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Host Adapters

Going the SAS Storage Way
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Unlike SATA controllers, SAS components will only be found on server class motherboards or as separate add-in cards for PCI-X or PCI Express. If you take another step forward and look at RAID controllers (Redundant Array of Inexpensive Drives), these are mostly sold as add-in cards due to their complexity. RAID cards do not only include the actual controller, but they have a hardware accelerator for parity calculation (called the XOR engine) as well as some cache memory. A limited amount is either soldered onto the controller card (usually up to 128 MB), or some cards allow for installing a DIMM or SO-DIMM instead.

You should identify your requirements first when selecting a host adapter or RAID controller, as there is an increasing range of products. Simple multi-port host adapters will be relatively cheap, while powerful RAID cards usually are a considerable investment. Think of where to place your storage devices as well: Using storage boxes requires at least one external connector. Rack mount server scenarios sometimes require low profile add-in cards.

If you think RAID, determine whether or not you need hardware acceleration. Some RAID cards burden the system processor(s) with XOR calculation for RAID 5 or 6; others have an XOR offload engine. Accelerated RAID is required for environments in which a server is not dedicated to storage only, such as databases and Web services.

All the host adapter cards that we present on the following pages support 300 MB/s speed per SAS port and are as flexible as described above when it comes to storage infrastructures. External ports are pretty much standard already, and both SAS and SATA hard drives are supported. All three cards use the PCI-X interface, but PCI Express versions are already in development.

Although all cards in this article are eight port models, this does not limit the maximum number of disk drives. Using a SAS expander (externally), you can attach all types of storage appliances to any of these adapters. As long as the quad link connection is fast enough for your applications, up to 122 drives are supported by the SAS protocol. Due to the performance impact of parity added RAID 5 or RAID 6, typical RAID setups will not be able to saturate the bandwidth of these quad links even when multiple drives are being used.

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    wdanforth , September 10, 2008 2:52 AM
    Thank you for the SAS lesson.
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    vinay_20 , December 12, 2011 11:44 AM
    It was a nice jest for SAS.