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Log-structured file system

A log-structured file system (LFS) is a file system architecture that appends all modifications to files and metadata sequentially to a single log on disk, rather than performing in-place updates, thereby treating the disk as an append-only circular buffer of ordered records. This design leverages sequential writes to boost performance on rotating disks and flash storage, while enabling atomic operations and simplified recovery from crashes by replaying the log from the most recent checkpoint.^[1] The concept was pioneered in 1991 by Mendel Rosenblum and John K. Ousterhout at the University of California, Berkeley, as part of their work on the Sprite operating system, with the seminal paper published in 1992 detailing its design and prototype implementation.^[1] The Sprite LFS prototype achieved write throughputs up to an order of magnitude higher than contemporary Unix file systems for small-file workloads, utilizing up to 70% of available disk bandwidth compared to 5-10% for traditional systems.^[1] Key mechanisms include dividing the log into fixed-size segments (typically 1 MB), maintaining an in-memory or disk-based inode map to track current file locations within the log, and employing a background segment cleaner to garbage-collect obsolete data by copying live blocks to new segments and erasing old ones.^[1]^[2] LFS offers several advantages, particularly for write-intensive applications: sequential appending eliminates random seeks, enabling high write bandwidth; inherent journaling ensures crash consistency without separate recovery logs; and it avoids free-space fragmentation by reusing cleaned segments.^[1]^[3] However, drawbacks include potential read amplification from scattered file fragments (mitigated by caching but worsening on cache misses), increased overhead from segment cleaning as the disk fills (potentially halving performance above 50% utilization), and higher complexity in managing the cleaner to minimize write amplification.^[2]^[4] These trade-offs make LFS especially suitable for environments with large caches and sequential access patterns, but less ideal for random-read-heavy workloads on mechanical disks.^[3] Notable implementations include the original Sprite LFS, a 1993 port to Unix by Margo Seltzer and colleagues that integrated with the vnode interface for broader compatibility and robustness, and specialized variants for flash memory such as JFFS2 (developed for embedded Linux systems).^[1] In modern systems, pure or hybrid LFS designs persist in commercial storage like NetApp's WAFL, Oracle's ZFS (via copy-on-write logging), and Linux's Btrfs and NILFS2, while nearly all solid-state drives (SSDs) employ LFS-like flash translation layers (FTL) for wear leveling and write optimization.^[5]^[4] Recent research continues to refine LFS for emerging storage, such as garbage-collection-free variants to reduce overhead on high-capacity SSDs.^[6]

History and Development

Origins and Invention

The log-structured file system (LFS) was invented by Mendel Rosenblum and John K. Ousterhout, along with colleagues in the Sprite operating system project at the University of California, Berkeley.^[1] Developed in the late 1980s and implemented by mid-1990, it emerged as part of the broader Sprite research effort to create an efficient network operating system for distributed computing environments.^[1] The Sprite LFS prototype was designed specifically to address inefficiencies in traditional file systems, particularly for workloads involving frequent small writes, which were common in Unix-like systems of the era.^[1] The invention was motivated by the growing disparity between rapidly advancing CPU and memory speeds and the relatively stagnant performance of disk storage during the 1980s.^[1] Disk access times improved only modestly compared to the exponential gains in processor performance, while transfer rates for sequential operations began outpacing random access capabilities due to evolving magnetic disk technologies.^[1] These trends highlighted the limitations of conventional file systems like the Berkeley Fast File System (FFS), which suffered from high seek times and fragmentation when handling small, random writes—operations that dominated many real-world workloads.^[1] By the late 1980s, large file caches in memory further shifted disk I/O toward write-dominated patterns, amplifying the need for a storage approach optimized for sequential throughput.^[1] The core innovation of LFS lay in treating the entire disk as a sequential log for all modifications, thereby minimizing seek overhead and leveraging the full bandwidth of emerging disk drives.^[1] This concept was first detailed in the seminal paper "The Design and Implementation of a Log-Structured File System," presented by Rosenblum and Ousterhout at the 13th ACM Symposium on Operating Systems Principles (SOSP) in 1991.^[1] The publication outlined the Sprite LFS prototype's goals of dramatically improving write performance for small files while simplifying crash recovery through log-based structures.^[1] As the authors noted, this sequential writing paradigm allowed LFS to achieve near-optimal disk utilization, marking a foundational shift in file system design.^[1]

