Fine-tune storage networks


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When to use an SRM tool
Storage resource management (SRM) tools such as Hewlett-Packard Co.'s Storage Essentials and EMC Corp.'s ControlCenter offer optional features that collect and correlate performance data from SAN devices. In some cases, the tool can manage the devices or even dynamically tune them based on pre-set policies. Yet when compared to the free or low-cost tools that ship with storage devices, it's sometimes difficult to justify their purchase. Here are some tips to help you decide if and when deploying an SRM tool is warranted:

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In a large environment with multiple SAN devices, the hassle of learning and managing each vendor-specific tool makes a comprehensive SRM application that's capable of tuning all of the devices a cost-effective alternative.

Check to see if the SRM tool can act as a dashboard that launches device-specific management tools.

The biggest benefit of an SRM tool is its ability to visualize, correlate and analyze performance data from all of the SAN devices that the application data touches or passes through.

SRM tools generally operate under the assumption that one IT group has control of all the devices in a SAN--from the host down to the storage array. This may not be the way your shop operates.

While SRM vendors provide flexible product licensing, you'll still have to negotiate. Consider deals that tie SRM licensing to the total number of managed SAN ports or total amount of managed storage capacity--those are the numbers that tend to outstrip server and CPU growth over time.

Too many server ports in the same zone can also degrade SAN performance. This will most likely occur during server boot ups, and while probing the SAN for new storage ports or existing storage ports for new LUNs. When the server initiates these probes, the server HBA starts to probe each port it has access to on the SAN. Although the server HBA software should recognize the difference between server HBAs ("initiators") and storage and tape HBAs ("targets"), some HBA drivers are problematic and probe server HBAs until they timeout, delaying server boots and new volume discoveries. The best way to avoid this is to keep each server HBA in its own zone with its own set of targets.

How and where application servers are attached to a growing SAN can also affect performance. Administrators may be tempted to simply attach new servers to a new FC switch or director, connect the new switch to an existing SAN director using an inter-switch link (ISL) and allocate storage. This is a bad idea for two reasons. First, new directors will generally have higher performance characteristics, and storage and tape devices generally should have first dibs on the highest performing ports. Second, the technology on new servers may support higher levels of SAN performance, such as 2Gb/sec or 4Gb/sec speeds, and if forced to access storage on the core SAN director via an ISL, performance will degrade to the level of the slowest component in the storage path.

With Cisco Systems Inc. SANs, the MDS 9000's SAN-OS remote switched port analyzer (RSPAN) functionality can identify these types of performance bottlenecks. The RSPAN utility mirrors traffic from one or more FC interfaces to another port in the fabric, and monitors traffic without the insertion of a probe. The traffic may also be captured and encapsulated by Cisco's optional Port Analyzer Adapter and sent to a PC for SCSI I/O and FC frame-level analysis.

The best way to avoid these performance problems is to keep the highest performing technology at the core. This typically involves moving the highest performing application servers, storage arrays, virtual tape libraries and tape drives to the new core director and relocating lower performing applications to edge switches. When allocating core director ports, priority should be given to the highest performing applications, such as backup, databases and file servers.

An ongoing process
Of course, SAN tuning is a work in progress. You need to constantly monitor and tinker with your storage fabric (see "Performance tips for arrays and tape drives," and "When to use an SRM tool") on a regular basis to get the best performance. By taking simple steps to tune SAN components before they're installed in the SAN, and knowing which ones to analyze first when performance hiccups occur, the number of performance issues will be greatly reduced down the road.

This was first published in April 2006

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