Documentation/filesystems/nilfs2.rst

Source file repositories/reference/linux-study-clean/Documentation/filesystems/nilfs2.rst

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Documentation/filesystems/nilfs2.rst
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Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.

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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

Annotation

Implementation Notes