Evolution and Key Publications

Following the introduction of the Sprite Log-structured File System (LFS) in 1991, subsequent research in the 1990s focused on practical implementations and optimizations to address cleaning overheads and integration challenges. In 1993, Margo Seltzer and colleagues developed an LFS prototype for Unix systems, demonstrating improved write throughput and crash recovery while highlighting the need for efficient segment cleaning to mitigate performance degradation from live data relocation.^[7] This work built on the original cleaning policy by introducing heuristic approaches that prioritized segments based on utilization and age, reducing cleaning costs in workloads with high update rates.^[8] Concurrently, early Linux experiments, such as the Linux LFS project initiated in the late 1990s, explored porting LFS concepts to open-source environments, though full implementations like LinLogFS emerged in 2000, emphasizing fast recovery and ordered writes.^[9]^[10] The 2000s saw LFS principles extend to specialized storage, particularly for flash memory and high-availability systems. Matthew Dillon's HAMMER file system, first prototyped in 2005 and detailed in a 2008 paper, incorporated LFS-style logging with B+ trees for metadata, enabling features like snapshots and history tracking in DragonFly BSD, which improved reliability for large-scale storage up to exabytes.^[11] For flash-optimized variants, systems like JFFS2 (2001) and YAFFS (2002) adapted log-structured writing to NAND flash constraints, using sequential appends to minimize erase cycles and support wear leveling by distributing writes evenly across blocks. These adaptations addressed flash's out-of-place update nature. In the 2010s and 2020s, LFS ideas influenced distributed and SSD-centric storage, evolving into hybrid structures for scalability and durability. The Log-Structured Merge-Tree (LSM-tree), proposed by Patrick O'Neil et al. in 1996 but widely adopted in the 2000s through systems like LevelDB (2011), extended LFS sequential writing to key-value stores, enabling high-ingestion rates in databases like RocksDB by merging sorted logs to bound read amplification. Google's Colossus, deployed around 2010 as a successor to GFS, incorporated log-structured elements for append-only files and metadata logging, supporting exabyte-scale clusters with sub-millisecond latencies in cloud environments.^[12] SSD-specific evolutions, such as Samsung's F2FS (introduced in Linux kernel 3.8 around 2013), refined log structuring with multi-log zones to align with flash translation layers, reducing garbage collection overhead and enhancing wear leveling through hot/cold data separation.^[13] Key publications marking this progression include the seminal 1991 SOSP paper by Rosenblum and Ousterhout on Sprite LFS design, the 1993 USENIX implementation by Seltzer et al., and the 1995 heuristics paper on cleaning by Seltzer.^[14]^[7]^[8] The 2008 HAMMER overview by Dillon, the 2011 LevelDB technical notes, and the 2015 FAST paper on F2FS represent 2000s advancements.^[11]^[13] A comprehensive 2018 survey by Luo and Carey in The VLDB Journal analyzed LSM-based extensions of LFS, covering over 50 works and emphasizing their role in modern NoSQL systems with write amplification trade-offs.^[15] These contributions underscore LFS's enduring impact on sequential-write optimization amid shifting hardware paradigms.

Core Concepts and Design

Fundamental Principles

A log-structured file system (LFS) treats the entire storage medium as an append-only log, where all file system modifications—including file creations, deletions, and updates—are written sequentially to the end of this log rather than in place. This design mimics a write-ahead logging mechanism but applies it to the whole file system, ensuring that every change, from metadata updates to data blocks, is appended as a contiguous sequence.^[14] The rationale for this log structuring stems from the inherent geometry of disk drives, where sequential writes significantly outperform random ones by minimizing mechanical seeks and rotational delays. Traditional file systems suffer from fragmented updates that require scattering small writes across the disk, leading to inefficient access patterns; LFS amortizes these costs by batching operations into sequential streams, thereby exploiting the full sequential bandwidth of disks even for workloads dominated by small, random-like modifications.^[14] At its core, the key abstraction in an LFS is the disk viewed as a circular log composed of fixed-size segments, typically 512 KB to 1 MB, which serve as atomic units for writing and management. Old data within these segments is invalidated through non-reference rather than immediate overwriting, allowing the log to wrap around circularly once it reaches the disk's end, thus maintaining a monotonically growing structure without gaps.^[14] This approach represents a fundamental conceptual shift from conventional file systems, which rely on in-place updates to fixed locations for files and inodes. In an LFS, no such overwrites occur; instead, entirely new versions of affected structures are appended to the log, with the current valid state reconstructed via dynamic maps or in-memory pointers that track the latest references, enabling efficient reads while deferring space reclamation.^[14]

