Written by Christian Ahmer | 11/08/2023


NILFS (New Implementation of a Log-structured File System) and its successor NILFS2 are log-structured file systems that were developed to provide continuous snapshotting capabilities, thereby ensuring data integrity and quick recovery following system crashes. NILFS was created at the NTT Cyber Space Laboratories, with its first release in 2005, and NILFS2, which introduced significant improvements, was merged into the Linux kernel in version 2.6.30.

Log-structured file systems such as NILFS differ markedly from traditional file systems in how they manage data writing to disk. Instead of overwriting data in place, NILFS writes all changes in a log-like format to a continuous append-only segment, a method that optimizes write performance, especially in environments with heavy write loads. This sequential writing approach reduces seek times on traditional spinning hard drives and is also beneficial for solid-state drives (SSD), which prefer sequential write patterns for longevity and performance.

NILFS implements a versioning system that takes snapshots of the file system at a particular point in time. These snapshots are created at regular intervals and on-demand, enabling users to access historical data quickly. This continuous snapshotting is not only useful for data protection and system restores but also for version control and archival purposes.

One of the standout features of NILFS2, in particular, is its garbage collection mechanism. Since the file system is always writing new data sequentially, space occupied by obsolete or deleted data needs to be reclaimed. The garbage collector in NILFS2 is designed to work in the background without disrupting normal operations, cleaning up blocks that are no longer needed and consolidating free space.

NILFS2 also offers near-instantaneous snapshot creation. The snapshots are immutable, which means they do not change once created, and this consistency is critical for reliable backups and restores. Because snapshots in NILFS2 are lightweight and do not duplicate data, they can be created frequently without a significant overhead.

Another advantage of NILFS2 is its resistance to data corruption. Traditional file systems that overwrite data in place can suffer from corruption due to partial writes caused by power failures or system crashes. In contrast, the write-once nature of NILFS2 ensures that either a write operation is fully completed or not done at all, leaving no window for partial overwrites and the resultant data corruption.

NILFS2 also supports continuous checksumming of data and metadata. Each block written includes a checksum, allowing the file system to detect and respond to data corruption caused by hardware issues, ensuring the integrity of the data stored on disk.

Regarding scalability, NILFS can handle large volumes and large files efficiently due to its design. It supports 64-bit timestamps, which addresses the year 2038 problem and allows for precise timestamps well into the future. NILFS2 also utilizes B-tree-like data structures for directories, improving the performance of operations on large directories.

The file system is managed through a user-space tool called 'nilfs_cleanerd', which handles garbage collection and snapshot management. It allows administrators to configure retention policies for snapshots, ensuring that the file system does not become overburdened with historical data.

Despite its advantages, NILFS is not widely adopted as a primary file system. This is partly due to the specialized nature of its advantages, which are most beneficial in scenarios that require robust versioning and snapshotting capabilities. It is also due to competition from other file systems that offer similar features, such as Btrfs, which has gained traction due to its broader feature set.

In conclusion, NILFS and NILFS2 represent significant advancements in log-structured file system technology, particularly in the area of continuous snapshotting and data integrity. Their design philosophies reflect a forward-thinking approach to file system development, anticipating the need for robust data protection and quick recovery in increasingly complex computing environments.