Hard Disk Drives

The last chapter introduced the general concept of an I/O device and showed you how the OS might interact with such a beast. In this chapter, we dive into more detail about one device in particular: the hard disk drive. These drives have been the main form of persistent data storage in computer systems for decades and much of the development of file sys-tem technology (coming soon) is predicated on their behavior. Thus, it is worth understanding the details of a disk’s operation before building the file system software that manages it. Many of these details are avail-able in excellent papers by Ruemmler and Wilkes [RW92] and Anderson, Dykes, and Riedel [ADR03]. CRUX: HOW TO STORE AND ACCESS DATA ON DISK How do modern hard-disk drives store data? What is the interface? How is the data actually laid out and accessed? How does disk schedul-ing improve performance? 37.1 The Interface Let’s start by understanding the interface to a modern disk drive. The basic interface for all modern drives is straightforward. The drive consists of a large number of sectors (512-byte blocks), each of which can be read or written. The sectors are numbered from 0 to n − 1 on a disk with n sectors. Thus, we can view the disk as an array of sectors; 0 to n − 1 is thus the address space of the drive. Multi-sector operations are possible; indeed, many file systems will read or write 4KB at a time (or more). However, when updating the disk, the only guarantee drive manufactures make is that a single 512-byte write is atomic (i.e., it will either complete in its entirety or it won’t complete at all); thus, if an untimely power loss occurs, only a portion of a larger write may complete (sometimes called a torn write)