As the turn of the last millennium approached, it was clear that parallel SCSI had run its course as the interconnect of choice in network server storage. Ultra320 SCSI was the last generation of parallel SCSI and we will likely see the last of any new shipments this year.

A number of replacement technologies once considered for the volume server segment of the market, including a variety of SATA and Fibre Channel (FC) derivatives gave way, and Serial Attached SCSI (SAS) ultimately prevailed. Beginning late 2006, major network server platform releases embraced 3Gb/s SAS and there has been no looking back.

Server platform releases are significant because they establish a volume foothold in the mainstream segment of the multi-user server market, and provide a launching point for further market penetration. Now successfully rooted in these platforms, SAS is poised to further its penetration.

We will look at the opportunities that exist in adjacent markets and show how SAS intends to capture a larger portion of the storage enterprise.

SAS expands into external storage
First generation, 3Gb/s SAS solutions were more than adequate for storage environments that were essentially targeted as parallel SCSI replacements, but often more capable. They provided a sufficient range of RAID and JBOD storage solutions located in or near the server itself.

As shown in Figure 1 below, we refer to these connections as "Intra-Server Storage Connections". Emerging opportunities for SAS in external storage solutions exist at the "Host-Storage Connection" and the "Drive Interface Connection". The transition of SAS into the "Drive Interface connection" is well underway and examined first.

Figure 1: Network Server Storage Connections

Drive-Interface Connection
While not extensively covered in this article, it is important to review a few of the basic SAS attributes. SAS is the only storage connect that embraces both the high-performance and high reliability disk drive offered with SAS connected drives, as well as providing native compatibility with low-cost per gigabyte SATA drives.

This capability allows common storage components to span a variety of storage environments, including tiered storage solutions. The ability of the storage system to scale is a critical attribute for enterprise critical data storage solutions.

In addition, SAS offers other features that make it attractive external to the server:

- Scalability for 1000's of connections
- Cost effective, connection based switching elements (expanders)
- Preserves Logical SCSI
- Familiar Direct Attached Storage (DAS) software model
- Dual-ported connections supporting fault tolerant designs
- Aggregating multiple SAS ports into "wide-ports" for increased bandwidth
- External cascadable cabling capable of reaching 10's of meters

With 3Gb/s SAS firmly established in the mainstream server architecture, SAS began to seep into external storage architectures as a legitimate alternative to Fibre Channel (FC), and as a method for delivering large amounts of cost optimized storage with SATA drives.

Because 70 percent of enterprise drives are consumed in the volume network server segment of the market; the combination of multiple suppliers, drive selection (including small form-factor), and competitive pricing made SAS a viable and significant option for "back-side" connections on a variety of SAN and NAS storage solutions