page cache
also: page buffer, disk cache, filesystem cache
The page cache is the Linux kernel's in-memory cache of file data from disk, automatically managed to speed up repeated file access without additional system calls.
The page cache stores copies of file contents (and other disk blocks) in RAM after they are first read. When a program reads a file, the kernel checks the page cache first before accessing the slow disk. On subsequent accesses to the same data, the kernel retrieves it from RAM instead, dramatically improving performance.
The kernel automatically manages the page cache size, growing it to use available memory and shrinking it when memory is needed for other purposes. You can observe page cache statistics in /proc/meminfo under the Buffers and Cached fields, or inspect details with tools like free -h or vmstat.
For example, reading a large log file the first time causes disk I/O, but reading it again is much faster because the pages are cached in memory. You can clear the page cache manually with echo 3 > /proc/sys/vm/drop_caches (requires root), though the kernel will repopulate it as needed.