Log Structure Mechanics

In a log-structured file system (LFS), the entire disk is organized as a single sequential log, divided into fixed-size segments, typically ranging from 512 KB to 1 MB in length, to which all modifications are appended in a contiguous manner.^[14] These segments serve as the fundamental units of storage and include a variety of content: file data blocks, inodes, indirect blocks for larger files, portions of the inode map, segment summary blocks, and dedicated checkpoint regions.^[14] A superblock is maintained at a fixed disk location to provide initial boot information, such as the location of the most recent checkpoint, while each segment begins with a header followed by a summary block that records metadata for every data block within it, including the file number, block offset, and version number to facilitate quick identification of contents during recovery or cleaning.^[14] Central to the LFS mechanics are the mapping structures that translate virtual file addresses to physical locations in the log, ensuring efficient access without in-place updates. The inode map (IMAP) is a key component, consisting of fixed-size entries that point to the current location and version of each file's inode within the log; it is primarily cached in memory for fast lookups but periodically flushed to disk as part of checkpointing.^[14] Complementing the IMAP is the segment usage table (SUT), which maintains per-segment information such as the number of live bytes, the age of the oldest data, and timestamps for cleaning decisions, helping the system track which portions of the log contain valid versus obsolete data.^[14] Additional cleaner metadata, derived from segment summaries and the SUT, supports the identification and relocation of live data during space reclamation, with these structures themselves stored as appends within the log to maintain consistency.^[14] Checkpoints provide atomic snapshots of the volatile mapping tables, enabling reliable crash recovery and rapid mount-time reconstruction of the active file system state.^[14] Generated periodically—often after writing a certain volume of data or at shutdown—a checkpoint involves a two-phase process: first, all modified data and partial maps are appended to the log, followed by the complete IMAP and SUT written to one of two fixed checkpoint regions on disk, alternating between them to avoid overwriting active structures.^[14] Upon mounting, the system reads the latest checkpoint to load the IMAP and SUT into memory, then scans recent segment summaries to update mappings for any post-checkpoint writes, ensuring the file system view reflects the state at the last consistent point.^[14] File operations in an LFS leverage these structures through a virtual-to-physical addressing scheme that promotes append-only writes for atomicity and efficiency. When creating or modifying a file, new blocks—including data, inodes, and directory entries—are appended sequentially to the current log segment, with their virtual addresses (file ID and offset) mapped to physical log positions via an in-memory block map that extends the IMAP for individual blocks.^[14] This mapping is updated dynamically in memory and committed durably through subsequent checkpoints, while a separate log of directory operations ensures that name-to-inode linkages remain consistent even if a crash occurs mid-update, as all changes are grouped into indivisible log appends rather than scattered updates.^[14]

Operational Mechanisms

Writing and Allocation

In a log-structured file system (LFS), the writing process begins by buffering small, asynchronous writes in a kernel-level file cache to aggregate them into larger, segment-sized units, typically ranging from 512 KB to 1 MB, before committing them sequentially to disk.^[1] This batching ensures that modifications to data blocks, inodes, and other metadata are grouped together and written atomically as a single log segment, maximizing disk bandwidth utilization by converting random small writes into sequential large I/O operations.^[1] For instance, when updating a file, both the new data and the corresponding inode revisions are appended to the log in this bundled fashion, maintaining consistency without immediate on-disk reorganization.^[1] Space allocation for these writes relies on the Segment Usage Table (SUT), a data structure maintained in memory and periodically checkpointed to disk, which tracks the status of each disk segment by recording the number of live bytes and the last modification time for each.^[1] To allocate a new segment, the system scans the SUT to identify free or sufficiently clean segments—those with minimal live data—and reserves one for the impending write, ensuring that the append occurs at the log's current tail without fragmentation.^[1] This approach allows for rapid allocation, as the SUT provides an efficient index into the log's overall structure, where segments are written contiguously.^[1] Allocation policies in LFS guide the selection of segments for writing and future reuse, with common strategies including a greedy policy that prioritizes the least-utilized (cleanest) segments to quickly reclaim space, and a more sophisticated cost-benefit policy that weighs the potential reduction in cleaning overhead against the effort required.^[1] The cost-benefit approach, for example, selects segments based on a metric that favors those with low utilization u (the ratio of live bytes to segment size) and high age (time since last modification), using a formula like benefit/cost = [(1 - u) × age] / (1 + u), to minimize the long-term cost of relocating live data during space recovery.^[1] These policies are applied proactively during write allocation to balance immediate performance with sustained free space availability.^[1] When handling overwrites or modifications to existing files, LFS invalidates the old versions of affected blocks directly in the inode map or block mapping structures without physically erasing them from their original log positions, thereby marking space as obsolete for later reclamation.^[1] The new data is then written to a fresh log address within the newly allocated segment, and the mapping tables are updated to point to this new location; for actively modified files, a forwarding mechanism in the inodes ensures that subsequent reads access the most recent version by chaining pointers through the log.^[1] This out-of-place update strategy avoids the seek penalties of in-place revisions, treating overwrites as append-only operations.^[1] Integration with the operating system occurs through kernel-managed buffering that groups disparate write operations—such as file data updates, directory changes, and inode modifications—into cohesive log segments, enforcing sequential I/O patterns even for mixed workloads.^[1] For example, a metadata-intensive operation like creating a new file involves buffering the directory entry update alongside the initial data write, allowing both to be flushed atomically to the log without separate synchronous disk accesses, which reduces latency and enhances throughput in multi-user environments.^[7] This buffering layer, often implemented as an extension to the virtual file system interface, ensures that LFS presents a standard POSIX-like API while internally optimizing for log appends.^[7]

