Microsoft clarifies Exchange/NAS compatibility

Microsoft finally explains its Exchange/NAS position. The software giant says it will do its best to make the technology work, but believes NAS has major limitations. Upshot: It's not making any promises.

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In a new support bulletin posted on Microsofts' Web site Monday, the company has attempted to clarify the somewhat confusing situation for users attempting to store Exchange 2000 databases on network attached storage (NAS) devices. While not opening the gates wide to NAS, the restated policy stops short of categorically banning NAS.

Although Exchange 5.5 can be made to work effectively with NAS devices, Exchange 2000, the latest version of the popular mail server, presents new challenges. As the bulletin says, "For Exchange 2000, if access to a disk resource is processed through the network redirector, the disk storage system is not supported as a location for Exchange 2000 database files."

That's because Exchange 2000 requires a block level interface with the storage subsystem (normally found in direct attached and SAN storage) and most NAS boxes ? being file-level storage devices -- don't have that.

David Siroky, product manager for Exchange, says that the company has been in talks with at least three different NAS vendors and has established a path for NAS vendors to qualify their products to work with Exchange. According to Siroky, any of the technologies that put a block-level interface on top of the NAS filer should qualify. There are at least two ways to do that: software that presents a block map to the server (such as Network Appliance's Snap Manager series) or an iSCSI interface.

Microsoft will support Exchange with any NAS devices that accomplish that, according to the latest bulletin. They will give full support to those that meet the Windows Hardware Quality Lab certification for block mode storage devices. Siroky says Microsoft will give "best effort" support for non-storage device related issues to Exchange customers who use NAS devices that have the right technical characteristics but are not officially certified.

According to Siroky, the need for block level access stems from design decisions taken in 1997 to speed up message transport within Exchange. The IFS technology provides a way for mail and news server messages to be written and read from disk in a streamlined fashion. He claimed that message routing is 66% more efficient than in Exchange 5.5.

Don't look for the situation to change any time soon. IFS is an integral part of Exchange for at least the next two to three years, said Siroky.

Microsoft's clarification may not end the controversy over its conditional support of NAS, however.

"I think it's good that Microsoft will qualify NAS boxes," says Marc Farley, author of Building Storage Networks and an expert on network storage. "But this announcement is much more about Microsoft limiting its support costs than about any weakness in NAS technology."

Farley was particularly critical of Microsoft's implication that NAS is prone to corruption because data could be lost in cache during a power failure.

"Protecting cache during a power failure is a problem with any form of storage that uses cache," Farley points out. "It's insulting to imply that the industry can't power condition a NAS box. And it shows a complete lack of trust of their customers, including enterprise customers, who probably understand more about optimizing file I/O for storage than Microsoft. They may want to use large caches, whether that be with NAS, DAS, or SANs."

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