NILFS or NILFS2 (New Implementation of a Log-structured File System) is a log-structured file system implementation for the Linux kernel. It was 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).

"NILFS is a log-structured file system, in that the storage medium is treated like a circular buffer and new blocks are always written to the end.[…]Log-structured file systems 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 file system snapshots. The NILFS developers talk about the creation of "continuous snapshots" which can be used to recover from user-initiated file system problems[…]."[2]

Using a copy-on-write technique known as a log-structured file system, 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.