Garbage Collection and Cleaning

In log-structured file systems (LFS), garbage collection, often referred to as segment cleaning, is the background process responsible for reclaiming space in log segments that contain a mix of live and invalid (dead) data blocks. The algorithm scans candidate segments to determine their live-to-dead ratios, typically using metadata like segment summary blocks to identify live blocks by file identifiers and offsets. Live data is then relocated and compacted into fewer new, clean segments, after which the original segments are erased and marked as available for reuse. This process is triggered proactively when the number of clean segments drops below a configurable threshold, such as maintaining tens of segments free, and continues until a higher threshold is met, like 50-100 clean segments, to ensure steady-state operation without interrupting foreground writes.^[14] The efficiency of cleaning hinges on cost models that quantify the overhead of relocation, as analyzed by Rosenblum in 1992. A key insight is write amplification, where rewriting live data increases total I/O beyond the original writes; for a segment with utilization u (fraction of live data), the write cost multiplier is given by \frac{1 + u}{1 - u}, assuming all data in N read segments is processed. Optimal performance targets segments with u < 0.5, yielding multipliers around 2 or less, though real implementations achieve effective write costs of 1.2-1.6 by selecting low-utilization segments, enabling 65-75% of disk bandwidth to be used for writing new data (with the remainder for cleaning). Building briefly on segment usage tracking via summary blocks, these models enable predictive selection to minimize amplification.^[14] Cleaning policies vary to balance overhead and effectiveness, with two primary approaches: age-based and cost-based. Age-based policies prioritize the oldest segments, segregating hot (frequently updated) and cold (static) data to reduce repeated rewrites of hotspots, such as active files that could otherwise amplify costs in localized workloads. Cost-based policies, like the cost-benefit heuristic, select segments by maximizing a score of \frac{(1 - u) \times \text{age}}{1 + u}, favoring high-dead-ratio (low u) and aged segments to create a bimodal distribution—mostly full cold segments cleaned at ~75% utilization and sparse hot ones at ~15%—outperforming greedy selection (by utilization alone) by up to 50% in simulations. These policies handle hotspots by deprioritizing recently written hot data, preventing thrashing where high live ratios lead to inefficient cycles.^[14] Practical challenges include the burstiness of cleaning I/O, which can spike during intensive reclamation and compete with user requests, and the risk of thrashing if live ratios remain high across segments, exacerbating write amplification. Mitigations involve running cleaning in the background during idle periods to smooth bursts, and policy-driven selection to maintain a steady supply of low-cost segments, as demonstrated in production traces where 69% of cleaned segments were nearly empty, with average utilizations of 0.133-0.535.^[14]

