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New connections: SAS and iSCSI HBAs

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SAS' shortcomings
SAS HBAs present their own set of management issues. Despite support within the SAS protocol for more than 16,000 storage devices in a SAS domain, the most storage devices that the firmware on many SAS HBAs can address at one time is a few hundred.

Zoning and disk drive security are other functions that are available only in rudimentary forms in SAS domains at this time. To ensure SAS HBAs can access only assigned LUNs, administrators must first assign LUNs to a port on a SAS array, control access to the LUNs by using partitions or LUN masking on the SAS storage array, and then implement zoning on the SAS expanders. (There are two types of SAS expanders: the edge expander and the fan-out expander. A SAS edge expander, which plugs into the SAS port on a server, will support up to 128 devices without enhancement over distances of up to eight meters. A fan-out expander, which offers more routing capability and other features at a higher price, can also connect up to 128 devices.)

LSI Logic recommends that users limit the number of zones on SAS expanders to no more than three, as the expanders must arbitrate the flow of data to the different storage devices attached to them. The number and ability to create SAS zones is limited to fan-out expanders and hybrid expanders (combination of fan-out and edge

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expanders), although some of these zoning limitations are addressed in the current working draft of the SAS-2 specification.

This was first published in April 2007

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