[deliverable/linux.git] / Documentation / filesystems / nilfs2.txt

NILFS2
------

NILFS2 is a log-structured file system (LFS) supporting continuous
snapshotting.  In addition to versioning capability of the entire file
system, users can even restore files mistakenly overwritten or
destroyed just a few seconds ago.  Since NILFS2 can keep consistency
like conventional LFS, it achieves quick recovery after system
crashes.

NILFS2 creates a number of checkpoints every few seconds or per
synchronous write basis (unless there is no change).  Users can select
significant versions among continuously created checkpoints, and can
change them into snapshots which will be preserved until they are
changed back to checkpoints.

There is no limit on the number of snapshots until the volume gets
full.  Each snapshot is mountable as a read-only file system
concurrently with its writable mount, and this feature is convenient
for online backup.

The userland tools are included in nilfs-utils package, which is
available from the following download page.  At least "mkfs.nilfs2",
"mount.nilfs2", "umount.nilfs2", and "nilfs_cleanerd" (so called
cleaner or garbage collector) are required.  Details on the tools are
described in the man pages included in the package.

Project web page:    http://www.nilfs.org/en/
Download page:       http://www.nilfs.org/en/download.html
Git tree web page:   http://www.nilfs.org/git/
List info:           http://vger.kernel.org/vger-lists.html#linux-nilfs

Caveats
=======

Features which NILFS2 does not support yet:

	- atime
	- extended attributes
	- POSIX ACLs
	- quotas
	- fsck
	- defragmentation

Mount options
=============

NILFS2 supports the following mount options:
(*) == default

barrier(*)		This enables/disables the use of write barriers.  This
nobarrier		requires an IO stack which can support barriers, and
			if nilfs gets an error on a barrier write, it will
			disable again with a warning.
errors=continue		Keep going on a filesystem error.
errors=remount-ro(*)	Remount the filesystem read-only on an error.
errors=panic		Panic and halt the machine if an error occurs.
cp=n			Specify the checkpoint-number of the snapshot to be
			mounted.  Checkpoints and snapshots are listed by lscp
			user command.  Only the checkpoints marked as snapshot
			are mountable with this option.  Snapshot is read-only,
			so a read-only mount option must be specified together.
order=relaxed(*)	Apply relaxed order semantics that allows modified data
			blocks to be written to disk without making a
			checkpoint if no metadata update is going.  This mode
			is equivalent to the ordered data mode of the ext3
			filesystem except for the updates on data blocks still
			conserve atomicity.  This will improve synchronous
			write performance for overwriting.
order=strict		Apply strict in-order semantics that preserves sequence
			of all file operations including overwriting of data
			blocks.  That means, it is guaranteed that no
			overtaking of events occurs in the recovered file
			system after a crash.
norecovery		Disable recovery of the filesystem on mount.
			This disables every write access on the device for
			read-only mounts or snapshots.  This option will fail
			for r/w mounts on an unclean volume.
discard			This enables/disables the use of discard/TRIM commands.
nodiscard(*)		The discard/TRIM commands are sent to the underlying
			block device when blocks are freed.  This is useful
			for SSD devices and sparse/thinly-provisioned LUNs.

Ioctls
======

There is some NILFS2 specific functionality which can be accessed by applications
through the system call interfaces. The list of all NILFS2 specific ioctls are
shown in the table below.

Table of NILFS2 specific ioctls
..............................................................................
 Ioctl			        Description
 NILFS_IOCTL_CHANGE_CPMODE      Change mode of given checkpoint between
			        checkpoint and snapshot state. This ioctl is
			        used in chcp and mkcp utilities.

 NILFS_IOCTL_DELETE_CHECKPOINT  Remove checkpoint from NILFS2 file system.
			        This ioctl is used in rmcp utility.

 NILFS_IOCTL_GET_CPINFO         Return info about requested checkpoints. This
			        ioctl is used in lscp utility and by
			        nilfs_cleanerd daemon.