Advantages and Limitations

Performance Benefits

Log-structured file systems (LFS) provide significant performance improvements primarily through their sequential write mechanism, which batches modifications into large, contiguous log appends rather than scattering them across the disk as random updates. This approach dramatically increases write throughput for small-file operations, such as creates and deletes, by converting costly random I/O into efficient sequential transfers. In benchmarks from the Sprite LFS implementation, small-file (1 KB) creates and deletes achieved rates of approximately 160 files per second, compared to about 20 files per second in the SunOS Unix File System (FFS), representing up to a 10-fold speedup.^[14] Similarly, random writes to large files reached approximately 700 KB/s in Sprite LFS, outperforming SunOS's 400 KB/s, while sequential writes reached 800 KB/s versus 600 KB/s.^[14] By amortizing disk seeks through batching, LFS reduces the average seek time per operation, particularly benefiting metadata-intensive workloads like directory listings and file attribute updates. This is achieved by grouping related operations—such as multiple small appends or metadata changes—into single log segments, minimizing the number of head movements on mechanical disks. Production measurements of Sprite LFS demonstrated that it could utilize 65-75% of the disk's raw bandwidth for writing new data, in contrast to the 5-10% typical for traditional Unix file systems.^[14] Trace-driven simulations further illustrated this efficiency, showing LFS write costs as low as 1.2-1.6 (in terms of disk bandwidth consumed per unit of useful data written), compared to 10-20 for FFS, effectively yielding 6- to 16-fold improvements in write efficiency for production workloads.^[14] Read performance in LFS remains generally comparable to traditional systems, supported by in-memory mapping structures like the inode map that enable quick location of current file versions within the log. For small-file reads, Sprite LFS delivered about 180 files per second, slightly exceeding SunOS's 140 files per second. Sequential log scans also benefit from the ordered structure, facilitating efficient traversal for applications requiring full-file or directory scans. LFS proves particularly suitable for Unix-like environments characterized by frequent small appends and metadata operations, such as in office or engineering workloads, where simulations indicated up to 70% overall write speedup relative to FFS.^[14]

Drawbacks and Challenges

One significant drawback of log-structured file systems (LFS) is the substantial cleaning overhead required to reclaim space occupied by invalid data. In steady-state operation, cleaning can consume 20-50% of the disk's bandwidth, as it involves reading partially filled segments, copying live data to new locations, and writing them back, often leading to write bursts that cause performance stalls. For instance, simulations demonstrate that when segments contain only 30% live data, the write amplification factor reaches 1.4x, meaning 1.4 units of data must be written to the disk for every unit of new user data.^[5] This overhead arises from the core cleaning process, where the system periodically compacts data to maintain free space for sequential writes.^[14] LFS also suffers from space inefficiency due to the accumulation of invalid blocks until cleaning occurs, necessitating the reservation of extra disk space to avoid frequent or inefficient cleanups. To achieve acceptable performance, LFS typically requires 20-50% of the disk to remain free, as lower utilization levels reduce write amplification but leave less usable capacity for user data.^[14] This inefficiency is particularly sensitive to workload patterns; for example, frequent rewrites of large files exacerbate fragmentation, increasing the proportion of invalid data and demanding more reserved space.^[5] Recovery in LFS adds complexity, relying on periodic checkpoints to reconstruct the current file system map during mount time after a crash or unclean shutdown. This process scans the log from the last checkpoint, replaying updates to rebuild in-memory structures, which can take significant time—up to 132 seconds for a 50 MB log with small files—and risks data loss or inconsistency if corruption affects the checkpoint or log tail.^[14] Unclean shutdowns heighten vulnerability, as partial writes may leave the system in an inconsistent state without atomic commit mechanisms beyond checkpoints.^[14] Finally, LFS performance is highly dependent on underlying hardware characteristics. On traditional hard disk drives (HDDs), the random reads during segment selection for cleaning can lead to seek overhead, and the system is less effective on zoned block devices where writes must follow sequential zone constraints, potentially requiring adaptations to avoid zone overflows.^[6] On solid-state drives (SSDs), while sequential writes align well, the lack of TRIM support prevents the SSD controller from efficiently garbage-collecting invalid blocks, resulting in accelerated wear through unnecessary erases and writes.^[4]

