SCSI Express – Advancements in PCIe Storage

By: Bret Gibbs, Product Marketing Manager, Hewlett-Packard Company, SCSI Trade Association

SCSI Express is an emerging storage interface that runs Small Computer System Interface (SCSI) commands over a PCI Express (PCIe) link. It combines the enterprise-proven SCSI command set with the low latency of PCIe to create a powerful enterprise-class interface for the high-performance capabilities of SSD/flash storage.

The SCSI Trade Association (STA), a member-run industry association established to support and promote SCSI technology, has taken ownership of defining and marketing SCSI Express. STA is composed of member companies with expertise in SCSI technology who have a vested interest in developing and promoting technologies such as 12Gb/s SAS and SCSI Express to meet the growing storage performance needs of data centers. SCSI Express is being developed by the INCITS T10 Technical Committee, an ANSI accredited standards development organization.

So what has driven the need for a new storage interface like SCSI Express? Businesses not only need more storage capacity to meet data growth needs – they also need better performance out of their storage. One way to improve storage performance is to have the storage device communicate directly to the processor in a server via low-latency PCIe lanes. PCIe was never designed around storage and therefore a suitable storage protocol is needed to ensure the storage device properly comprehends storage commands while maximizing data integrity.

One option would be to build a brand new storage protocol from the ground up that is specifically designed for running over PCIe. However, that would take years of development and could potentially take several more years of early adopters testing out the new storage protocol to ensure it was rugged enough to handle the evolving needs of datacenters. What is really needed for PCIe storage is a storage protocol that has widespread adoption among Original Equipment Manufacturers (OEMs) and customers and is one of the most reliable and trusted protocols in the industry.

So what storage protocol is best suited to port over to PCIe? The SCSI protocol has been in the market over 30 years and Serial Attached SCSI (SAS) is the storage technology of choice for critical data storage. OEMs, storage vendors and customers are familiar with the SCSI protocol and SCSI devices. The tools used to manage SCSI devices are widely deployed, well understood and offer a rich management environment. SCSI Express preserves these benefits while requiring little modification of these storage management tools. With SAS drives being the current standard for enterprise storage, SCSI Express can easily coexist with existing SAS storage infrastructure. While SCSI commands have been transported across the PCIe bus for many years, these implementations have been vendor unique. Therefore, combining the robust and proven SCSI protocol with the low-latency features of PCIe as an industry-standard interface is the best solution to meet the performance needs of enterprise customers.

How does all of this technology talk translate into meeting business needs? SCSI Express is targeted at smaller block translation, high-frequency applications such as financial trading, databases, virtualization servers, online transactions and other applications that require lower latency from critical data storage. A primary goal of SCSI Express is to seamlessly integrate into a customer’s datacenter for simplified deployment and coexist with SAS and SATA storage to protect investments into existing storage infrastructure.

SCSI Express will also leverage a newly defined connector based on the SFF-8639 specification. This multi-function connector supports many common enterprise storage devices used today as well as offers an unprecedented flexible design to ensure support for future devices. In addition, SCSI Express devices will be supported in a common serviceable drive bay that supports multiple protocols such as SAS, SATA and SCSI Express. Figure 1 illustrates the common drive bay concept which demonstrates serviceable storage devices connected to a common drive backplane and multiple storage protocols coexisting within the same drive bay.

Figure 1: Illustration of a potential common drive bay configuration showing serviceable drives and multiple storage protocols coexisting within the same drive bay.

How does all of this technology work together? SCSI Express uses two T10 standards: SCSI over PCIe (SOP) and a PCIe Queuing Interface (PQI) – and utilizes the SFF-8639 connector. The SOP standard defines the rules for exchanging information between SCSI devices using a PCIe queuing mechanism. The PQI standard defines the transport methods for exchanging information between SCSI devices using a PCIe interconnect. This PQI standard defines the queuing layer used by SOP. A serviceable SCSI Express device connected to an SFF-8639 connector uses an SOP and PQI software driver and controller to communicate using the SCSI Express protocol. Figure 2 illustrates the communication paths of SAS/SATA/SCSI Express devices in a potential common drive bay configuration.

Figure 2: Communication paths of SAS/SATA/SCSI Express devices in common drive bay configuration

STA member companies are actively working with the INCITS T10 Technical Committee to enhance the SOP and PQI standards so that products can be delivered to market in 2013. The objective is to enable delivery of enterprise-class SCSI Express storage solutions that provide outstanding performance and reliability while preserving investment protection in existing storage technologies like SAS and SATA by deploying within a common drive bay. SCSI Express is clearly designed to accelerate business while seamlessly co-existing with traditional storage technology with a flexible design to meet future needs.

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