 NILFS_IOCTL_GET_CPSTAT         Return checkpoints statistics. This ioctl is
			        used by lscp, rmcp utilities and by
			        nilfs_cleanerd daemon.

 NILFS_IOCTL_GET_SUINFO         Return segment usage info about requested
			        segments. This ioctl is used in lssu,
			        nilfs_resize utilities and by nilfs_cleanerd
			        daemon.

 NILFS_IOCTL_SET_SUINFO         Modify segment usage info of requested
				segments. This ioctl is used by
				nilfs_cleanerd daemon to skip unnecessary
				cleaning operation of segments and reduce
				performance penalty or wear of flash device
				due to redundant move of in-use blocks.

 NILFS_IOCTL_GET_SUSTAT         Return segment usage statistics. This ioctl
			        is used in lssu, nilfs_resize utilities and
			        by nilfs_cleanerd daemon.

 NILFS_IOCTL_GET_VINFO          Return information on virtual block addresses.
			        This ioctl is used by nilfs_cleanerd daemon.

 NILFS_IOCTL_GET_BDESCS         Return information about descriptors of disk
			        block numbers. This ioctl is used by
			        nilfs_cleanerd daemon.

 NILFS_IOCTL_CLEAN_SEGMENTS     Do garbage collection operation in the
			        environment of requested parameters from
			        userspace. This ioctl is used by
			        nilfs_cleanerd daemon.

 NILFS_IOCTL_SYNC               Make a checkpoint. This ioctl is used in
			        mkcp utility.

 NILFS_IOCTL_RESIZE             Resize NILFS2 volume. This ioctl is used
			        by nilfs_resize utility.

 NILFS_IOCTL_SET_ALLOC_RANGE    Define lower limit of segments in bytes and
			        upper limit of segments in bytes. This ioctl
			        is used by nilfs_resize utility.

NILFS2 usage
============

To use nilfs2 as a local file system, simply:

 # mkfs -t nilfs2 /dev/block_device
 # mount -t nilfs2 /dev/block_device /dir

This will also invoke the cleaner through the mount helper program
(mount.nilfs2).

Checkpoints and snapshots are managed by the following commands.
Their manpages are included in the nilfs-utils package above.

  lscp     list checkpoints or snapshots.
  mkcp     make a checkpoint or a snapshot.
  chcp     change an existing checkpoint to a snapshot or vice versa.
  rmcp     invalidate specified checkpoint(s).

To mount a snapshot,

 # mount -t nilfs2 -r -o cp=<cno> /dev/block_device /snap_dir

where <cno> is the checkpoint number of the snapshot.

To unmount the NILFS2 mount point or snapshot, simply:

 # umount /dir

Then, the cleaner daemon is automatically shut down by the umount
helper program (umount.nilfs2).

Disk format
===========

A nilfs2 volume is equally divided into a number of segments except
for the super block (SB) and segment #0.  A segment is the container
of logs.  Each log is composed of summary information blocks, payload
blocks, and an optional super root block (SR):

   ______________________________________________________
  | |SB| | Segment | Segment | Segment | ... | Segment | |
  |_|__|_|____0____|____1____|____2____|_____|____N____|_|
  0 +1K +4K       +8M       +16M      +24M  +(8MB x N)
       .             .            (Typical offsets for 4KB-block)
    .                  .
  .______________________.
  | log | log |... | log |
  |__1__|__2__|____|__m__|
        .       .
      .               .
    .                       .
  .______________________________.
  | Summary | Payload blocks  |SR|
  |_blocks__|_________________|__|

The payload blocks are organized per file, and each file consists of
data blocks and B-tree node blocks:

    |<---       File-A        --->|<---       File-B        --->|
   _______________________________________________________________
    | Data blocks | B-tree blocks | Data blocks | B-tree blocks | ...
   _|_____________|_______________|_____________|_______________|_


Since only the modified blocks are written in the log, it may have
files without data blocks or B-tree node blocks.

