Explain how the Linux kernel handles I/O with the page cache.

The Linux kernel uses the page cache to cache file data in RAM to speed up I/O. When you read a file, the kernel copies it into page cache. Subsequent reads are served from RAM (microseconds) instead of disk (milliseconds).

Writes are also cached: data is written to the page cache first and then persisted to disk asynchronously (write-back). This is why free -h shows most RAM as “used” on a healthy server — the kernel aggressively caches. This is not a memory leak.

Relevant commands: vmstat, iostat, /proc/meminfo (Cached, Buffers), echo 3 > /proc/sys/vm/drop_caches to flush cache (dangerous in production).