Documentation / filesystems / nilfs2.rst


Based on kernel version 5.10.1. Page generated on 2020-12-14 21:14 EST.

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.. SPDX-License-Identifier: GPL-2.0

======
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:    https://nilfs.sourceforge.io/
:Download page:       https://nilfs.sourceforge.io/en/download.html
: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 nilfs2_ondisk.h
located at include/uapi/linux directory.

There are no patents or other intellectual property that we protect
with regard to the design of NILFS2.  It is allowed to replicate the
design in hopes that other operating systems could share (mount, read,
write, etc.) data stored in this format.