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Documentation / filesystems / nilfs2.txt




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Based on kernel version 3.9. Page generated on 2013-05-02 23:06 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		- defragmentation
44	
45	Mount options
46	=============
47	
48	NILFS2 supports the following mount options:
49	(*) == default
50	
51	barrier(*)		This enables/disables the use of write barriers.  This
52	nobarrier		requires an IO stack which can support barriers, and
53				if nilfs gets an error on a barrier write, it will
54				disable again with a warning.
55	errors=continue		Keep going on a filesystem error.
56	errors=remount-ro(*)	Remount the filesystem read-only on an error.
57	errors=panic		Panic and halt the machine if an error occurs.
58	cp=n			Specify the checkpoint-number of the snapshot to be
59				mounted.  Checkpoints and snapshots are listed by lscp
60				user command.  Only the checkpoints marked as snapshot
61				are mountable with this option.  Snapshot is read-only,
62				so a read-only mount option must be specified together.
63	order=relaxed(*)	Apply relaxed order semantics that allows modified data
64				blocks to be written to disk without making a
65				checkpoint if no metadata update is going.  This mode
66				is equivalent to the ordered data mode of the ext3
67				filesystem except for the updates on data blocks still
68				conserve atomicity.  This will improve synchronous
69				write performance for overwriting.
70	order=strict		Apply strict in-order semantics that preserves sequence
71				of all file operations including overwriting of data
72				blocks.  That means, it is guaranteed that no
73				overtaking of events occurs in the recovered file
74				system after a crash.
75	norecovery		Disable recovery of the filesystem on mount.
76				This disables every write access on the device for
77				read-only mounts or snapshots.  This option will fail
78				for r/w mounts on an unclean volume.
79	discard			This enables/disables the use of discard/TRIM commands.
80	nodiscard(*)		The discard/TRIM commands are sent to the underlying
81				block device when blocks are freed.  This is useful
82				for SSD devices and sparse/thinly-provisioned LUNs.
83	
84	NILFS2 usage
85	============
86	
87	To use nilfs2 as a local file system, simply:
88	
89	 # mkfs -t nilfs2 /dev/block_device
90	 # mount -t nilfs2 /dev/block_device /dir
91	
92	This will also invoke the cleaner through the mount helper program
93	(mount.nilfs2).
94	
95	Checkpoints and snapshots are managed by the following commands.
96	Their manpages are included in the nilfs-utils package above.
97	
98	  lscp     list checkpoints or snapshots.
99	  mkcp     make a checkpoint or a snapshot.
100	  chcp     change an existing checkpoint to a snapshot or vice versa.
101	  rmcp     invalidate specified checkpoint(s).
102	
103	To mount a snapshot,
104	
105	 # mount -t nilfs2 -r -o cp=<cno> /dev/block_device /snap_dir
106	
107	where <cno> is the checkpoint number of the snapshot.
108	
109	To unmount the NILFS2 mount point or snapshot, simply:
110	
111	 # umount /dir
112	
113	Then, the cleaner daemon is automatically shut down by the umount
114	helper program (umount.nilfs2).
115	
116	Disk format
117	===========
118	
119	A nilfs2 volume is equally divided into a number of segments except
120	for the super block (SB) and segment #0.  A segment is the container
121	of logs.  Each log is composed of summary information blocks, payload
122	blocks, and an optional super root block (SR):
123	
124	   ______________________________________________________
125	  | |SB| | Segment | Segment | Segment | ... | Segment | |
126	  |_|__|_|____0____|____1____|____2____|_____|____N____|_|
127	  0 +1K +4K       +8M       +16M      +24M  +(8MB x N)
128	       .             .            (Typical offsets for 4KB-block)
129	    .                  .
130	  .______________________.
131	  | log | log |... | log |
132	  |__1__|__2__|____|__m__|
133	        .       .
134	      .               .
135	    .                       .
136	  .______________________________.
137	  | Summary | Payload blocks  |SR|
138	  |_blocks__|_________________|__|
139	
140	The payload blocks are organized per file, and each file consists of
141	data blocks and B-tree node blocks:
142	
143	    |<---       File-A        --->|<---       File-B        --->|
144	   _______________________________________________________________
145	    | Data blocks | B-tree blocks | Data blocks | B-tree blocks | ...
146	   _|_____________|_______________|_____________|_______________|_
147	
148	
149	Since only the modified blocks are written in the log, it may have
150	files without data blocks or B-tree node blocks.
151	
152	The organization of the blocks is recorded in the summary information
153	blocks, which contains a header structure (nilfs_segment_summary), per
154	file structures (nilfs_finfo), and per block structures (nilfs_binfo):
155	
156	  _________________________________________________________________________
157	 | Summary | finfo | binfo | ... | binfo | finfo | binfo | ... | binfo |...
158	 |_blocks__|___A___|_(A,1)_|_____|(A,Na)_|___B___|_(B,1)_|_____|(B,Nb)_|___
159	
160	
161	The logs include regular files, directory files, symbolic link files
162	and several meta data files.  The mata data files are the files used
163	to maintain file system meta data.  The current version of NILFS2 uses
164	the following meta data files:
165	
166	 1) Inode file (ifile)             -- Stores on-disk inodes
167	 2) Checkpoint file (cpfile)       -- Stores checkpoints
168	 3) Segment usage file (sufile)    -- Stores allocation state of segments
169	 4) Data address translation file  -- Maps virtual block numbers to usual
170	    (DAT)                             block numbers.  This file serves to
171	                                      make on-disk blocks relocatable.
172	
173	The following figure shows a typical organization of the logs:
174	
175	  _________________________________________________________________________
176	 | Summary | regular file | file  | ... | ifile | cpfile | sufile | DAT |SR|
177	 |_blocks__|_or_directory_|_______|_____|_______|________|________|_____|__|
178	
179	
180	To stride over segment boundaries, this sequence of files may be split
181	into multiple logs.  The sequence of logs that should be treated as
182	logically one log, is delimited with flags marked in the segment
183	summary.  The recovery code of nilfs2 looks this boundary information
184	to ensure atomicity of updates.
185	
186	The super root block is inserted for every checkpoints.  It includes
187	three special inodes, inodes for the DAT, cpfile, and sufile.  Inodes
188	of regular files, directories, symlinks and other special files, are
189	included in the ifile.  The inode of ifile itself is included in the
190	corresponding checkpoint entry in the cpfile.  Thus, the hierarchy
191	among NILFS2 files can be depicted as follows:
192	
193	  Super block (SB)
194	       |
195	       v
196	  Super root block (the latest cno=xx)
197	       |-- DAT
198	       |-- sufile
199	       `-- cpfile
200	              |-- ifile (cno=c1)
201	              |-- ifile (cno=c2) ---- file (ino=i1)
202	              :        :          |-- file (ino=i2)
203	              `-- ifile (cno=xx)  |-- file (ino=i3)
204	                                  :        :
205	                                  `-- file (ino=yy)
206	                                    ( regular file, directory, or symlink )
207	
208	For detail on the format of each file, please see include/linux/nilfs2_fs.h.
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