3.1 Current Mass Storage Architectures

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3.1 Current Mass Storage Architectures

Current data transfer protocols (such as IPI, HIPPI, and SCSI) have problems that limit their effectiveness in mass storage. The limitations, illustrated in Figure 3-1, are:

Figure 3-1. Current Limitations
graphics/03fig01.gif
  1. Limited speed

  2. Limited distance between devices

  3. Limited number of devices supported

For example, the Small Computer System Interface (SCSI) is restricted to:

  • the bus being no longer than 25 meters

  • 32 SCSI devices per bus

  • a double cable system

In today s modern computer system environments, these restrictions are both very limiting to design and confining in space ”and it gets worse . The two-byte wide SCSI P-cable limits configurations to 16 devices.

The single-ended SCSI protocol is limited to eight IDs or addresses per bus (seven devices and one controller), and wide differential SCSI is limited to 16 IDs per bus (15 devices and one controller) to configure a one-terabyte disk storage unit. A fully redundant disk array would require 30 SCSI IDs (two per bus).

These concerns about limited speed, distance, and number of devices caused the industry to start thinking about alternatives. The alternative is Fibre Channel.

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Storage Area Networks. Designing and Implementing a Mass Storage System
Storage Area Networks: Designing and Implementing a Mass Storage System
ISBN: 0130279595
EAN: 2147483647
Year: 2000
Pages: 88

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