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Documentation / filesystems / nilfs2.txt




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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:03 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://nilfs.sourceforge.net/
29	Download page:       http://nilfs.sourceforge.net/en/download.html
30	List info:           http://vger.kernel.org/vger-lists.html#linux-nilfs
31	
32	Caveats
33	=======
34	
35	Features which NILFS2 does not support yet:
36	
37		- atime
38		- extended attributes
39		- POSIX ACLs
40		- quotas
41		- fsck
42		- defragmentation
43	
44	Mount options
45	=============
46	
47	NILFS2 supports the following mount options:
48	(*) == default
49	
50	barrier(*)		This enables/disables the use of write barriers.  This
51	nobarrier		requires an IO stack which can support barriers, and
52				if nilfs gets an error on a barrier write, it will
53				disable again with a warning.
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	norecovery		Disable recovery of the filesystem on mount.
75				This disables every write access on the device for
76				read-only mounts or snapshots.  This option will fail
77				for r/w mounts on an unclean volume.
78	discard			This enables/disables the use of discard/TRIM commands.
79	nodiscard(*)		The discard/TRIM commands are sent to the underlying
80				block device when blocks are freed.  This is useful
81				for SSD devices and sparse/thinly-provisioned LUNs.
82	
83	Ioctls
84	======
85	
86	There is some NILFS2 specific functionality which can be accessed by applications
87	through the system call interfaces. The list of all NILFS2 specific ioctls are
88	shown in the table below.
89	
90	Table of NILFS2 specific ioctls
91	..............................................................................
92	 Ioctl			        Description
93	 NILFS_IOCTL_CHANGE_CPMODE      Change mode of given checkpoint between
94				        checkpoint and snapshot state. This ioctl is
95				        used in chcp and mkcp utilities.
96	
97	 NILFS_IOCTL_DELETE_CHECKPOINT  Remove checkpoint from NILFS2 file system.
98				        This ioctl is used in rmcp utility.
99	
100	 NILFS_IOCTL_GET_CPINFO         Return info about requested checkpoints. This
101				        ioctl is used in lscp utility and by
102				        nilfs_cleanerd daemon.
103	
104	 NILFS_IOCTL_GET_CPSTAT         Return checkpoints statistics. This ioctl is
105				        used by lscp, rmcp utilities and by
106				        nilfs_cleanerd daemon.
107	
108	 NILFS_IOCTL_GET_SUINFO         Return segment usage info about requested
109				        segments. This ioctl is used in lssu,
110				        nilfs_resize utilities and by nilfs_cleanerd
111				        daemon.
112	
113	 NILFS_IOCTL_SET_SUINFO         Modify segment usage info of requested
114					segments. This ioctl is used by
115					nilfs_cleanerd daemon to skip unnecessary
116					cleaning operation of segments and reduce
117					performance penalty or wear of flash device
118					due to redundant move of in-use blocks.
119	
120	 NILFS_IOCTL_GET_SUSTAT         Return segment usage statistics. This ioctl
121				        is used in lssu, nilfs_resize utilities and
122				        by nilfs_cleanerd daemon.
123	
124	 NILFS_IOCTL_GET_VINFO          Return information on virtual block addresses.
125				        This ioctl is used by nilfs_cleanerd daemon.
126	
127	 NILFS_IOCTL_GET_BDESCS         Return information about descriptors of disk
128				        block numbers. This ioctl is used by
129				        nilfs_cleanerd daemon.
130	
131	 NILFS_IOCTL_CLEAN_SEGMENTS     Do garbage collection operation in the
132				        environment of requested parameters from
133				        userspace. This ioctl is used by
134				        nilfs_cleanerd daemon.
135	
136	 NILFS_IOCTL_SYNC               Make a checkpoint. This ioctl is used in
137				        mkcp utility.
138	
139	 NILFS_IOCTL_RESIZE             Resize NILFS2 volume. This ioctl is used
140				        by nilfs_resize utility.
141	
142	 NILFS_IOCTL_SET_ALLOC_RANGE    Define lower limit of segments in bytes and
143				        upper limit of segments in bytes. This ioctl
144				        is used by nilfs_resize utility.
145	
146	NILFS2 usage
147	============
148	
149	To use nilfs2 as a local file system, simply:
150	
151	 # mkfs -t nilfs2 /dev/block_device
152	 # mount -t nilfs2 /dev/block_device /dir
153	
154	This will also invoke the cleaner through the mount helper program
155	(mount.nilfs2).
156	
157	Checkpoints and snapshots are managed by the following commands.
158	Their manpages are included in the nilfs-utils package above.
159	
160	  lscp     list checkpoints or snapshots.
