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The case of the disappearing devices

Hubs disappear in FC-AL SANS. Here's how to find them.

A hub-based network -- a number of devices or other hubs attached to a central hub in a star topology -- is a popular arrangement for FC-AL SANs. However this arrangement can present problems for SAN management software, which is trying to inventory devices on the network as the first stage of managing the SAN.

To manage the SAN, the software has to know what devices are connected to it. Commonly, the software polls the devices using SNMP over IP (an out-of-band technique) and builds an SNMP Management Information Base (MIB) with the data. From this the software extracts the information necessary to identify the topology and presents the administrator with a topology map to the user.

In a conventional FC-AL SAN, the devices are connected in series in a ring with the devices arbitrating among themselves for control of the bus. Hubs, in effect, collapse the loop -- or part of it -- into a single device. This allows the SAN to bypass failed devices, but not all hubs provide information on all the devices connected to them. In that case the management software may see the hub but not the connected devices. The result is that the map presented to the administration is inaccurate and the software cannot manage the SAN properly.

The good news is that this is becoming less common as SAN hubs become more sophisticated. The bad news is that because FC-AL SANs are generally simple, small storage networks that are often used by organizations lacking sophisticated management tools and personnel commonly found on larger, fabric-based SANs, the person using the management software may not understand what is happening.

There are two ways to handle the problem of disappearing devices. More and more hubs provide the necessary information for the management software to untangle the real topology. Implementing a "link table" in the hub that complies with the Fibre Alliance MIB standard 2.2 or later usually does this. The table gives the management software the information it needs to know which devices are connected to the hub.

The second solution is simply to go in and manually provide the information. Assuming the management software cooperates, this is a straightforward, if time-consuming, method of handling the problem. To make this work the software must allow the administrator to adjust the representation of the topology. Ideally, the software should give the administrator an intuitive view of the loop topology of the SAN and its devices as the software has found it and also present any hubs that are found without connection information. The administrator should be able to match each orphan hub with the devices attached and the connections between them. The information should be stored as part of the SAN representation in the software.

A white paper discussing the issue is available at

Rick Cook has been writing about mass storage since the days when the term meant an 80K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.

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