buffercache

Buffercache is a term used in operating systems to describe a portion of main memory set aside to temporarily store data blocks read from or written to storage devices. The goal is to accelerate I/O operations by keeping recently accessed disk blocks nearby, reducing the need to fetch data from slower storage media on every access. Buffercache is part of the broader file system cache and is managed by the kernel.

In Unix-like systems, the cached data is divided into two related components: a block buffer cache for block device I/O and a page cache for file data and metadata. Modern kernels may treat these caches as a unified memory management resource, reclaiming pages when memory pressure rises. Data in the buffercache can be considered recyclable and is kept only as long as it improves performance.

The kernel writes back modified data in the cache to storage asynchronously, using dirty buffers or pages. When memory is tight, the system will evict cached data to free RAM, potentially increasing latency for subsequent reads. Cache effectiveness depends on workload characteristics; read-heavy workloads generally benefit from larger caches, while memory-constrained systems may need to balance cache size with application needs.

Monitoring and tuning can vary by OS. On Linux, metrics such as Buffers and Cached appear in /proc/meminfo and related tools, with tunables like vm.vfs_cache_pressure and read-ahead settings influencing behavior. Cross-platform, buffercache remains a fundamental mechanism to speed up disk I/O by leveraging local RAM, at the cost of occupying nonportable memory that can be reclaimed when needed.