The organization of the blocks is recorded in the summary information
blocks, which contains a header structure (nilfs_segment_summary), per
file structures (nilfs_finfo), and per block structures (nilfs_binfo):

  _________________________________________________________________________
 | Summary | finfo | binfo | ... | binfo | finfo | binfo | ... | binfo |...
 |_blocks__|___A___|_(A,1)_|_____|(A,Na)_|___B___|_(B,1)_|_____|(B,Nb)_|___


The logs include regular files, directory files, symbolic link files
and several meta data files.  The mata data files are the files used
to maintain file system meta data.  The current version of NILFS2 uses
the following meta data files:

 1) Inode file (ifile)             -- Stores on-disk inodes
 2) Checkpoint file (cpfile)       -- Stores checkpoints
 3) Segment usage file (sufile)    -- Stores allocation state of segments
 4) Data address translation file  -- Maps virtual block numbers to usual
    (DAT)                             block numbers.  This file serves to
                                      make on-disk blocks relocatable.

The following figure shows a typical organization of the logs:

  _________________________________________________________________________
 | Summary | regular file | file  | ... | ifile | cpfile | sufile | DAT |SR|
 |_blocks__|_or_directory_|_______|_____|_______|________|________|_____|__|


To stride over segment boundaries, this sequence of files may be split
into multiple logs.  The sequence of logs that should be treated as
logically one log, is delimited with flags marked in the segment
summary.  The recovery code of nilfs2 looks this boundary information
to ensure atomicity of updates.

The super root block is inserted for every checkpoints.  It includes
three special inodes, inodes for the DAT, cpfile, and sufile.  Inodes
of regular files, directories, symlinks and other special files, are
included in the ifile.  The inode of ifile itself is included in the
corresponding checkpoint entry in the cpfile.  Thus, the hierarchy
among NILFS2 files can be depicted as follows:

  Super block (SB)
       |
       v
  Super root block (the latest cno=xx)
       |-- DAT
       |-- sufile
       `-- cpfile
              |-- ifile (cno=c1)
              |-- ifile (cno=c2) ---- file (ino=i1)
              :        :          |-- file (ino=i2)
              `-- ifile (cno=xx)  |-- file (ino=i3)
                                  :        :
                                  `-- file (ino=yy)
                                    ( regular file, directory, or symlink )

