Take a Byte Out of Your Storage Problems

Continued from last week

LOOP? FABRIC? BOTH? 






You can deploy Fibre Channel SANs in one or both of the following topologies:

Arbitrated loop

Fabric (also called switched fabric) 





In the Loop

The arbitrated loop topology enables you to connect up to 127 Fibre Channel-enabled devices. For example, you can use a Fibre Channel hub to connect 100 network servers to 27 storage devices.

The arbitrated loop topology is similar to the token ring networking topology. Each device on the loop must arbitrate with the other devices to gain control of the transmission medium. When one device gains control, other devices are blocked from transmitting data until that device's I/O cycle is complete (no matter how long that cycle takes).

However, devices in an arbitrated loop do not pass tokens to determine which device can transmit data, as devices in a token ring networking topology do. Instead, devices in an arbitrated loop transmit a signal that tells other devices that they want to gain control of the loop. This signal contains the address of the device and an x value, which determines the order in which arbitration signals are forwarded through the loop. When a device receives its own signal back again--that is, after this signal has been forwarded all the way around the loop--that device gains control of the loop.

The main advantage of deploying the arbitrated loop topology is cost: In a Fibre Channel loop, you can use hubs, which are significantly less expensive than Fibre Channel switches. For example, a seven-port Fibre Channel hub from Compaq retails for $ 1,477. In contrast, a Compaq eight-port Fibre Channel Arbitrated Loop switch retails for $ 5,398.

The main disadvantage of deploying the arbitrated loop technology is that servers connected in this topology must share the available bandwidth. For example, if the arbitrated loop has a total available bandwidth of 1 GB and ten servers are attached to the loop, each server has an available bandwidth of only 100 MB.

Switch to High Gear

Just as Ethernet switches provide a virtual dedicated connection between devices on a LAN, so Fibre Channel fabric switches provide a virtual dedicated connection between network servers and the SAN. (See Figure 3.) That is, the fabric topology (which is sometimes called a switched or switched fabric topology) does not preclude several servers asking for, and receiving, access to the SAN simultaneously--even if these servers are requesting access to the same storage device.

If several servers request access to a particular storage device--such as a RAID controller--the Fibre Channel fabric switch interleaves requests from and responses to these servers. For example, if ten servers want to access a storage device that has a 1 GB connection, each server receives 100 MB of bandwidth.

On the other hand, if each of these servers wants to access a separate storage device, each server can use the full 1 GB of bandwidth. In other words, in the worst case--when all of the servers want to access the same storage device at the same time--the fabric topology gives each server a portion of the available bandwidth. (However, these servers still would not have to arbitrate for control of the loop, as they would if they were deployed in an arbitrated loop topology.) In the best case--when all of these servers want to access different storage devices--these servers have the entire bandwidth available on the Fibre Channel connection.

In either case, the fabric topology offers better SAN performance than the arbitrated loop topology offers. Therefore, if your company has applications that need fast, highly available access to storage, you probably need to deploy a fabric topology.

However, the fabric topology has some disadvantages, including the cost of Fibre Channel fabric switches and directors. In addition, using Fibre Channel switches, you can connect only as many network devices as the switch has ports. Of course, if your switch supports cascading, you can extend this number of connections by simply connecting additional switches to one or more of these ports.

You can also extend the number of connections by connecting an arbitrated loop to one of these ports. In fact, you can add switches and loops to support an almost unlimited number of connections. To the extent you do this, however, you increase both the cost and complexity of managing the resulting SAN.

Cheryl Walton works for Niche Associates

Reprinted with the permission of Novell Connection

(To be continued)