Section 4.3. Advantages of Fibre Channel | Inside Windows Storage: Server Storage Technologies for Windows 2000, Windows Server 2003 and Beyond

4.3 Advantages of Fibre Channel

Before diving into the details of Fibre Channel, let's look at the advantages of deploying Fibre Channel “based storage area networks. Although the advantages discussed in Sections 4.3.1 through 4.3.7 are currently accurate, keep in mind that none of these features are permanent and they need to be continuously reevaluated, especially in view of technological advances and adoption patterns.

Table 4.1. Comparison of Network-Attached Storage and Storage Area Networks

Network-Attached Storage	Storage Area Networks
NAS provides access to files over a network. Because users are aware of the existence of files ”for example, Microsoft Excel spreadsheet files or Word user document files ”NAS devices are intuitively easier to understand.	SANs provide access to disk blocks over a network. Users are typically aware of the existence of files. A file system maintains the abstraction between files and disk blocks as to which particular disk blocks are used to store which particular files and so on.
NAS uses network file access protocols such as Common Internet File System (CIFS) or Network File System (NFS). CIFS and NFS typically are the payload of TCP/IP, which itself is typically the payload of Ethernet.	SANs provide access to disk blocks typically using Fibre Channel protocol, which has SCSI as its upper-layer protocol.
NAS devices appear to be servers from which clients access files.	Devices on a SAN appear to be storage units that are accessible to initiators.
NAS devices can be fairly easily plugged into an existing LAN. NAS does not imply adding more network complexity, but at the cost of leaving all traffic on an existing LAN.	A SAN implies creating a new network architecture that is typically FC based. The new network relieves network congestion and allows new applications such as LAN-free backup.
Adding NAS devices to a LAN is not costly.	Adding SAN to an existing LAN-based network is costly.
NAS typically works on Ethernet at 100 Mbps (megabits per second) and also at gigabit speeds when the LAN is based on Gigabit Ethernet.	Fibre Channel SANs typically work at a rate of 1 Gbps or 2 Gbps.

4.3.1 Scalability

SANs are highly scalable in terms of the volume of data and throughput they can handle. One can simply add switches as needed and grow from a departmental loop that connects multiple loops to a backbone that cascades a hierarchy of switches. Of course, IP networks are also scalable, and that is one reason why IP storage (described in Chapter 8) is also developing rapidly .

4.3.2 Segregation of Storage

When properly configured and managed, SANs offer the best of both worlds : The data storage is decoupled from the application server, yet data storage can be segregated to ensure data privacy and integrity. For example, data may be segregated on a departmental basis, yet exist in a common storage pool.

4.3.3 Centralization and Management of Storage

SANs allow storage to be consolidated, used optimally, and managed more easily. The biggest advantage is that a lot of data duplication that happens without SANs is avoided. Another big advantage is storage allocation, avoiding situations in which one particular server has too much storage while another starves. Efficient storage allocation also minimizes costs that are needlessly incurred to add storage to the server that is starved (of storage). For example, if all disks are centrally administered, storage may be reassigned from a server that has too much to a server that has too little. Another example would be storage needed to create a temporary snapshot for backup purposes. In the classical way, each server would have its own dedicated snapshot disk. With a SAN, all the servers can share a couple of disks that are used for creating temporary snapshots.

4.3.4 Legacy Device Support

SANs offer the ability to protect existing investment in legacy devices ”for example, SCSI storage ”or even, from an ultramodern point of view, to support legacy devices such as loop hubs. Bridge devices that provide connectivity between older devices and FC are often the backbone of legacy device support. Of course, not all legacy devices can be supported.

4.3.5 Support for More Devices

SANs provide support for more devices. Whereas an arbitrated loop can support a theoretical maximum of 127 ports (up to 15 in typical installations, and a practical maximum of about 50 in terms of realistic latency problems), a fabric SAN can theoretically support approximately 15 million (2 ²⁴ ) ports.

4.3.6 Distance

SANs allow storage to be located at increasingly greater distances from the hosts (servers or workstations) that access them. Whereas SCSI can support distances of only a few tens of meters , with FC we're typically talking about distances of tens of kilometers.

4.3.7 New Functionality Enabled

FC enables some brand-new functionality. One example is LAN-free backup, in which all backup traffic is routed onto the SAN, leaving the LAN free for clients or servers to access the storage. Another example of new functionality is that data can be moved directly between two storage subsystems without first having to be moved to a server. Yet another example is SAN file systems (described in Chapter 6) that allow multiple servers to directly access and simultaneously share the same file system volumes .

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