Configuration of a large number of devices

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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.

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:
  • Configuring a large number of devices requires storage of more information in the ODM device-configuration database. It can also require more device special files. As a result, more space and more i-nodes are required of the file system.
  • Some devices require more space than others in the ODM device-configuration database. The number of special files or i-nodes used also varies from device to device. As a result, the amount of space and i-nodes required of the file system depends on the types of devices on the system.
  • Multipath I/O (MPIO) devices require more space than non-MPIO devices because information is stored in the ODM for the device itself as well as for each path to the device. As a rough guideline, assume that each path takes up the space of one-fifth of a device. For example, an MPIO device with five paths will have the space equivalent to two non-MPIO devices.
  • AIX includes both logical devices and physical devices in the ODM device-configuration database. Logical devices include volume groups, logical volumes, network interfaces, and so on. In some cases, the relationship between logical and physical devices can greatly affect the total number of devices supported. For example, if you define a volume group with two logical volumes for each physical disk that is attached to a system, this will result in four AIX devices for each disk. On the other hand, if you define a volume group with six logical volumes for each physical disk, there will be eight AIX devices for each disk. Therefore, only half as many disks could be attached.
  • Changing device attributes from their default settings results in a larger ODM device-configuration database and could lead to fewer devices that can be supported.
  • More devices require more real memory.
Two file systems are used by AIX to support devices:
  • The RAM file system is used during boot in an environment that has no paging space and no disk file systems mounted. The size of the RAM file system is 25% of the system memory size up to a maximum of 128 MB. One i-node is allocated for every KB in the RAM file system. The minimum system memory requirement for the AIX operating system is 256 MB, which translates into a minimum RAM file system size of 64 MB with 65536 i-nodes. If the system memory size is 512 MB or larger, then the RAM file system will be at its maximum size of 128 MB with 131072 i-nodes. If either the amount of RAM file system space or number of i-nodes needed to support the attached devices exceeds what has been allocated to the RAM disk, the system might not boot. If this is the case, you must remove some of the devices.
  • The space and i-nodes of the root file system (rootvg) on the disk can be increased as long as there are unallocated partitions in the rootvg. With the maximum RAM file system size, it is likely that up to 25,000 AIX devices can be configured. These numbers include both physical and logical devices. Depending on the various factors mentioned in this section, your system might be able to configure more or fewer devices than this number.
Note: With a large number of devices in the system, the longer configuration time contributes to a longer boot time.