Disk buffer

The drive circuitry usually has a small amount of memory, used to store the data going to and coming from the disk platters.

The disk buffer is physically distinct from and is used differently from the page cache typically kept by the operating system in the computer's main memory.

The disk buffer is usually quite small, ranging between 8 MB to 4 GB, and the page cache is generally all unused main memory.

When a disk's controller executes a physical read, the actuator moves the read/write head to (or near to) the correct cylinder.

Consistency can be maintained, however, by using a battery-backed memory system for caching data, although this is typically only found in high-end RAID controllers.

Alternatively, the caching can simply be turned off when the integrity of data is deemed more important than write performance.

Another option is to send data to disk in a carefully managed order and to issue "cache flush" commands in the right places, which is usually referred to as the implementation of write barriers.

[5] An operating system will send FLUSH CACHE and STANDBY IMMEDIATE command to hard disk drives in the shutdown process.

FUA is more fine-grained as it allows a single write operation to be forced to stable media and thus has smaller overall performance impact when compared to commands that flush the entire disk cache, such as the ATA FLUSH CACHE family of commands.