Page cache.

the two-minute refresher

• read() is served from RAM on a hit (about 1 µs) and from disk on a miss (about 10 ms on an HDD).
• Writes are write-back by default: they land in the cache and are flushed later, so a crash can lose them.
• fsync() forces the flush; databases call it to make commits durable.
• Databases that run their own cache use O_DIRECT to avoid caching the same data twice.