Anatomy of a Block Device

The smallest addressable unit on a block device is known as a sector. Sectors come in various powers of two, but 512 bytes is the most common size. The sector size is a physical property of the device and the sector is the fundamental unit of all block devicesthe device cannot address or operate on a unit smaller than the sector, although many block devices can transfer multiple sectors at once. Although most block devices have 512-byte sectors, other sizes are common (for example, many CD-ROM discs have 2-kilobyte sectors).

Software has different goals, however, and therefore imposes its own smallest logically addressable unit, which is the block. The block is an abstraction of the filesystemfilesystems can be accessed only in multiples of a block. Although the physical device itself is addressable at the sector level, the kernel performs all disk operations in terms of blocks. Because the device's smallest addressable unit is the sector, the block size can be no smaller than the sector and must be a multiple of a sector. Furthermore, the kernel (like hardware with the sector) needs the block to be a power of two. The kernel also requires that a block be no larger than the page size (see Chapter 11, "Memory Management," and Chapter 19, "Portability")^[1]. Therefore, block sizes are a power-of-two multiple of the sector size and are not greater than the page size. Common block sizes are 512 bytes, 1 kilobyte, and 4 kilobytes.

^[1] This is an artificial constraint that could go away in the future. Forcing the block to remain equal to or smaller than the page size, however, simplifies the kernel.

Somewhat confusingly, some people refer to sectors and blocks with different names. Sectors, the smallest addressable unit to the device, are sometimes called "hard sectors" or "device blocks." Meanwhile, blocks, the smallest addressable unit to the filesystem, are sometimes referred to as "filesystem blocks" or "I/O blocks." This chapter continues to call the two notions "sectors" and "blocks," but you should keep these other terms in mind. Figure 13.1 is a diagram of the relationship between sectors and buffers.

Other terminology, at least with respect to hard disks, is commonterms such as clusters, cylinders, and heads. Those notions are specific only to certain block devices and, for the most part, are invisible to user-space software. The reason that the sector is important to the kernel is because all device I/O must be done in units of sectors. In turn, the higher-level concept used by the kernelblocksis built on top of sectors.