For detail on the format of each file, please see include/linux/nilfs2_fs.h.
Commit	Line	Data
962281a7 RK	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/
6aff43f8	31	List info: http://vger.kernel.org/vger-lists.html#linux-nilfs
962281a7 RK	32
	33	Caveats
	34	=======
	35
	36	Features which NILFS2 does not support yet:
	37
	38	- atime
	39	- extended attributes
	40	- POSIX ACLs
	41	- quotas
fb6e7113	42	- fsck
962281a7 RK	43	- defragmentation
	44
	45	Mount options
	46	=============
	47
	48	NILFS2 supports the following mount options:
	49	(*) == default
	50
773bc4f3 RK	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.
277a6a34 RK	55	errors=continue Keep going on a filesystem error.
277a6a34 RK	56	errors=remount-ro(*) Remount the filesystem read-only on an error.
962281a7 RK	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.
0234576d RK	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.
802d3177 RK	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.
962281a7	83
d623a942 VD	84	Ioctls
	85	======
	86
	87	There is some NILFS2 specific functionality which can be accessed by applications
	88	through the system call interfaces. The list of all NILFS2 specific ioctls are
	89	shown in the table below.
	90
	91	Table of NILFS2 specific ioctls
	92	..............................................................................
	93	Ioctl Description
	94	NILFS_IOCTL_CHANGE_CPMODE Change mode of given checkpoint between
	95	checkpoint and snapshot state. This ioctl is
	96	used in chcp and mkcp utilities.
	97
	98	NILFS_IOCTL_DELETE_CHECKPOINT Remove checkpoint from NILFS2 file system.
	99	This ioctl is used in rmcp utility.
	100
	101	NILFS_IOCTL_GET_CPINFO Return info about requested checkpoints. This
	102	ioctl is used in lscp utility and by
	103	nilfs_cleanerd daemon.
	104
	105	NILFS_IOCTL_GET_CPSTAT Return checkpoints statistics. This ioctl is
	106	used by lscp, rmcp utilities and by
	107	nilfs_cleanerd daemon.
	108
	109	NILFS_IOCTL_GET_SUINFO Return segment usage info about requested
	110	segments. This ioctl is used in lssu,
	111	nilfs_resize utilities and by nilfs_cleanerd
	112	daemon.
	113
2cc88f3a AR	114	NILFS_IOCTL_SET_SUINFO Modify segment usage info of requested
	115	segments. This ioctl is used by
	116	nilfs_cleanerd daemon to skip unnecessary
	117	cleaning operation of segments and reduce
	118	performance penalty or wear of flash device
	119	due to redundant move of in-use blocks.
	120
d623a942 VD	121	NILFS_IOCTL_GET_SUSTAT Return segment usage statistics. This ioctl
	122	is used in lssu, nilfs_resize utilities and
	123	by nilfs_cleanerd daemon.
	124
	125	NILFS_IOCTL_GET_VINFO Return information on virtual block addresses.
	126	This ioctl is used by nilfs_cleanerd daemon.
	127
	128	NILFS_IOCTL_GET_BDESCS Return information about descriptors of disk
	129	block numbers. This ioctl is used by
	130	nilfs_cleanerd daemon.
	131
	132	NILFS_IOCTL_CLEAN_SEGMENTS Do garbage collection operation in the
	133	environment of requested parameters from
	134	userspace. This ioctl is used by
	135	nilfs_cleanerd daemon.
	136
	137	NILFS_IOCTL_SYNC Make a checkpoint. This ioctl is used in
	138	mkcp utility.
	139
	140	NILFS_IOCTL_RESIZE Resize NILFS2 volume. This ioctl is used
	141	by nilfs_resize utility.
	142
	143	NILFS_IOCTL_SET_ALLOC_RANGE Define lower limit of segments in bytes and
	144	upper limit of segments in bytes. This ioctl
	145	is used by nilfs_resize utility.
	146
962281a7 RK	147	NILFS2 usage
	148	============
	149
	150	To use nilfs2 as a local file system, simply:
	151
	152	# mkfs -t nilfs2 /dev/block_device
	153	# mount -t nilfs2 /dev/block_device /dir
	154
	155	This will also invoke the cleaner through the mount helper program
	156	(mount.nilfs2).
	157
	158	Checkpoints and snapshots are managed by the following commands.
	159	Their manpages are included in the nilfs-utils package above.
	160
	161	lscp list checkpoints or snapshots.
	162	mkcp make a checkpoint or a snapshot.
	163	chcp change an existing checkpoint to a snapshot or vice versa.
	164	rmcp invalidate specified checkpoint(s).
	165
	166	To mount a snapshot,
	167
	168	# mount -t nilfs2 -r -o cp=<cno> /dev/block_device /snap_dir
	169
	170	where <cno> is the checkpoint number of the snapshot.
