Understanding the Linux Kernel

13.5. Block Device Handling

Typical block devices like hard disks have very high average access times. Each operation requires several milliseconds to complete, mainly because the hard disk controller must move the heads on the disk surface to reach the exact position where the data is recorded. However, when the heads are correctly placed, data transfer can be sustained at rates of tens of megabytes per second.

In order to achieve acceptable performance, hard disks and similar devices transfer several adjacent bytes at once. In the following discussion, we'll say that groups of bytes are adjacent when they are recorded on the disk surface in such a manner that a single seek operation can access them.

The organization of Linux block device handlers is quite involved. We won't be able to discuss in detail all the functions that have been included in the kernel to support the handlers. But we'll outline the general software architecture and introduce the main data structures. Kernel support for block device handlers includes the following features:

• Offers a uniform interface through the VFS

• Implements efficient read-ahead of disk data

• Provides disk caching for the data

The kernel basically distinguishes two kinds of I/O data transfer:

Buffer I/O operations

Here the transferred data is kept in buffers, the kernel's generic memory containers for disk-based data. Each buffer is associated with a specific block identified by a device number and a block number. Linux misleadingly ...