2.8 Storage Area Networks (SANs)

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2.8 Storage Area Networks (SANs)

The evolution of storage brings us to Storage Area Networks (SANs), described in detail in Chapter 4.

A Storage Area Network is based on the principle of networking devices together. As you learned in Chapter 1, the SAN consists of a group of mass storage devices (disk arrays and tape libraries) connected indirectly to host computer systems and each other by means of interconnecting devices such as hubs or switches.

Figure 2-10. Example of a SAN

Where LANs connect host computer systems together to share the access to servers and stored data, SANs connect storage devices together to provide host computer systems (and users) high-speed and immediate access to data. SANs also provide host computer systems multiple routes to data, as a measure of protection if failures happen. In some LANs where SANs are employed, the host computer system in control of some specific data can be failed over to another host system to guarantee access to data in case of a system failure. It can be configured so that any host can access any storage unit.

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Chapter 3. A Brief Review of Fibre Channel

This chapter discusses:

Limitations of current mass storage architectures
How Fibre Channel answers these limitations
Advantages of Fibre Channel
Fibre Channel basics
Topologies
Fibre Channel functional levels and protocols
Fibre Channel Arbitrated Loop (FC-AL)
Hubs and topologies

Fibre Channel is the enabling technology of the SAN. This chapter explains the significant concepts.

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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

Limited speed
Limited distance between devices
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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3.2 How Does Fibre Channel Help?

Fibre Channel is a new communications protocol designed to overcome the limitations of existing architectures. It is a generic data transport mechanism with the primary task of transporting data at the fastest rate possible using current technology. Fibre Channel is a scalable interface for achieving high-speed data transfer rates among heterogeneous systems and peripherals. System types include supercomputers, mainframes, workstations, and desktop PCs.

Peripherals include mass storage devices such as disk arrays and possibly tape libraries. The main purpose of Fibre Channel is to have any number of existing protocols over a variety of physical media and existing cable options.

Table 3-1 shows the various speeds and distances that can be attained using different cable types.

Table 3-1. Cable Types, Speeds, and Distances

SPEED (Mbps)	9 µm Single Mode	50 µm Multimode	62.5 µm Multimode	COAX	Mini COAX	TWINAX	STP
133				100 m	42 m	93 m	80 m
266	10 km	2 km	1 km	100 m	28 m	66 m	57 m
533	10 km	1 km	1km	71 m	19 m	46 m	48 m
1063	10 km	500 m	175 m	50 m	14 m	33 m	28 m
2125	2 km	500 m
4250	2 km	175 m

3.2.1 Fibre Channel Use for Mass Storage

Since Fibre Channel is a generic data transport mechanism, it can transmit a number of existing networking and I/O protocols:

I/O protocols:

SCSI
HIPPI
IPI

Network protocols:

IP
IEEE 802.2

Hewlett-Packard has chosen to support the SCSI-3 protocol over Fibre Channel for its mass storage environment. Mass storage consists of several device classes: