Serial Storage Wire

System Benefits of 6Gb/s SAS

By Jesse Molina, Product Marketing Engineer, Enterprise Storage Division,
PMC-Sierra

Introduction

The next generation of Serial Attached SCSI (SAS), 6Gb/s SAS, doubles the previous generation bandwidth for each link and adds link multiplexing to enable a 6Gb/s link rate to share two 3Gb/s connections. The Serial Management Protocol (SMP) is enhanced to provide more configuration, faster initialization and greater reporting for diagnostic and status monitoring.

The performance- and robustness-related features of 6Gb/s SAS include:

Performance — doubles the link rate and bandwidth
Multiplexing — optimizes bandwidth by enabling two 3Gb/s links to share a 6Gbit/s port
Increased zoning capabilities — enables partitioning of a domain into smaller sets of accessible devices
Self-configuring expander devices — accelerates system initialization and change detection
Diagnostics and robustness — improves status reporting and error notification

At 6Gb/s, the second-generation SAS controllers are optimized to take full advantage of the 5Gb/s per-link speeds of PCIe 2.0. These 6Gb/s SAS improvements ensure that SAS systems are faster, provide better bandwidth utilization, are easier to manage and enable peerless system robustness.

6Gb/s SAS Multiplexing

Second-generation SAS expander devices provide the connectivity fabric to complement high-performance controllers. 6Gb/s links provide double bandwidth availability to support more disk drives and better throughput. 6Gb/s SAS multiplexing enables a single link to be shared by two 3Gb/s SAS or SATA drives, leveraging full advantage of 6Gb/s throughput while reducing the number of cable attachments. The diagram below shows how a 6Gb/s link is multiplexed to provide two interleaved 3Gb/s links sharing a single port. This enables very high-throughput controllers to concurrently access more disk drives with fewer connections.

Figure 1: Link Multiplexing

In a system where both 3Gbit/s and 6Gbit/s devices are used, some ports may be configured as multiplexed ports while others run at native 6Gbit/s rates to optimize the system performance and retain backward compatibility.

6Gb/s SAS Zoning

6Gb/s SAS zoning enhances the SAS fabric by providing a hardware mechanism to increase device segregation. SAS data storage systems may include a variety of device types, such as SAS and SATA, as well as data protection mechanisms, such as RAID and encryption. Zoning enables segregation of these storage types at the system level to simplify partitioning, provisioning and overall system management. Zoning can be optionally secured by password to prevent unauthorized access, malicious attacks and corruption of data by operator or application error on the server. Having this control ensures that if a server is compromised, only the data that is accessible on the compromised server is at risk of being lost, instead of losing all the data on the network.

An example of where 6Gb/s SAS zoning can be used to achieve a greater level of security is within a blade server that utilizes a SAS fabric. By separating host access, local storage can easily be managed between email servers, database servers and application servers. In combination with Controller Based Encryption (CBE), 6Gb/s SAS zoning adds an additional layer of security to protected data.

Self-Configuring Expanders

First-generation SAS expanders do not assist with the SAS topology discovery process. SAS controllers process all of the device identification and mapping of route tables within each expander device.

However, second-generation expanders implement self-configuration features. Each expander device discovers the devices attached to it and completes its own route table. Since all expanders are initializing at the same time, the overall system topology is resolved quickly. This is most apparent with large topologies or where there are multiple hosts in a domain.

Backward Compatibility

To protect 3Gb/s SAS investments, 6Gb/s SAS is built to be backward compatible to 3Gb/s and 1.5Gb/s link rates. With the use of 6Gb/s SAS controllers, performance is optimized when 6Gb/s SAS drives run natively and 3Gb/s SAS and SATA drives use SAS multiplexing to keep host rate at 6Gb/s. Enterprise data consists of many different types of data that demand different storage handling. For example, Sarbanes Oxley demands data archiving and the payment card industry (PCI) demands that credit card data be encrypted. The SAS infrastructure supports a mix of SAS and SATA drives and link rates. Combining the storage device types with a mix of data protection mechanisms in a SAS system allows for information lifecycle management (ILM), whereby data migrates from primary 24/7 storage using SAS devices to secondary/nearline storage using SATA devices as it ages and is accessed less frequently. Additionally, Solid State Disks may be used to support high-transaction applications, creating a new tier of high performance data storage.

Conclusion

The first generation of SAS brought ease in serviceability, with serial connectors replacing bulking parallel connectors and support for both SAS and SATA with a single interface. The second generation of SAS, 6Gb/s SAS, builds on this legacy, providing the next level of reliability, availability and serviceability to SAS and SATA storage systems. This results in less time spent fixing system errors and more time available to manage your data. An hour of down time could mean everything from millions lost on the stock market to a fantasy football loss, and avoiding either scenario will ensure that all your clients remain happy customers.