I have read a document which tells me the following 4 things are done by the RAM embedded on disk driver controller. But I don't know what's difference between buffer and cache. Thanks!

RAM on disk drive controllers
1 firmware
2 speed matching buffer
3 prefetching buffer
4 cache

Firmware is usually in flash. Some old systems stored it in reserved areas on the disk.

Prefetch buffer = read cache = RAM. This is memory used to satisfy reads more quickly than having to read from the disk.

Write cache in a high end system is mirrored and non-volatile. It can be battery+RAM or MRAM. It is used to hold write data safely until it can be written to disk.

Both read cache and write cache are used to match the speed of the host connection to the speed of the disk(s).

Thanks for your reply.

Before you replying, I suppose the write cache embedded in disk controller is also used for read cache and the disk will schedule the capacity of cache to satisfy the requirements of reading and writing dynamically . If the disk is off power suddenly, the date in the RAM transferred from bus will disappear.
What you mean seems that the write cache and the read cache exists in two RAM　separately. If the disk is off power suddenly, the data in write cache will not disappear.
So which viewpoint is right ? thanks!

RAID controllers either separate read cache and write cache or else provide battery backup for both. On the RAID controllers that I've used, if the batteries expire then write cache is disabled and the controllers switch to write-through mode so that no write data is lost once it is written to the controller. In write-through mode, the write is not completed until the data is on disk.

Sorry, I don't mean the RAID controllers, but the controller embedded in disk which is the circuit inner the disk.