Serial Attached SCSI emerges: profound changes ahead

Computer Technology Review, June-July, 2005 by Harry Mason

Nearly four years after work on the Serial Attached SCSI (SAS) specification began, SAS products have now entered the market. Thousands of meeting hours and specification writing, plus component and infrastructure development, have all culminated in the first round of servers, host bus adapters, disk drives, enclosures and production-capable infrastructure.

Initial Deployment of SAS

Initial SAS deployments will take on one of two evolutionary forms, the first being a simple replacement for parallel SCSI. SCSI is still the dominant enterprise drive connection scheme nearly 25 years after initial shipments and preserving this industry investment is paramount in sustaining many business critical environments. Most of the initial SAS deployments, performing as a parallel SCSI replacement, will be found in traditional Direct Attached Storage (DAS) environments located in or near the server.

Parallel SCSI Replacement

As a parallel SCSI replacement, SAS maintains legacy SCSI middleware compatibility and extends its value by improving overall system scalability (distance and addressing), improved availability, (dual-porting), improved performance (full-duplex and port aggregation) and improved reliability (point-to-point architecture).

An incremental, but incredibly significant value addition to SAS, is the ability for both enterprise-class SAS performance-oriented drives and desktop-class Serial ATA (SATA) low-cost bulk storage drives, to co-exist within the same SAS infrastructure. This dramatically augments the legacy value of SCSI by allowing architectures that can support a variety of tiered storage application needs. These deployments will leverage common building block components to span the diverse storage needs in terms of performance and cost (Table 1).

SATA Uncompromised

In the second type of early deployments, SAS will emerge as a "super-sized" version of SATA. SAS has often been called SATA without the compromises. SATA deployments in the enterprise have faced numerous obstacles. Routing SATA signaling across back planes, scaling SATA to meaningful drive counts, providing robust failover mechanisms, as well as supporting multi-initiator environments come with their own set of challenges.

SAS neutralizes a number of these challenges by providing an industry-standard method of bridging between the two technologies. Using the SATA tunneling protocol (inherent to the SAS standard), SATA connections can be managed at the endpoints of the topology, usually between a SAS expander and the SATA drive, without requiring significant routing of SATA lines across backplanes. This can greatly improve signal integrity within these SATA-based subsystem solutions and can significantly enhance overall system reliability. Additionally, SATA cabling schemes offer significant limitations in terms of providing meaningful storage expansion external to the server. Routing SATA protocol over the enterprise-capable SAS infrastructure removes these system-scaling barriers for SATA-based systems. Finally, the ability of SAS to scale beyond 16,000 connections makes it suitable for implementing systems requiring large numbers of SATA drives.

Supporting SATA drives through SAS expanders also provides a means of supporting the enterprise necessities of failover and multi-initiator when implementing SATA-based storage systems. SATA drives are primarily cost driven and as such, most new capabilities in SATA II are considered to be optional (so as not to impose further costs on the primary volume desktop market segments). SAS functionality encompasses all of these SATA enhancements and provides an interoperability framework for SATA and SATA II offerings with dissimilar feature sets. In addition, SAS RAID controllers will support RAID 6 capabilities, allowing a second drive to fail during the rebuild cycle of an initial drive failure--an especially important and significant feature for "fat" and relatively slow SATA drives.

We expect to see initial "super-sized" SATA deployments occurring in some traditional DAS-based infrastructures, but mostly into new categories of tiered storage solutions including disk-to-disk copy, near-line storage and other archiving solutions. A bulk of these deployments will occur in the emerging segments of the Storage Area Network (SAN) and Network Attached Storage (NAS) subsystem markets that have begun to embrace SATA in the enterprise.

SAS Impact on Server Architecture

These initial deployment models, while very meaningful, are just the beginning. SAS promises to make an even more profound impact on server architecture, as the technology matures and becomes more widely deployed. Some key themes that will influence how servers are implemented and deployed are discussed in the paragraphs below.

Size Matters

SAS was architected to embrace small form factor (SFF) 2.5" enterprise drives. Cable and connector form factors allow for new system capabilities that are just now beginning to be explored. These smaller drives allow for greater airflow in temperature-challenged server environments, allow for full RAID 5 or 6 implementations that can be housed in IU "rackable" form factors and allow for a dramatically large number of drives to be configured in storage racks and shelves, thereby permitting an overall reduction in computer room floor space. Furthermore, server blades are often configured with dualboot drives and the 2.5" form factor promises to greatly improve the size, cooling and rackability for this category of server blades.