From SCSI to SAS with integrated Intel I/O processors

Author: Brad Corrion, Product Marketing Engineer
Intel Corporation

The evolution and innovation of a new technology generally is paced by the demands of its user base. New technologies can grow and adapt quickly, rapidly responding to the needs of a small user base. Ironically, when the technology becomes popular and entrenched, the innovations focus on extending the shelf life of the current technology to minimize user disruption. For example, parallel shared bus architectures such as PCI were made wider and clocked faster in order to move more data more quickly, but without significantly altering their underlying architecture to support backward compatibility.

When simple extensions no longer address fundamental technological challenges, radical reinvention occurs. The best bits are retained and the rest is reconsidered. Again, consider the parallel shared PCI bus that is quickly being replaced with the serial, point-to-point PCI Express bus. They share many conventions and compatibilities, but the technologies are fundamentally different.

Similarly, the storage industry is witnessing SCSI’s big leap to Serial Attached SCSI, or SAS. Like PCI Express, SAS drastically reduces the scalability and design challenges of parallel SCSI while also bringing new capabilities which parallel SCSI could not offer. Timed to leverage SAS adoption momentum, the recently launched Intel IOP34x Storage Processor Family integrates SAS/SATA controllers into the sixth generation of Intel’s I/O processor line to provide end-product ease of use, data protection, and the flexibility of multi-protocol storage technologies.

Ease of Use

With SAS, parallel SCSI cabling headaches are a thing of the past. Point-to-point serial technologies don’t require bus termination. And where the SCSI shared bus required coordinated jumper settings on the attached drives, SAS drives simply plug into the SAS cables and backplanes. No jumpers are required.

Increased clock frequencies caused signal integrity issues on parallel SCSI product designs, requiring additional engineering expense to resolve. In addition, the shared bus architecture had practical limits to the number of devices that could be attached to a bus. SAS departs from the wide, flat parallel SCSI cables and instead relies on differential pairs of serial cables. The pairings work together to reduce signal integrity challenges, such as crosstalk, resulting in cables that can span many meters and create spatially large drive arrays and topologies.

Cabling and signal routing may be invisible to many end users but are of great concern to system designers. The simpler cabling requirements of SAS both reduce board design complexity and, perhaps more importantly, improve internal ventilation in storage systems, as the smaller SAS cables enable greater airflow than the wide, flat SCSI cables. Better airflow leads to reduced fan noise and quieter systems, or systems with more available processing power and drive capacity due to increased cooling potential.

By departing from a shared bus cabling model, point-to-point SAS cabling may require expanders to attach additional storage devices. For example, the Intel® IOP348 I/O Processor allows eight SAS and SATA devices to be directly attached to its ports, but the IOP348 also supports expander-attached SAS and SATA drives for larger drive arrays. Note that not all SAS products support both direct- and expander-attached SAS and SATA connections, but the IOP348 supports these modes with expander trees up to seven levels deep and addressing 256 total target devices.

High Availability

IT departments demand the highest availability of storage devices in their network infrastructure. A key way to ensure availability is through the use of redundancy in storage systems. The core innovation of RAID focused around using redundant disk drives to maximize drive array capacity while managing for drive failure without data loss or system downtime. Entire drive arrays may be duplicated with redundant storage controllers; in the event that one controller fails, the other is still operational. Power supplies often come in pairs so that one is ready to cover the failure of another.

Going a step further, SAS supports dual-port drives so that a device may be controlled by two simultaneously-connected I/O controllers. Should one controller go down, the other controller simply takes over control of the drive array. Using the PCI Express host interfaces of the Intel® IOP34x Storage Processor Family, two storage processors can synchronize data to ensure that a RAID cache is consistent between the processors should one go offline. This use of interfaces enables high bandwidth cache synchronization with the industry-standard PCI Express bus while maximizing SAS/SATA device connectivity from the IOP348.

SAS delivers another form of high availability at the physical level. While SAS connections utilize paired copper signals, multiple pairs of connections can be grouped together into logical “wide-port” groupings. The IOP348 supports a maximum of two x4 wide-port connections. While the wide-port connections are logical and have the same performance as the sum of the individual pairs, when treated as a wide-port connection an individual pair can go offline without affecting availability. The wide-port link will accept the lost pair with a drop in performance, but availability is maintained between the two end-points.

SATA Compatibility

Serial ATA (SATA) disk technologies have been evolving over the past few years, primarily in the desktop computer and workstation segments. As SAS is doing for parallel SCSI, SATA is improving on challenges facing parallel ATA disk technologies such as cabling signal issues and bandwidth. The SAS standards body has worked on a number of measures to simplify SAS compatibility with SATA devices. The same data cables can be used when cabling drives, controllers and SAS backplanes. While SAS drives may not be used in SATA enclosures, it is typical to see SATA drives attached to SAS backplanes without any sort of electrical or mechanical adapters. Tunneling protocols were defined so that SATA devices can be attached behind SAS expanders as well as directly attached to SAS controllers and processors.

While SAS and SATA disks are physically interchangeable, it is important to recognize that most IT and storage administrators would not consider them functionally interchangeable. SAS disks, like their SCSI forefathers, tend to have higher performance characteristics and are built to withstand enterprise-class requirements. SATA disks, on the other hand, are typically not rated for continuous use nor do they have the performance enterprise storage systems require. Nevertheless, SATA devices have found homes in certain storage domains by utilizing RAID arrays for data protection, and the RAID5 and RAID6 capabilities offered by the Intel IOP348 I/O Processor- can deliver high-capacity arrays insulated from drive failure. Utilizing previous SCSI and SATA solutions, implementations would require parallel storage systems geared to each technology. Systems based on the IOP34x family can mix and match drive technologies as required, consolidating what would have required two systems into a single storage array to serve the needs of multiple clients.

The IT industry continues to benefit from technological innovation in storage standards that lead to improvements in product usability and functionality. Storage product developers, whether for DAS, SAN or NAS, are enjoying the fruits of one of those periods now. Intel’s introduction of the IOP34x Storage Processor family, which delivers integrated RAID-on-chip (ROC) with support for the latest storage technology, Serial Attached SCSI (SAS), provides the solution. Offering improved internal bus, memory controller and application accelerator performance, the IOP34x family is based on the fastest available Intel® XScale processor cores, and makes an ideal I/O processor for RAID offload and external storage designs.

For more information on the Intel IOP34x Storage Processor Family and other Intel storage products, please visit:

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