What Developers Need to Know About SAS

Author: Jesse Molina, Product Marketing Manager,

The transition from Parallel SCSI to Serial Attached SCSI (SAS) has been taking place for more than three years, since the first SAS hard disk drives (HDDs) were introduced to the market in 2005. Since then, SAS has made its way into a number of applications: from high performance workstations, to Small and Medium Business (SMB), external storage Just a Bunch of Disks (JBODs) and enterprise-class servers. SAS performance, matched with its support for Serial Advanced Technology Attachment (SATA), has made the transition to SAS economical and ideal for addressing many storage applications using a single storage networking infrastructure.

SAS and SATA Compatibility
One of the main reasons that SAS has been able to scale is that it is designed to be compatible with SATA HDDs, which provide the highest capacity at the lowest cost-per-gigabyte of any storage media. In addition, the use of a SATA Active/Active port selector to dual-port a SATA HDD enables fully redundant storage architectures with greater system fault tolerance.

The trade-off for SATA is reduced performance and lower reliability than enterprise-class SAS and Fibre Channel (FC) drives. Where SATA drives are used for infrequently accessed data, such as near-line storage or backup, Redundant Array of Independent Disks (RAID) is commonly used to mitigate the reliability risks of SATA storage.

SAS and SATA devices connect through compact serial connectors and cables that allow good airflow within a system. The disk drive connectors for SAS and SATA look very similar, as both include power and data connections in a single connector. The main physical difference between them is that the SATA connector has a small notch, or key, between the data and power connections. This allows a SATA drive to be connected to a SAS connector, however the reverse is not true as a SAS drive cannot be plugged into a SATA connector.

SAS for the Enterprise
For enterprise-level, mission-critical applications, SAS disk drives are ideal. SAS drives are highly reliable and are designed for 24/7 operation, or any application with very high duty cycles. SAS drives also provide high performance with RPM speeds up to 15K. However, capacity is currently limited compared to SATA, typically to 300GB per disk drive. The high reliability and performance also come at a high price, typically three to five times the price of a SATA disk drive.

The ability to support enterprise-quality data storage with SAS, and cost-effective, high-capacity storage with SATA, both using the same SAS infrastructure, has led to economical, scalable storage and server offerings. In a small business, low cost may be the number one requirement. These businesses can purchase a SATA JBOD today, and as the business grows, can use the same infrastructure to migrate to higher performance SAS drives.

In addition, having a mix of SAS and SATA drives 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. When data has completed its useful life it is moved to tape for archiving.

SAS as a Fabric
SAS expanders provide powerful switching functionality, enabling quick aggregation of many drives in a single SAS domain. Using switches, a SAS fabric can allow multiple hosts and disk arrays to share a common storage networking infrastructure. With enhancements made in the SAS-2 standard, this fabric can be scaled to support an unlimited number of devices. Using 3Gb/s SAS products today, a SAS fabric can combine multiple 300MB/s single links to form a wide port. For example, combining four single links delivers up to 1.2GB/s bandwidth. With next-generation SAS, touting speeds of 6Gb/s per link, a x4 wide port will run at 2.4GB/s, and for even higher throughput applications, a x8 wide port will provide up to 4.8GB/s.

Taking advantage of SAS as a fabric, the concept of diskless blade servers is emerging. In this configuration, the HDDs are moved off the server/CPU blade onto a local shared storage blade. Storage blades are available today in configurations supporting many SAS and SATA HDDs, such as a flexible pool of storage for blade servers. A global resource manager can assign and manage boot images, applications, and select server links to specific data volumes. The storage blade provides an advantage in price and performance by maximizing storage utilization across multiple blade servers instead of having dedicated drives on every blade server that cannot be shared.


Figure 1: A SAS Fabric is Many Links between SAS Controllers, Switches/Expanders and Disk Drives.


SAS zoning further enhances the SAS fabric by providing a means to increase security. It separates data traffic between hosts and resources, restricting host access to only a specific set of disk drives, as not all servers need to access all data. Zoning can be used 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.


Figure 2: Zoned Portion of Service Delivery Subsystem (ZPSDS)


The Future of SAS
The next generation of SAS – 6Gb/s SAS – includes a higher link rate, improved bandwidth utilization, and many features to improve the robustness and manageability of SAS topologies. For example, 6Gb/s SAS provides additional status and reporting information to facilitate diagnostic functions. This ensures that optimal system operation can be maintained. In the event of a fault, this status data can be used to identify, isolate, and analyze fault and error conditions.

The major improvements in 6Gb/s SAS include:

  • Performance — doubles the link rate and bandwidth
  • Multiplexing — optimizes bandwidth by enabling two 3Gb/s links to share a 6Gb/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, 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.

With its compatibility with high capacity SATA drives and its robust feature set, it is no surprise that SAS is continually finding more homes, scaling entry-level, cost-sensitive data systems to the enterprise where high reliability and performance are critical. Using SAS as a fabric, storage resources can be shared across multiple hosts and managed effectively through SAS zoning. The emergence of 6Gb/s SAS provides continued increases in performance and overall system reliability with the use of fault isolation diagnostics. SAS has found its place in the storage arena and the many applications it services today have made it more than just a replacement for parallel SCSI HDDs.


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