I/O

Neither systems architects nor computer architecture researchers have paid as much attention to I/O as they have to processors. The rationale for this lack of attention is completely unclear to us, since I/O is a key part of a system, being both a driver for system performance and an opportunity for value add—unlike processors, where the economies of scale predominate.

We provided a generic server model with the purpose of identifying the various interconnects of interest, mentioning the interconnects between processor, memory and I/O.

An I/O system comprises a number of elements.

As shown in this diagram, the various elements of a classical I/O subsystem are as follows:

» A system controller implementing the connection between processor(s), memory and the I/O bus(es). In practice, this is often a chip integrating two functions—a memory controller and an I/O controller.

» One or more I/O buses. The industry has converged on the PCI (Peripheral Component Interconnect) bus and its extensions.

» I/O controllers connected to an I/O bus. Peripheral devices—for example, disks directly connected to the system, long-distance network connections (WAN, for Wide Area Network) and local network connections (LAN, for Local Area Network) are connected to these controllers.

» Directly-attached magnetic peripherals (a configuration called DAS, for Directly Attached Storage), on specialized buses such as SCSI, which today is the standard for this purpose.

» Specialized networks are used for the connection of peripheral subsystems, such as disk subsystems (e.g., SAN, for Storage Area Network) or communications subsystems (WAN or LAN). In this domain, the industry is moving to Fibre Channel.

We will first be interested in buses used for I/O (principally PCI and its extensions) and in the connections between controllers and magnetic peripherals (SCSI and Fibre Channel—Arbitrated Loop, or FC-AL) and subsequently in Fibre Channel, used to connect to peripheral subsystems.

Later, we will look at the InfiniBand I/O proposal, which has some chance of gaining traction in the years to come. For this success to come about, widespread industry support will be vital; at the time of writing, we are still waiting for confirmation of such commitment from an appropriate spectrum of industry players. For InfiniBand, we will concentrate on the functionality of I/O interfaces and the optimizations appropriate for communications in a loosely-coupled system, with an emphasis on clusters. Since data is the vital and central element of business information technology, the characteristics of the storage systems used are important factors in the choice of servers. Since data needs to be directly accessible—that is, on-line—permanently, both within a company and outside it (through the Internet), storage systems have become an essential part of a server. Trends in storage subsystems are ever-increasing capacity, performance, and availability, coupled with steadily reducing cost per gigabyte.

Communications support—both local and wide area—also has increasing importance. Reflecting this, specialized subsystems connected to servers have been developed.

Source of Information : Elsevier Server Architectures 2005

I/O