161	  mkcp     make a checkpoint or a snapshot.
162	  chcp     change an existing checkpoint to a snapshot or vice versa.
163	  rmcp     invalidate specified checkpoint(s).
164	
165	To mount a snapshot,
166	
167	 # mount -t nilfs2 -r -o cp=<cno> /dev/block_device /snap_dir
168	
169	where <cno> is the checkpoint number of the snapshot.
170	
171	To unmount the NILFS2 mount point or snapshot, simply:
172	
173	 # umount /dir
174	
175	Then, the cleaner daemon is automatically shut down by the umount
176	helper program (umount.nilfs2).
177	
178	Disk format
179	===========
180	
181	A nilfs2 volume is equally divided into a number of segments except
182	for the super block (SB) and segment #0.  A segment is the container
183	of logs.  Each log is composed of summary information blocks, payload
184	blocks, and an optional super root block (SR):
185	
186	   ______________________________________________________
187	  | |SB| | Segment | Segment | Segment | ... | Segment | |
188	  |_|__|_|____0____|____1____|____2____|_____|____N____|_|
189	  0 +1K +4K       +8M       +16M      +24M  +(8MB x N)
190	       .             .            (Typical offsets for 4KB-block)
191	    .                  .
192	  .______________________.
193	  | log | log |... | log |
194	  |__1__|__2__|____|__m__|
195	        .       .
196	      .               .
197	    .                       .
198	  .______________________________.
199	  | Summary | Payload blocks  |SR|
200	  |_blocks__|_________________|__|
201	
202	The payload blocks are organized per file, and each file consists of
203	data blocks and B-tree node blocks:
204	
205	    |<---       File-A        --->|<---       File-B        --->|
206	   _______________________________________________________________
207	    | Data blocks | B-tree blocks | Data blocks | B-tree blocks | ...
208	   _|_____________|_______________|_____________|_______________|_
209	
210	
211	Since only the modified blocks are written in the log, it may have
212	files without data blocks or B-tree node blocks.
213	
214	The organization of the blocks is recorded in the summary information
215	blocks, which contains a header structure (nilfs_segment_summary), per
216	file structures (nilfs_finfo), and per block structures (nilfs_binfo):
217	
218	  _________________________________________________________________________
219	 | Summary | finfo | binfo | ... | binfo | finfo | binfo | ... | binfo |...
220	 |_blocks__|___A___|_(A,1)_|_____|(A,Na)_|___B___|_(B,1)_|_____|(B,Nb)_|___
221	
222	
223	The logs include regular files, directory files, symbolic link files
224	and several meta data files.  The mata data files are the files used
225	to maintain file system meta data.  The current version of NILFS2 uses
226	the following meta data files:
227	
228	 1) Inode file (ifile)             -- Stores on-disk inodes
229	 2) Checkpoint file (cpfile)       -- Stores checkpoints
230	 3) Segment usage file (sufile)    -- Stores allocation state of segments
231	 4) Data address translation file  -- Maps virtual block numbers to usual
232	    (DAT)                             block numbers.  This file serves to
233	                                      make on-disk blocks relocatable.
234	
235	The following figure shows a typical organization of the logs:
236	
237	  _________________________________________________________________________
238	 | Summary | regular file | file  | ... | ifile | cpfile | sufile | DAT |SR|
239	 |_blocks__|_or_directory_|_______|_____|_______|________|________|_____|__|
240	
241	
242	To stride over segment boundaries, this sequence of files may be split
243	into multiple logs.  The sequence of logs that should be treated as
244	logically one log, is delimited with flags marked in the segment
245	summary.  The recovery code of nilfs2 looks this boundary information
246	to ensure atomicity of updates.
247	
248	The super root block is inserted for every checkpoints.  It includes
249	three special inodes, inodes for the DAT, cpfile, and sufile.  Inodes
250	of regular files, directories, symlinks and other special files, are
251	included in the ifile.  The inode of ifile itself is included in the
252	corresponding checkpoint entry in the cpfile.  Thus, the hierarchy
253	among NILFS2 files can be depicted as follows:
254	
255	  Super block (SB)
256	       |
257	       v
258	  Super root block (the latest cno=xx)
259	       |-- DAT
260	       |-- sufile
261	       `-- cpfile
262	              |-- ifile (cno=c1)
263	              |-- ifile (cno=c2) ---- file (ino=i1)
264	              :        :          |-- file (ino=i2)
265	              `-- ifile (cno=xx)  |-- file (ino=i3)
266	                                  :        :
267	                                  `-- file (ino=yy)
268	                                    ( regular file, directory, or symlink )
269	
270	For detail on the format of each file, please see include/linux/nilfs2_fs.h.
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