LogStructured

Logstructured refers to a design principle in which storage systems treat data as an append-only log rather than performing in-place updates. In a log-structured approach, new writes are appended to an ever-growing sequence of log records. Reads use an index to locate the current position of data within the log, and updates or deletions are implemented by writing new records (and, if necessary, tombstones) and by reclaiming space through a separate cleaning or compaction process.

History and design principles

Log-structured designs were proposed to optimize write performance and crash resilience, particularly on devices with slow random writes such as traditional spinning disks. Log-structured file systems (LFS) organize the disk as a sequence of segments; all new data is appended to the active segment, and a background cleaner reclaims space by discarding or rewriting obsolete data. The approach can improve sequential write throughput and simplify recovery after crashes, but it introduces read amplification and the overhead of garbage collection.

Variants and applications

A prominent extension of the log-structured idea is the log-structured merge-tree (LSM-tree), used in many modern databases. In an LSM-tree, writes land in an in-memory structure and are flushed to disk as immutable, sorted runs. Over time, runs are merged or compacted into larger levels to maintain order and control space. This yields high write throughput and scalable performance, at the cost of more complex reads and periodic compaction overhead.

Usage and considerations

Log-structured methods are found in file systems, databases, and storage engines. Benefits include high write throughput and straightforward crash recovery in append-only writes. Trade-offs involve read amplification, garbage collection or compaction costs, and space management. With the rise of solid-state drives and large-memory architectures, log-structured variants are continually adapted to balance throughput, latency, and resource usage. See also log-structured file system and log-structured merge-tree.