Mix SAS/SATA drives for speed or capacity


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Anticipated benefits
One of the key benefits of mixed SAS and SATA architectures is that the same disk-drive socket can accommodate SAS or SATA disk drives. This differs from the current generation of mixed FC and SATA storage arrays that don't permit the insertion of SATA disk drives into FC sockets or vice versa. Using both types of drives in the same socket allows users to grow performance or capacity using existing free sockets in the storage arrays while simplifying the planning process around obtaining more sockets for disk drives.

The SAS protocol also possesses characteristics that, while not as strong as FC, are satisfactory for the demands most enterprises place upon their storage arrays. For instance, switched FC supports 4Gb/sec speeds and can address more than 16 million devices, while SAS offers a switched 3Gb/sec back end and can address 4,032 devices per port. Even though SAS speeds and the number of addressable units are lower than those for FC, they're sufficiently large to allow mixed SAS and SATA storage arrays to grow to meet most enterprise requirements for speed and capacity.

NEC Corp. of America's D-Series storage line is one of the first to capitalize on the benefits SAS offers. The D-Series is available in six models from the entry-level 72-drive 54TB D1-10 to the enterprise 1,536-drive 1.15 petabyte (PB) D8-1040. Being fully modular, the D-Series lets users upgrade from the D1-10 model to the D8-1040 model without a rip-and-replace

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strategy. SAS makes these types of modular configurations possible and, in so doing, helps to eliminate the need to deploy different storage models for midrange and enterprise requirements and recreate storage islands.

Products like Dot Hill Systems Corp.'s 2730T, Excel Meridian's SecurStor Astra ES and Overland Storage's Ultamus RAID 1200 similarly capitalize on the SAS protocol's ability to address larger numbers of disk drives, as these models include expansion packs that let users grow each of their base units from 12 disk drives to 60 disk drives. However, these are only JBODs and they don't allow users to introduce more controllers to handle additional performance requirements, although with Excel Meridian you can upgrade to a more powerful controller. This is where NEC maintains a distinct advantage for now, as users can scale its D-Series up to 16 controllers on NEC's D8-1040 to handle the increase in performance required by the introduction of more disk drives. Yet what NEC's D-Series and most other storage systems in this class don't yet accomplish is automatically placing data on the most appropriate tier of disk.

Click here for an overview of Clustered SAS/SATA disk drives (PDF).

This was first published in July 2007

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