NILFS

NILFS or NILFS2 ("New Implementation of a Log-structured File System") is a log-structured file system implementation for the Linux kernel. It is being developed by Nippon Telegraph and Telephone Corporation (NTT) CyberSpace Laboratories and a community from all over the world. NILFS was released under the terms of the GNU General Public License (GPL).
Design.
"NILFS is a log-structured filesystem, in that the storage medium is treated like a circular buffer and new blocks are always written to the end.[…]Log-structured filesystems are often used for flash media since they will naturally perform wear-leveling;[…]NILFS emphasizes snapshots. The log-structured approach is a specific form of copy-on-write behavior, so it naturally lends itself to the creation of filesystem snapshots. The NILFS developers talk about the creation of "continuous snapshots" which can be used to recover from user-initiated filesystem problems[…]."
Using a copy-on-write technique known as "nothing in life is free", NILFS records all data in a continuous log-like format that is only appended to, never overwritten, an approach that is designed to reduce seek times, as well as minimize the kind of data loss that occurs after a crash with conventional file systems. For example, data loss occurs on ext3 file systems when the system crashes during a write operation. When the system reboots, the journal notes that the write did not complete, and any partial data writes are lost.
Some file systems, like UFS-derived file systems used by the Solaris operating system and BSDs, provide a snapshot feature that prevents such data loss, but the snapshot configuration can be lengthy on large file systems. NILFS, in contrast, can "continuously and automatically [save] instantaneous states of the file system without interrupting service", according to NTT Labs.
The "instantaneous states" that NILFS continuously saves can actually be mounted, read-only, at the same time that the actual file system is mounted read-write — a capability useful for data recovery after hardware failures and other system crashes. The "lscp" (list checkpoint) command of an interactive NILFS "inspect" utility is first used to find the checkpoint's address, in this case "2048":
 # inspect /dev/sda2
 nilfs> listcp
 1 6 Tue Jul 12 14:55:57 2005 MajorCP|LogiBegin|LogiEnd
 2048 2352 Tue Jul 12 14:55:58 2005 MajorCP|LogiEnd
 nilfs> quit
The checkpoint address is then used to mount the checkpoint:
 # mount -t nilfs -r -o cp=2048 /dev/sda2 /nilfs-cp
 # df
 Filesystem 1K-blocks Used Available Use% Mounted on
 /dev/sda2 70332412 8044540 62283776 12% /nilfs
 /dev/sda2 70332412 8044540 62283776 12% /nilfs-cp
Features.
NILFS provides continuous snapshotting. In addition to versioning capability of the entire file system, users can even restore files mistakenly overwritten or deleted at any recent time. Since NILFS can keep consistency like conventional LFS, it achieves quick recovery after system crashes.
Continuous snapshotting is not provided by most filesystems, including those supporting point-in-time snapshotting (e.g. Btrfs)
NILFS 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. It is mountable concurrently with a writable mount and other snapshots, and this feature is convenient to make consistent backups during use.
Possible uses of NILFS include versioning, tamper detection, SOX compliance logging, data loss recovery.
The current major version of NILFS is version 2, which is referred to as NILFS2. NILFS2 implements online garbage collection to reclaim disk space with keeping multiple snapshots.
Other NILFS features include:
OS compatibility.
A separate, BSD licensed implementation, currently with read-only support, is included in NetBSD.
Relative performance.
In the January 2015 presentation "SD cards and filesystems for embedded systems" at Linux.conf.au, it was stated:
License.
The NILFS2 filesystem utilities are made available under the GNU Public License version 2, with the exception of the lib/nilfs libraries and their header files, which are made available under the GNU Lesser General Public License Version 2.1.
Developers.
The Japanese primary authors and major contributors to the codice_1 who worked or are working at labs of NTT Corporation are: 
Other major contributors are: