Devices include hardware components such as, printers, drives, adapters, buses, and enclosures, as well as pseudo-devices, such as the error special file and null special file. Device drivers are located in the /usr/lib/drivers directory.
This chapter provides an overview of the following methods used by the operating system to manage these devices:
The number of devices that AIX can support can vary from system to system, depending on several important factors. The following factors have an impact on the file systems that support the devices:
Two file systems are used by AIX to support devices:
Devices are organized into clusters known as nodes. Each node is a logical subsystem of devices, where lower-level devices have a dependency on upper-level devices in child-parent relationships. For example, the system node is the highest of all nodes and consists of all the physical devices in the system. The system device is the top of the node and below that are the bus and adapters that have a dependency on the higher-level system device. At the bottom of the hierarchy are devices to which no other devices are connected. These devices have dependencies on all devices above them in the hierarchy.
At startup time, parent-child dependencies are used to configure all devices that make up a node. Configuration occurs from the top node down and any device having a dependency on a higher-level device is not configured until the higher-level device is configured.
Multiple-path I/O (MPIO) is a feature available with AIX 5.2 and later. If a device has an MPIO-capable device driver, it can have more than one parent within this hierarchy, which allows multiple, simultaneous communication paths between the device and a given machine or logical partition within a machine. For information about how this feature works, see Device Configuration Database.
Managing devices requires the operating system to comprehend what device connections are allowed. The operating system classifies devices hierarchically into three groups:
Functional classes consist of devices that perform the same function. Printers, for example, comprise a functional class. Functional classes are grouped into subclasses according to certain device similarities. For example, printers have a serial or parallel interface. Serial printers are one subclass and parallel printers are another. Device types are classified according to their model and manufacturer.
Device classes define valid parent-child connections for the operating system. The hierarchy defines the possible subclasses that can be connected for each of the possible child connection locations. For example, the term RS-232 8-port adapter specifies that only devices belonging to the RS-232 subclass can be connected to any of the eight ports of the adapter.
Device classes and their hierarchical dependencies are maintained in an Object Data Manager (ODM) Device Configuration database.
Device information is contained in a predefined database or a customized database that makes up the device configuration database. The predefined database contains configuration data for all possible devices supported by the system. The hierarchical device class information is contained in this database. The customized database contains configuration data for all currently defined and configured devices in the system. A record is kept of each device currently connected to your system.
The Configuration Manager is a program that automatically configures devices on your system during system startup and run time. The Configuration Manger uses the information from the predefined and customized databases during this process, and updates the customized database afterwards.
Beginning with AIX 5.2, the Multiple-path I/O (MPIO) feature uses a unique device identifier (UDID) to identify each MPIO-capable device, regardless of the path on which it was discovered. The UDID is saved in the device configuration database. When a device is discovered, the UDIDs in the database are checked to determine whether the device is new or whether the discovery is another path to an existing device. When multiple paths to a device are detected, the device driver or the Path Control Manager kernel extension decides which path to use for a particular request. For information on managing MPIO-capable devices, see Managing MPIO-Capable Devices in AIX 5L Version 5.2 System Management Guide: Operating System and Devices.
Devices that are connected to the system can be in one of four states:
If a tty device and a printer alternately use the same tty connector, both a tty device and a printer are defined on the same parent and port in the device configuration database. Only one of these devices can be configured at a time. When the tty connector is configured, the printer specific setup information is retained until it is configured again. The device is not removed; it is in the defined state. Maintaining a device in defined state retains customized information for a device that is not currently in use, either before it is first made available or while it is temporarily removed from the system.
If a device driver exists for a device, the device can be made available through the device driver.
Some devices, in particular TCP/IP pseudo-devices, need the stopped state.
You can use the Web-based System Manager Devices application, SMIT, or operating system commands to perform device management tasks such as deleting, adding, or configuring a device.