Mixing it Up with SAS, SATA

Author; Kent Smith, Product Manager,
LSI Corporation

Exponentially increasing storage requirements in data centers, combined with the implications of regulations such as Sarbanes Oxley and HIPAA, entails large amounts of “business-important” information that must be periodically backed up and accessed while remaining readily available. IT managers are under extreme pressure to find storage solutions that deliver more capacity for less money – without sacrificing reliability or data availability.

In order to address this requirement, nearline enterprise disk array systems are emerging that retain the Fibre Channel (FC) infrastructure on the front-end of the array (in order to fit into existing SANs), while using a mix of Serial Attached SCSI (SAS) and Serial ATA (SATA) drives instead of more expensive FC drives. Such mixed-technology systems reduce costs while retaining the availability, manageability, data integrity and functionality achieved with FC-based storage systems. Though SATA-based storage solutions are ideally suited for the specific workload, capacity, and cost requirements of “secondary storage”, SAS-based storage solutions provide performance and reliability comparable to current FC storage solutions.

The SAS Advantage
SAS provides a number of advantages over parallel SCSI and FC. SAS addresses the high frequency, immediate random data access required for transactional applications that is conventionally addressed by FC. SAS also combines the proven reliability and functionality of parallel SCSI with the performance and design advantages of serial technology to deliver performance, flexibility, scalability, reliability and availability. Yet perhaps the most significant advantage of SAS is that its backplane design and protocol interface allow the use of both SAS and SATA drives in the same system. The ability to mix and match different drive types appropriate for the application is a benefit for both integrators and users alike.

SAS/SATA compatibility enables systems integrators to design hybrid storage systems using common connectors and cabling. SAS’ backward compatibility with previous-generation SCSI software and middleware also makes it easy to incorporate legacy components, hosts, and drives into evolving SAS topologies, minimizing or even eliminating training or integration costs and the need for modifications to legacy software.

SAS also provides a broad address range to physical devices, long cables with small connectors, and connectivity to external storage systems. SAS uses expander hardware as a switch to simplify configuration of large external storage systems that can be easily scaled with minimal latency while preserving bandwidth for increased workloads. Expanders enable highly flexible storage topologies of up to 16,256 mixed SAS/SATA drives.


Figure 1 – Scalability and SAS/SATA Compatibility Using SAS Expanders


The Need for Dual-Port Disk Drives
Numerous configurations are supported by SAS to provide disk drive access to multiple hosts and/or host bus adapters and to ensure continuous disk access in case a system fails. With SAS, dual-port devices can be used to build high-availability systems with no single point of failure. Fault tolerance can also be increased with SAS by using expanders to connect multiple devices to multiple initiators. The SAS expander, in combination with dual-port SAS drives or the use of interposer cards to add dual-port functionality to SATA drives, makes it easy to design redundant systems for maximum fault tolerance and high availability.

The SAS backplane connector has both SAS Port 1 and SAS Port 2 signal groups, along with power traces. The SATA backplane connector looks exactly like the SAS connector except that it provides only one signal port and has a key that mates with the notch in the SATA disk drive connector. This key is removed in the SAS backplane connector and replaced with the signals for the second SAS port (see the figure below). Because of the absence of this key, SAS backplane connectors will accept both SAS and SATA disk drives, while SATA backplane connectors will only accept SATA disk drives.


Figure 2 – SAS and SATA Disk Drive Connectors and SAS Backplane Connector


Because SATA drives offer only a single port, an interposer card is required to support dual-port functionality with a SATA drive.

Shown below is a block diagram of an interposer card designed to interface a single-port SATA drive to a SAS backplane driven by dual SAS expanders.


Figure 3 – Block Diagram of an Interposer Card based on an Active-Active Multiplexer


The interposer card is implemented using an active-active multiplexer chip. The chip supports a SATA connector on the drive side that connects with the single-port SATA drive and a SAS connector that plugs into a SAS backplane or midplane, providing two SAS/STP-compliant ports to the expander. The interposer card implements two separate physical ports from the upstream SAS expander to each single-port SATA HDD, creating redundancy via two independent paths, as required in a high availability environment.

With an active-active multiplexer, two hosts can access a single SATA storage device independently, each through its own SATA interface. In this case, the active-active multiplexer emulates dual-port SAS capability with a single-port SATA drive. The active-active multiplexer provides seamless dual-host support with practically no switching latency, while transparently supporting enhanced SATA drive features like native command queuing (NCQ). This dual-port SAS emulation results in simplification of the storage management software from a failover, clustering, and high availability perspective.

SAS-based storage systems combine the proven reliability and functionality of parallel SCSI with the performance and design advantages of serial storage technology to deliver performance, flexibility, scalability, reliability and availability. Because SAS systems accommodate the SATA serial transport interface, SAS provides a level of flexibility to intermix SAS and SATA drives in the same array, meeting next generation enterprise storage needs. The availability of dual-port capability for SATA drives, through the use of interposer cards based on active-active multiplexer technology, provides a lower cost, higher capacity alternative to FC or SAS drives for SAS-based systems targeted at cost-sensitive nearline storage applications. As a result, many new SAS-based storage array designs will implement active-active multiplexers as the architecture of choice for enterprise SATA drive connectivity.


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