Implementations and Applications

Early and Research Systems

The Sprite Log-structured File System (LFS), developed at the University of California, Berkeley, served as a pioneering prototype implementation in 1991 within the Sprite operating system, a Unix-like environment running on Sun workstations. It employed a 4 KB block size and configurable segment sizes of 512 KB or 1 MB, with all modifications written sequentially to the log to optimize disk throughput. Dynamic segment allocation was handled via an in-memory segment map that tracked free space, complemented by a segment cleaner that copied live data from partially filled segments to reclaim space and maintain utilization above 70%. This design emphasized high write performance for small files and rapid crash recovery through checkpoints, though it required substantial memory for mapping structures.^[16] In 1993, Margo Seltzer and colleagues implemented a port of LFS to 4.4BSD Unix, integrating it with the vnode interface for improved compatibility and robustness. This BSD-LFS retained core log-structured principles, including sequential writes and checkpoint-based recovery, and was later ported to derivatives like NetBSD, where it became functional again with the NetBSD 4.0 release in 2007. Primarily used for research, it demonstrated portability but required kernel modifications and was limited to specific architectures.^[7] Early LFS systems like Sprite and BSD-LFS were inherently tied to their host operating systems, complicating portability to other Unix variants or non-BSD kernels. Experimental status often meant incomplete feature sets, such as limited support for legacy tools and persistent issues with cleaner overhead under low-utilization scenarios, restricting them to controlled research settings rather than broad adoption.^[16]^[7]

Modern and Commercial Uses

One prominent open-source implementation of log-structured file systems is the Flash-Friendly File System (F2FS), developed by Samsung and integrated into the Linux kernel starting with version 3.8 in 2012.^[17] F2FS is optimized for NAND flash storage such as eMMC and SSDs, employing an append-only logging scheme with multi-head log structures that separate hot and cold data into distinct sections to minimize write amplification.^[13] It incorporates adaptive cleaning algorithms that dynamically adjust garbage collection based on segment utilization and device characteristics, improving performance on flash media by reducing random writes.^[13] ZFS, originally developed by Sun Microsystems in 2005 and now available in open-source distributions like OpenZFS, integrates log-structured elements through its copy-on-write (CoW) design, where all modifications append new blocks rather than overwriting existing ones. This approach ensures data immutability and enables efficient snapshots and clones, with the ZFS Intent Log (ZIL) serving as a dedicated log for synchronous writes to enhance reliability on storage pools spanning multiple devices.^[18] In commercial environments, Apple's Apple File System (APFS), introduced in 2017 with macOS High Sierra, adopts LFS-like mechanisms including CoW for snapshots and cloning operations, allowing space-efficient versioning without duplicating data.^[19] APFS structures its container-based volumes to support these features natively on SSDs, facilitating features like Time Machine backups through immutable snapshots.^[19] Google's File System (GFS), deployed in 2003, employs a log-structured approach for metadata management via an operation log that records all critical changes, enabling fault-tolerant append-only writes across distributed clusters.^[20] Its successor, Colossus, introduced around 2010 and evolved for petabyte-scale storage, extends this with log-structured replication and append-only file semantics to handle massive workloads in Google's data centers, supporting services like BigQuery and Spanner.^[12] For embedded and cloud applications, Microsoft's Resilient File System (ReFS), available since Windows Server 2012, incorporates integrity streams that enable a log-structured mode for data validation using checksums, particularly when file integrity is enforced to detect and repair corruption in virtualized or storage-space configurations.^[21] Key-value stores like LevelDB (developed by Google in 2011) and its derivative RocksDB (by Facebook in 2012) utilize log-structured merge-trees (LSM-trees), which organize data in immutable levels of sorted runs for write-optimized persistence in databases and NoSQL systems.^[22]^[23] Recent advancements in the 2020s include hybrid LFS designs tailored for NVMe-optimized storage, such as those leveraging the Zoned Namespaces (ZNS) standard ratified by NVM Express in 2020, which divides SSD capacity into sequential-write zones to reduce internal fragmentation and wear leveling overhead.^[24] Systems like Z-LFS build on this by adapting log-structured allocation to ZNS constraints, achieving up to 33x performance gains in small-zone SSDs through zone-aware segment management and reduced host-side garbage collection.^[25]

