SAS and the SFF Revolution

Author: Franco Castaldini and Kent Bransford

Seagate Technology

Serial Attached SCSI (SAS)/Serial ATA (SATA) compatibility produces unprecedented storage synergies and efficiencies, enabling enterprises to meet both performance and capacity requirements with a single SAS infrastructure. SAS drives deliver the speed, reliability and scalability demanded in high-availability enterprise environments, while high-capacity SATA nearline drives are purpose-built for bulk storage applications, combining low cost per GB and enhanced reliability.

And now the SAS value proposition grows even stronger with the arrival of small form factor (SFF) 2.5-inch enterprise drives. These drives are spearheading a new class of high-I/O-density storage solutions that facilitate rapid and cost-effective storage consolidation. SFF 2.5-inch drives are 70 percent smaller than conventional 3.5-inch 10K drives, enabling them to deliver 130-150 percent greater system performance in the same footprint. They also use 40 percent less power and run cooler, reducing airflow requirements and enabling greater storage density.

SAS is an ideal complement to SFF 2.5-inch drives, as it seamlessly accommodates the greater drive counts found in high-density storage solutions. SAS delivers exceptional expandability by enabling direct cascading of multiple edge expanders (each expander connecting up to 128 devices). For still greater scalability, a single fan-out expander can aggregate up to 128 edge expanders, yielding a theoretical total of over 16,000 devices in a single SAS domain.

Furthermore, SAS proves an optimal match for the SFF 2.5-inch drive’s focus on maximum IOPS/U (Input Output Per Second/Unit), a metric that factors in both a storage device’s performance and its space requirements. SAS combines superior throughput (3.0 Gb/s) with compact cabling and connectors that use less enclosure space, enhancing airflow/cooling in high-density computing solutions such as 1U servers.

Benefits of SAS/SFF Synergies
The real-world benefits of these SAS/SFF synergies are significant: A 1U server equipped with six 2.5-inch SAS drives (such as Seagate’s new Savvio 10K.1) delivers full RAID 5 functionality in one-half the space required by a 2U server equipped with six 3.5-inch 10K RPM disk drives—and the 1U server will provide a 150 percent performance improvement.

But SAS goes beyond optimizing the efficiency and performance of 1U servers; it also ensures enormous flexibility when adding more storage capacity to them. SAS expanders work in concert with the Serial ATA Tunneling Protocol (STP) to enable a 1U server’s RAID controller to identify and communicate with SATA devices. When the server directs data to a SATA drive that’s connected to a SAS backplane with an edge expander, an STP connection is immediately opened to allow SATA frames to pass through the connection to the drive.

STP enables a 1U SAS server to seamlessly access high-capacity nearline drives, in order to periodically migrate data off the server’s SFF SAS drives and free up capacity for online, transactional applications. SAS storage environments (whether internal storage sub-systems or external storage arrays) can freely intermix 3.5-inch nearline-ready SATA drives (e.g., the new Seagate NL35 SATA) with both 3.5-inch high-performance SAS drives (such as Seagate’s Cheetah 15K.4) and SFF 2.5-inch high-performance SAS drives. A 1U SAS server can thus have ready access to both abundant bulk storage and additional high-performance storage.

SAS and 1U SFF Server Connectivity
A typical 1U SFF server might employ a SAS RAID controller with eight ports. These ports can accommodate the six SFF SAS drives in the RAID 5 array, while allowing each of the two remaining ports to connect one or more SAS storage arrays, whether internal or external.

Internal server storage sub-systems are less demanding of edge expanders because they generally entail a relatively small number of drives. In addition to requiring fewer expander ports than external arrays (see Figure 1 below), internal storage sub-systems typically do not cascade expanders, eliminating the need for large routing tables or exceptionally low latency. This enables use of lower-priced expanders that are more cost-effective when a 1U server requires only a modest number of additional drives.

External SAS arrays are intended for higher drive quantities, and consequently utilize edge expanders with more generous port counts. Incorporating table routing capabilities and extremely low latency, these more costly expanders can be cascaded together—even across separate enclosures—yielding a remarkable capability for storage expansion. A single 1U server port can thus communicate with a multitude of drives carried in several enclosures.

As the number of drives escalates, the potential for a single RAID controller port to be saturated increases. In such cases the dual-port architecture of SAS facilitates 1U server expandability with the improved performance of bandwidth aggregation. Constructing wide links from dual ports enables transmissions from multiple drives to be aggregated over a single large pipe to the 1U server’s RAID controller, eliminating the performance bottleneck that can arise when too many drives are connected to a single host port (Figure 1).

Figure 1


SAS compatibility with SATA has rewritten the rules for enterprise-class storage efficiency, and now the partnership of SFF 2.5-inch enterprise drives and SAS promises to raise the bar for storage value yet again. Whether in 1U servers or storage arrays, SAS will play an integral role in SFF-based storage solutions, its compelling blend of performance, scalability and compact cabling/connectors ideally complementing SFF’s emphasis on performance and efficiency.

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