and an optional super root block (SR):
117	
118	   ______________________________________________________
119	  | |SB| | Segment | Segment | Segment | ... | Segment | |
120	  |_|__|_|____0____|____1____|____2____|_____|____N____|_|
121	  0 +1K +4K       +8M       +16M      +24M  +(8MB x N)
122	       .             .            (Typical offsets for 4KB-block)
123	    .                  .
124	  .______________________.
125	  | log | log |... | log |
126	  |__1__|__2__|____|__m__|
127	        .       .
128	      .               .
129	    .                       .
130	  .______________________________.
131	  | Summary | Payload blocks  |SR|
132	  |_blocks__|_________________|__|
133	
134	The payload blocks are organized per file, and each file consists of
135	data blocks and B-tree node blocks:
136	
137	    |<---       File-A        --->|<---       File-B        --->|
138	   _______________________________________________________________
139	    | Data blocks | B-tree blocks | Data blocks | B-tree blocks | ...
140	   _|_____________|_______________|_____________|_______________|_
141	
142	
143	Since only the modified blocks are written in the log, it may have
144	files without data blocks or B-tree node blocks.
145	
146	The organization of the blocks is recorded in the summary information
147	blocks, which contains a header structure (nilfs_segment_summary), per
148	file structures (nilfs_finfo), and per block structures (nilfs_binfo):
149	
150	  _________________________________________________________________________
151	 | Summary | finfo | binfo | ... | binfo | finfo | binfo | ... | binfo |...
152	 |_blocks__|___A___|_(A,1)_|_____|(A,Na)_|___B___|_(B,1)_|_____|(B,Nb)_|___
153	
154	
155	The logs include regular files, directory files, symbolic link files
156	and several meta data files.  The mata data files are the files used
157	to maintain file system meta data.  The current version of NILFS2 uses
158	the following meta data files:
159	
160	 1) Inode file (ifile)             -- Stores on-disk inodes
161	 2) Checkpoint file (cpfile)       -- Stores checkpoints
162	 3) Segment usage file (sufile)    -- Stores allocation state of segments
163	 4) Data address translation file  -- Maps virtual block numbers to usual
164	    (DAT)                             block numbers.  This file serves to
165	                                      make on-disk blocks relocatable.
166	
167	The following figure shows a typical organization of the logs:
168	
169	  _________________________________________________________________________
170	 | Summary | regular file | file  | ... | ifile | cpfile | sufile | DAT |SR|
171	 |_blocks__|_or_directory_|_______|_____|_______|________|________|_____|__|
172	
173	
174	To stride over segment boundaries, this sequence of files may be split
175	into multiple logs.  The sequence of logs that should be treated as
176	logically one log, is delimited with flags marked in the segment
177	summary.  The recovery code of nilfs2 looks this boundary information
178	to ensure atomicity of updates.
179	
180	The super root block is inserted for every checkpoints.  It includes
181	three special inodes, inodes for the DAT, cpfile, and sufile.  Inodes
182	of regular files, directories, symlinks and other special files, are
183	included in the ifile.  The inode of ifile itself is included in the
184	corresponding checkpoint entry in the cpfile.  Thus, the hierarchy
185	among NILFS2 files can be depicted as follows:
186	
187	  Super block (SB)
188	       |
189	       v
190	  Super root block (the latest cno=xx)
191	       |-- DAT
192	       |-- sufile
193	       `-- cpfile
194	              |-- ifile (cno=c1)
195	              |-- ifile (cno=c2) ---- file (ino=i1)
196	              :        :          |-- file (ino=i2)
197	              `-- ifile (cno=xx)  |-- file (ino=i3)
198	                                  :        :
199	                                  `-- file (ino=yy)
200	                                    ( regular file, directory, or symlink )
201	
202	For detail on the format of each file, please see include/linux/nilfs2_fs.h.
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