The operating system's disk cache

Operating systems such as Ubuntu use free memory for caching previously accessed contents of the server’s disks. The operating system automatically releases memory used by the disk cache when that memory is needed by applications.

The disk cache speeds up file access by allowing disk contents to be read from memory without needing to access the disk every time a file is accessed. Even with a fast SSD disk, memory access is around 1000x faster than disk access.

The operating system’s disk cache is one of the reasons why it’s important for a server to have more memory than is normally used by your apps: your server will slow down due to increased disk access when your server is low on memory.