Understanding the Linux Kernel

13.7. Anticipating Linux 2.4

Linux 2.4 heavily changes how I/O device drivers are handled. The main improvement consists of a new Resource Management Subsystem used to allocate IRQ lines, DMA channels, I/O ports, and so on. Thanks to this new subsystem, Linux now fully supports hot-pluggable Plug-And-Play hardware devices, USB buses, and PCMCIA cards.

Linux 2.4 reorganizes the block device driver layer and adds support for the Logical Volume Manager. The Logical Volume Manager allows filesystems to span several disk partitions and to be resized dynamically. This new feature brings Linux closer to enterprise-class operating systems.

The new kernel introduces a class of character devices called raw I/O devices. These devices allow applications like DBMS to directly access disks without making use of the kernel caches.

Another significant addition is kernel support for Intelligent Input/Output (I2O) hardware. The goal of this new standard, derived from the PCI architecture, is to write OS-independent device drivers for several kind of devices like disks, SCSI devices, and network cards.

Finally, Linux 2.4 includes the devfs virtual filesystem, which replaces the old static /dev directory of device files. Virtual files appear only when the corresponding device driver is present in the kernel. The device filenames have also been changed. As an example, all disc devices are placed under the /dev/discs directory: /dev/hda might become /dev/discs/disc0, /dev/hdb might become /dev/discs/disc1 ...