F.1. The sysfs File System

The Linux 2.6 kernel introduces the sysfs file system. The sysfs file system is described as a union of the proc, devfs, and devpty file systems. The sysfs file system enumerates the devices and busses attached to the system into a file system hierarchy that can be accessed from user space. It is designed to handle the device and driver specific options that have previously resided in /proc/, and encompass the dynamic device addition previously offered by devfs. At this early point in the implementation of sysfs, there are many drivers and utilities that still refer to the older proc entries. However, it is understood that sysfs is the way of the future.

The sysfs file system is mounted at /sys/ and contains directories that organize the devices attached to the system in several different ways. The /sysfs/ subdirectories include:

1. The /devices/ directory

   This directory contains the /css0/ directory. Its subdirectories represent all the subchannels detected by the Linux kernel. Subchannel directories are named in the form 0.0.nnnn where nnnn is the subchannel number in hex between 0 and ffff. Subchannel directories in turn contain status files and another subdirectory which represents the actual device. The device directory is named 0.0.xxxx where xxxx is the unit address for the device. The /devices/ directory also contains status information as well as configuration options for the device.

2. The /bus/ directory

   This contains a /ccw/ subdirectory and a /ccwgroup/ subdirectory. CCW devices are accessed using channel command words. Devices in the /ccw/ directory only use one subchannel on the mainframe channel subsystem. CCW group devices are also accessed with channel command words, but they use more than one subchannel per device. For example, a 3390-3 DASD device uses one subchannel, while a QDIO network connection for an OSA adapter uses three subchannels. The /ccw/ and the /ccwgroup/ directories both contain directories called devices and drivers:

   The /devices/ directory contains a symbolic link to the device directories in the /sys/devices/css0/ directory.

   The /drivers/ directory contains directories for each device driver currently loaded on the system. The zFCP driver has a directory here. The /driver/ directory contains settings for the device driver, as well as symbolic links to the devices it is using (in the /sys/devices/css0/ directory).

3. The /class/ directory

   This contains directories that group together similar devices such as ttys, SCSI tape drives, network devices, and other miscellaneous devices.

4. The /block/ directory

   This directory contains directories for each of the block devices on the system. These are mostly disk type devices such as real DASD, loopback devices, and software raid block devices. The noticeable difference between older Linux systems and ones that use sysfs is the need to refer to devices by their sysfs name. On a 2.4 kernel image, the zFCP driver was passed as its device addresses. On the 2.6 Kernel image system the driver is passed as 0.0.1600.