Reiser4
Reiser4 is a journaling file system designed by Namesys, led by Hans Reiser. It is the successor to ReiserFS (version 3) and aims to improve upon its predecessor's performance, flexibility, and features. Reiser4 is designed to handle very large filesystems efficiently and incorporates a unique philosophical approach to filesystem design.
Key Features and Concepts:
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Transactions: Reiser4 uses a transaction-based approach to ensure data consistency and recovery in the event of system crashes. Transactions group multiple filesystem operations into a single atomic unit, either completing successfully or rolling back entirely.
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Write Anywhere Transaction Layer (WAL): Reiser4 employs a Write Anywhere Transaction Layer (WAL) that writes all filesystem changes to a journal before applying them to the main filesystem. This ensures durability and speeds up recovery.
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Plugins: A core design principle of Reiser4 is its plugin architecture. Virtually every aspect of the filesystem's behavior, including file layouts, hashing algorithms, and data compression methods, can be modified and extended using plugins. This makes Reiser4 highly customizable and adaptable to various workloads. These plugins are loaded at runtime and are selected based on file type or other criteria.
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Formatted Nodes: Reiser4's file system metadata is arranged into tree structures that use "formatted nodes" (also sometimes referred to as "items"). These nodes contain data and pointers to other nodes, allowing for efficient storage and retrieval of information.
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Object IDs: Reiser4 uses object IDs to identify files and directories, rather than relying solely on pathnames. This allows for faster lookups and greater flexibility in managing files.
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Atomic Operations: Reiser4 implements several atomic operations that allow for complex filesystem modifications to be performed in a single, indivisible step. This improves performance and simplifies application development.
Controversies and Adoption:
Reiser4 has been the subject of significant controversy due to the legal troubles of its creator, Hans Reiser. This has hindered its adoption by mainstream Linux distributions, despite its technical merits. Although it has been available as a patch for the Linux kernel for a long time, it has never been officially integrated into the mainline kernel tree. Due to lack of widespread adoption and continuous development it is rarely used.