Comparisons with Other File Systems

Versus Traditional Inode-Based Systems

Log-structured file systems (LFS) fundamentally differ from traditional inode-based systems, such as the Berkeley Fast File System (FFS) or ext2, in their approach to data and metadata updates. In LFS, all modifications—whether to file data, directories, or inodes—are appended sequentially to a log-like structure on disk, creating new versions of blocks rather than overwriting existing ones in place.^[14] This append-only model enables high write throughput by amortizing multiple small writes into large, sequential operations, achieving up to 65-75% of raw disk bandwidth for writes, compared to 5-10% in FFS due to scattered in-place updates that require multiple seeks per operation.^[14] However, this leads to higher space amplification in LFS, as obsolete data accumulates until garbage collection reclaims it, often operating effectively only at 85-90% disk utilization and consuming approximately 20% more space than FFS for equivalent workloads.^[14]^[26] Metadata handling in LFS contrasts sharply with the scattered inode tables of traditional systems. LFS batches inodes and related metadata into the log alongside data, using an inode map (a separate structure pointing to the latest versions) to locate current file attributes, which reduces random seeks during writes but requires additional indirection for reads.^[14] In contrast, FFS and ext2 maintain fixed inode tables distributed across cylinder groups or block groups, allowing direct access but necessitating separate I/O operations for metadata updates, such as up to five seeks for a simple file creation.^[14] Without effective caching, LFS reads can thus incur higher latency due to log scanning, though in practice, it performs comparably to FFS for sequential access patterns.^[26] Crash recovery mechanisms highlight another key divergence. LFS employs checkpointing to mark consistent log states, followed by rapid forward replay of post-checkpoint operations to reconstruct the file system, typically completing in under 1-132 seconds depending on the log size after a crash.^[14] Traditional inode-based systems like FFS rely on full-disk scans via tools such as fsck to detect and repair inconsistencies, a process that can take tens of minutes or more for large volumes, especially after write-heavy failures.^[14] This makes LFS particularly resilient for workloads with frequent small writes, localizing recovery to recent changes rather than rescanning the entire disk.^[26] LFS is optimized for workloads dominated by append-only operations, such as logging or creating many small files, where it delivers 1.5-10x faster write performance than inode-based systems—for instance, 0.28 MB/sec appends versus 0.11 MB/sec in FFS.^[14]^[26] Conversely, traditional systems like ext2 or UFS excel in random read/write patterns typical of databases or general-purpose use, where LFS's cleaning overhead can degrade throughput by up to 40% at high utilization, and read performance suffers without fragment support for small files under 8 KB.^[27] In benchmarks like the Andrew file system benchmark, LFS outperforms FFS in write-intensive phases (e.g., 1.30 seconds versus 3.30 seconds for file creation and writes) but trails in read-heavy scenarios.^[26]

Versus Other Journaling Approaches

Log-structured file systems (LFS) treat the entire disk as a single append-only log, where both data and metadata modifications are written sequentially without in-place updates. In contrast, traditional journaling file systems such as ext3 primarily log metadata changes in a separate, fixed-size journal (typically 1-32 MB), while data is written to fixed locations on disk in ordered or writeback modes; data journaling modes in ext3 include both data and metadata but still separate the journal from the main file system structure.^[28]^[29] NTFS similarly focuses on metadata-only journaling, recording structural changes in its $LogFile while performing in-place data writes.^[28] This unified logging in LFS promotes more complete sequential write patterns, enhancing performance on sequential media, but it increases the scope of garbage collection compared to the bounded journaling in ext3 and NTFS. Regarding overhead, LFS incurs write amplification from segment cleaning, with factors typically ranging from 1.2 to 1.6 times the user data volume under realistic workloads, though simulations indicate up to 2.5-3 times at higher utilization levels. Journaling systems like ext3 and NTFS maintain lower, more predictable overhead through their small, fixed journals—avoiding widespread rewriting—but data journaling in ext3 doubles writes by logging data temporarily before committing it to its final location.^[28] Despite this, LFS scales more favorably for small, random writes by aggregating them into sequential log appends, outperforming journaling approaches in such scenarios.^[28] For recovery and consistency, both LFS and journaling file systems provide atomic operations to prevent partial updates after crashes. LFS ensures this via periodic checkpoints, followed by a straightforward replay of the log tail from the last checkpoint, which restores the file system without needing transaction-specific rollbacks. Journaling systems like ext3 and NTFS rely on transaction logs for redo (replaying committed changes) or undo/rollback mechanisms for incomplete ones, scanning only the journal for quicker recovery in most cases.^[28] However, LFS's approach trades off with potential read amplification during normal operation to maintain current file maps, as the log structure requires indirect access via in-memory or on-disk indices. Modern hybrid systems like Btrfs incorporate log-structured copy-on-write mechanisms, blending LFS's append-only updates with B-tree indexing to mitigate fragmentation and cleaning overheads inherent in pure LFS designs.^[30] For instance, ext4's delayed allocation feature batches small writes before committing them to disk, akin to LFS's log batching for sequential efficiency, but within an inode-based framework rather than a full log.^[31]

