The same track on all disk surfaces forms a cylinder, usually called a cylinder. The heads on each cylinder are numbered from "0" from top to bottom. Data is read/written by cylinder, that is, when…

The same track on all disk surfaces forms a cylinder, usually called a cylinder. The heads on each cylinder are numbered from "0" from top to bottom. Data is read/written by cylinder, that is, when the head reads/writes data, it first starts from the "0" head in the same cylinder, and then operates on different surfaces of the same cylinder. Only after all the heads on the same cylinder have finished reading/writing, the head will move to the next cylinder (the cylinder further in the concentric circle), because the selection of the head only needs to be done through electronic switching, while the selection of the cylinder must be done through mechanical switching. Electronic switching is quite fast, much faster than the mechanical movement of the head to the adjacent track. Therefore, data is read/written by cylinder, not by disk surface. In other words, after a track is full of data, it will be written on the next disk surface of the same cylinder. After a cylinder is full, it will move to the next south area to start writing data. Reading data is also done in this way, which improves the read/write efficiency of the hard disk. When understanding the local write process of a storage system, an engineer believed that the host first writes a request to a controller engine and then the engine writes the value directly to the engine's local hard disk.