is available with previous bus architectures. In order to optimize performance, the PCI architecture strictly limits the number of loads (hence the number of adapters) on the bus. It therefore needs an I/O expansion bus to handle the more routine I/O devices.
The bus has 32 or 64 bits of address and data, is processor independent and is capable of speeds over 50 MHz.
Unique features of the PCI include parity on all bus lines and control lines. The parity is not optional as in other architectures, but is required. All PCI bus masters must support data streaming to memory devices.
1.6 Disk Subsystem
The disk subsystem is a critical element of server design. In this section we examine the controllers, the devices, and the interfaces between them. We will specifically address SCSI technology and also examine RAID technology in some detail.
1.6.1 Hard Disk Interfaces
The disk interface specifies the physical, electrical, and logical connections between the controller and the Direct Access Storage Devices (DASD). There have been four main interfaces developed thus far. Each possesses different characteristics and performance levels. The interfaces are:
1.ST506 - This interface was the original standard for microcomputers. It has a data transfer rate of 5 million bits per second (Mbps) between the controller and the DASD Device. It is a serial rather than a parallel interface. This interface is classified as a device level interface because the device itself has no logic to interpret commands. Functions such as formatting, head selection, and error detection are directed by the controller which is housed in an adapter card. A device level interface requires specific adapters and device drivers for each different type of device.
2.Enhanced Small Device Interface (ESDI) - This is an enhanced version of the ST506 interface. It provides a 10 Mbps data transfer rate (15 Mbps in some implementations). ESDI devices were the first to use a type of data encoding called Run Length Limited (RLL) which results in denser storage and faster data transfer than the older modified frequency modulation (MFM) technique. However, it is still a device level, serial interface.
3.Integrated Drive Electronics (IDE) - This is a bus level interface meaning that the device controller is built into the device itself. The IDE interface was designed for the low cost PC market segment. The interface is flexible and has been enhanced over time. The latest enhancements include caching at the adapter level, a
16NetWare Integration Guide
