SAN Topologies


Figure 12.1 shows the four different topologies you can use to connect to storage. The simplest topology is not a network topology at all but a straightforward bus connection. Here you connect to a disk within your server, which is also called captive disk. In high-performance servers, SCSI is the dominant interconnect technology. For low-end servers, captive disk tends to be connected using Serial ATA (SATA) or parallel ATA (PATA) drives. Fibre Channel and Gigabit Ethernet (GigE) rarely play in this space.

Figure 12.1. Four different storage connection topologies: DAS, point-to-point, loop (with and without a hub), and a fabric topology.


You can also connect to external storage by using a point-to-point topology, and that's called direct attached storage (DAS). Many people refer to internal disk as part of DAS, and the two are indeed rather similar.

SCSI dominates the DAS marketplace, but Fibre Channel is also used in many cases. There are fewer instances of DAS using GigE, but that's probably because very-high-speed GigE has only been available for a short period of time. DAS also isn't really a networking topology. With dual-homed Fibre Channel NICs you can create a loop topology, the same topology used in IBM's Token Ring network. A Fibre Channel loop of this kind is called a Fibre Channel Arbitrated Loop or (FC-AL).

With 126 theoretical nodes and 100MBps per channel, FC-AL dominates the market for this kind of topology. The advantages of FC-AL are that it is self-configuring, allows for hot-swapping of devices, and can use the same software as SCSI. With coaxial runs of 30 meters and fiber runs of up to 10km, these advantages have kept Fibre Channel in play, particularly in this simple kind of network. It's not necessary to daisy-chain together a Fibre Channel loop; you can achieve the same result by using a Fibre Channel hub.

The main problems with Fibre Channel in terms of storage networking are that it is relatively expensive and that it requires some specific expertise that most network administrators don't normally develop. For those reasons, desktop vendors have been promoting FireWire (IEEE 1394) and high-speed USB as desktop alternatives for small bus storage networks. Both of these alternatives are backbones and not loops, and neither offers the fault tolerance or performance of Fibre Channel. No other technology is implemented as a loop structure.

The last topology you find in SANs is a fabric technology. Nearly all modern SANs are built using a fabric connecting Fibre Channel or GigE together using a switch. The advantage of the fabric architecture is that it offers the most flexibility as well as the easiest manageability. Storage APIs located at the switch can act like a traffic cop, which gives the fabric topology a significant advantage over other topologies. When you build a fabric from Fibre Channel, you are creating two separate networks: an out-of-band Fibre Channel network for storage and an in-band Ethernet network for hosts and clients. However, when you build a network using an Ethernet technology, there is only one network for all system devices. This fact has led some people to suggest that the term storage area network should be reserved for the dual-network type of solution, whereas the term systems area network should be used when all devices are on the same wire protocol (Ethernet). Perhaps this is just semantics, but it is worth noting.

It is worth spending a moment considering how a fabric architecture influences your decision about where to focus your management efforts. A fabric has three intelligent devices: the host or server, the switch, and the storage server. Software can run on any one of these devices, so even if you are viewing the performance of your SAN devices from a host, it may actually be the routines at the switch that are doing the probing and collecting. The host-based software is simply displaying the data. Deciding what kind of software you buy and where your control and management functions for a SAN take place is a fundamental decision. It can be the difference between purchasing a very high-end storage array or a cheaper one, a fully managed switch or an unmanaged switch, and so on. Unfortunately, there are no right and wrong answers here. Each of these three SAN devices has advantages and disadvantages from a management standpoint, so your conclusion may be based entirely on the hardware you already have or are committed to using.




Upgrading and Repairing Servers
Upgrading and Repairing Servers
ISBN: 078972815X
EAN: 2147483647
Year: 2006
Pages: 240

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