	171
	172	To unmount the NILFS2 mount point or snapshot, simply:
	173
	174	# umount /dir
	175
	176	Then, the cleaner daemon is automatically shut down by the umount
	177	helper program (umount.nilfs2).
	178
	179	Disk format
	180	===========
	181
	182	A nilfs2 volume is equally divided into a number of segments except
	183	for the super block (SB) and segment #0. A segment is the container
	184	of logs. Each log is composed of summary information blocks, payload
	185	blocks, and an optional super root block (SR):
	186
	187	______________________________________________________
	188	\| \|SB\| \| Segment \| Segment \| Segment \| ... \| Segment \| \|
	189	\|_\|__\|_\|____0____\|____1____\|____2____\|_____\|____N____\|_\|
	190	0 +1K +4K +8M +16M +24M +(8MB x N)
	191	. . (Typical offsets for 4KB-block)
	192	. .
	193	.______________________.
	194	\| log \| log \|... \| log \|
	195	\|__1__\|__2__\|____\|__m__\|
	196	. .
	197	. .
	198	. .
	199	.______________________________.
	200	\| Summary \| Payload blocks \|SR\|
	201	\|_blocks__\|_________________\|__\|
	202
	203	The payload blocks are organized per file, and each file consists of
	204	data blocks and B-tree node blocks:
	205
	206	\|<--- File-A --->\|<--- File-B --->\|
	207	_______________________________________________________________
	208	\| Data blocks \| B-tree blocks \| Data blocks \| B-tree blocks \| ...
	209	_\|_____________\|_______________\|_____________\|_______________\|_
	210
211
212	Since only the modified blocks are written in the log, it may have
213	files without data blocks or B-tree node blocks.
214
215	The organization of the blocks is recorded in the summary information
216	blocks, which contains a header structure (nilfs_segment_summary), per
217	file structures (nilfs_finfo), and per block structures (nilfs_binfo):
218
219	_________________________________________________________________________
220	\| Summary \| finfo \| binfo \| ... \| binfo \| finfo \| binfo \| ... \| binfo \|...
221	\|_blocks__\|___A___\|_(A,1)_\|_____\|(A,Na)_\|___B___\|_(B,1)_\|_____\|(B,Nb)_\|___
222
223
224	The logs include regular files, directory files, symbolic link files
225	and several meta data files. The mata data files are the files used
226	to maintain file system meta data. The current version of NILFS2 uses
227	the following meta data files:
228
229	1) Inode file (ifile) -- Stores on-disk inodes
230	2) Checkpoint file (cpfile) -- Stores checkpoints
231	3) Segment usage file (sufile) -- Stores allocation state of segments
232	4) Data address translation file -- Maps virtual block numbers to usual
233	(DAT) block numbers. This file serves to
234	make on-disk blocks relocatable.
962281a7 RK	235
	236	The following figure shows a typical organization of the logs:
	237
	238	_________________________________________________________________________
	239	\| Summary \| regular file \| file \| ... \| ifile \| cpfile \| sufile \| DAT \|SR\|
	240	\|_blocks__\|_or_directory_\|_______\|_____\|_______\|________\|________\|_____\|__\|
	241
	242
	243	To stride over segment boundaries, this sequence of files may be split
	244	into multiple logs. The sequence of logs that should be treated as
	245	logically one log, is delimited with flags marked in the segment
	246	summary. The recovery code of nilfs2 looks this boundary information
	247	to ensure atomicity of updates.
	248
	249	The super root block is inserted for every checkpoints. It includes
	250	three special inodes, inodes for the DAT, cpfile, and sufile. Inodes
	251	of regular files, directories, symlinks and other special files, are
	252	included in the ifile. The inode of ifile itself is included in the
	253	corresponding checkpoint entry in the cpfile. Thus, the hierarchy
	254	among NILFS2 files can be depicted as follows:
	255
	256	Super block (SB)
	257	\|
	258	v
	259	Super root block (the latest cno=xx)
	260	\|-- DAT
	261	\|-- sufile
	262	`-- cpfile
	263	\|-- ifile (cno=c1)
	264	\|-- ifile (cno=c2) ---- file (ino=i1)
	265	: : \|-- file (ino=i2)
	266	`-- ifile (cno=xx) \|-- file (ino=i3)
	267	: :
	268	`-- file (ino=yy)
	269	( regular file, directory, or symlink )
	270
	271	For detail on the format of each file, please see include/linux/nilfs2_fs.h.