FileSystemCache

FileSystemCache is a memory-resident caching layer used by operating systems and applications to store recently accessed file data and filesystem metadata in RAM, in order to reduce disk I/O and improve performance. It is not a single universal component but a family of techniques implemented in different systems.

In many systems, the cache is divided into a data cache, which holds the actual contents of files (often organized as memory pages), and a metadata cache, which stores information such as inodes, directory entries, permissions and timestamps. The data cache can support read-ahead and prefetching to anticipate future reads.

When a file is read, the cache is checked first; on a hit, data is returned quickly. On a miss, data is read from disk and stored in the cache. Writes may be cached (write-back) and later flushed to disk, or written-through immediately, depending on the policy.

Eviction and memory management: The cache has finite size; when memory pressure increases, entries are evicted according to an eviction policy. Common policies approximate least-recently-used behavior and can be augmented with age-based or frequency-based components. In operating systems like Linux, the page cache and dentries/inodes are subject to such mechanisms.

Consistency: Cache coherence is maintained by invalidating or updating entries when underlying files change on disk or by other processes, and by coordinating with file system drivers and network file systems. Synchronization calls may flush dirty pages.

Performance and tuning: The size of the cache, policies, and cache pressure parameters influence I/O performance and memory usage. Administrators may tune system parameters to balance cache effectiveness against available RAM and application needs.

Examples: Linux uses a page cache for file data and separate inode and dentry caches in the VFS; Windows maintains a system cache that covers file and metadata caching; database systems and file servers implement their own caches.

Limitations: Stale data risks with certain workloads; cache can become a bottleneck if memory is misallocated; network file systems complicate coherence.