IO

I/O

One difference between server systems and other mid-range class computers is the I/O subsystem. Server systems support much larger numbers of I/O devices and more complex I/O topologies. Because servers typically host large amounts of data and need to make data available to consumers quickly, I/O throughput and bandwidth can play a key role in system performance.

I/O buses serve as a connection from I/O devices to the system memory. The prevalent I/O bus on modern servers is PCI. Servers have multiple PCI slots that allow PCI cards that interface to SCSI, Fibre Channel, networks, and other I/O devices.

A server configuration can vary from a single disk up to thousands of disks. Although the number of system disks is limited to 256 in the 2.4 kernel series, this limit has been increased for the 2.6 kernel. Servers with only one or two disks might use IDE technology for the disks, but systems with more than a few disks mostly use SCSI technology. Fibre Channel is also used, especially for larger storage networks that might encompass multiple Fibre Channel adaptors, switches, and enterprise-level storage arrays.

Multipath I/O (MPIO) provides more than one path to a storage device. In basic MPIO configurations, the extra path(s) to the storage device is (are) available for failover onlythat is, when the system detects an error attempting to reach the storage device over the primary path, it switches over and uses the secondary path to access the storage device. More advanced MPIO configurations use additional paths to increase the bandwidth between the storage device and memory. The operating system attempts to load- balance across the multiple paths to maximize the amount of data throughput between memory and disks.

A server needs at least one network connection, but again, depending on the type of work the server is performing, it might have multiple network connections, either all to the same network (with the multiple connections providing increased network bandwidth) or to multiple networks. Ethernet is the network technology most in use. Although 10Mbps Ethernet is still supported, most Ethernet today operates at the rate of at least 100Mbps, with Gigabit Ethernet becoming the default on modern servers. Other network technologies, such as ATM, exist and are supported by Linux but are not as widespread as Ethernet.

Other I/O devices are also connected to servers. Most servers have a CD-ROM device for loading software onto the system. They might also have a CD-writer. Some type of device to back up system data is also needed. Tape devices are usually used for this, although other backup mechanisms and strategies are possible.

Large servers have some form of service processor that is used for controlling the initial power-on reset sequence, booting the operating system, and monitoring the system (power supply, cooling, and so on) during normal operation. Service processors often participate in machine fault handling and provide other services for the normal functioning of the server.

The multiprocessors, memory, and I/O subsystems are the kernel components that make an enterprise server. The hardware of the Linux Enterprise Server provides the foundation to bring this high-powered functionality to life.