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[PDF] THE HAMMER FILESYSTEM - DragonFly BSD
THE HAMMER FILESYSTEM. Matthew Dillon, 21 June 2008. Section 1 – Overview. Hammer is a new BSD file-system written by the author. Design on the file-.Missing: 2005 | Show results with:2005
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A peek behind Colossus, Google's file system | Google Cloud Blog
Apr 19, 2021 · Colossus is the secret scaling superpower behind Google's storage infrastructure. Colossus not only handles the storage needs of Google Cloud services.Missing: LFS | Show results with:LFS
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[PDF] F2FS: A New File System for Flash Storage - USENIX
Feb 19, 2015 · In this paper, we present the design and implemen- tation of F2FS, a new file system optimized for mod- ern flash storage devices. As far as we ...Missing: FFS | Show results with:FFS
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[PDF] The design and implementation of a log-structured file system
A log-structured file system writes all modifications to disk sequentially in a log-like structure, thereby speeding up both file writing.Missing: original | Show results with:original
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[1812.07527] LSM-based Storage Techniques: A Survey - arXiv
Dec 18, 2018 · In this paper, we provide a survey of recent research efforts on LSM-trees so that readers can learn the state-of-the-art in LSM-based storage techniques.
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[PDF] The Design and Implementation of a Log-Structured File System
Jul 24, 1991 · Synchronous writes couple the application's performance to that of the disk and make it hard for the application to benefit from faster CPUs.
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LFS - Log-Structured Filesystem for NetBSD - hhhh.org
Jun 27, 2002 · Short history and Description. The LFS, or Log-structured File System, is an alternative filesystem design proposed by Rosenblum and Ousterhout ...
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A Brief Retrospective on the Sprite Network Operating System
By late 1991 virtually all of Sprite's dozen disks were using LFS. The final phase of the project started in late 1990 and will continue until Sprite shuts ...
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Flash-Friendly File System (F2FS) - The Linux Kernel documentation
F2FS is a file system exploiting NAND flash memory-based storage devices, which is based on Log-structured File System (LFS).Missing: 2006 | Show results with:2006
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[PDF] Apple File System Reference
Jun 22, 2020 · Apple File System is the default file format used on Apple platforms. Apple File System is the successor to HFS.
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[PDF] The Google File System
The operation log contains a historical record of critical metadata changes. It is central to GFS. Not only is it the only persistent record of metadata, but it ...
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Resilient File System (ReFS) overview - Microsoft Learn
Jul 28, 2025 · Integrity-streams - ReFS uses checksums for metadata and optionally for file data, giving ReFS the ability to reliably detect corruptions.
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NVMe Zoned Namespaces (ZNS) Command Set Specification
The NVMe Zoned Namespaces (ZNS) interface is a command set developed by NVM Express. By dividing an NVMe namespace into zones, which are required to be ...Missing: hybrid LFS 2020
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[PDF] Z-LFS: A Zoned Namespace-tailored Log-structured File System for ...
This paper presents a novel zoned namespace (ZNS) tailored log-structured file system (LFS) called Z-LFS for commod- ity small-zone ZNS SSDs.
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[PDF] An Implementation of a Log- Structured File System for UNIX
Research results [ROSE91] demonstrate that a log-structured file system (LFS) offers the potential for dramatically improved write performance, faster recovery ...
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[PDF] File System Logging versus Clustering: A Performance Comparison
This paper presents a detailed performance comparison of the 4.4 BSD Log-structured File System and the 4.4 BSD Fast File System. ... This paper analyzes the ...
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Analysis and Evolution of Journaling File Systems
### Summary of Differences Between Log-Structured File Systems and Journaling File Systems (ext3, NTFS)
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[PDF] EXT3, Journaling Filesystem - cs.wisc.edu
Jul 20, 2000 · The ext3 filesystem is a journaling extension to the standard ext2 filesystem on Linux. Journaling results in massively reduced time spent.
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[1707.08514] Analyzing IO Amplification in Linux File Systems - ar5iv
... journaling, we find XFS significantly more efficient for file updates. Similarly, though F2FS and btrfs are implemented based on the log-structured approach ...
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Extents and Extent allocation in Ext4 | linux - Oracle Blogs
Oct 15, 2024 · Ext4 supports delayed allocation, it is also the default allocation algorithm. In delayed allocation, on-disk blocks are not allocated ...Missing: batching | Show results with:batching