Serial Attached SCSI – Simplifying Storage Architecture with Serial Technology

Author: Paul Vogt, Director of Product Marketing,

With Serial Attached SCSI (SAS) now widespread in the marketplace, developers have new freedom to create a single storage solution that meets a variety of price, performance, capability, and reliability needs. By having one SAS storage system meet the full spectrum of application requirements, development budgets can be reduced from previous generations where multiple products were required to meet multiple application requirements.

In addition to this flexibility, SAS offers a number of important benefits, including new levels of scalability, performance, and robustness.

The key to implementing a scalable and high performance serial storage system starts with the type of controller used. Unlike SATA controllers that are typically limited to a 1:1 port-to-drive relationship, SAS controllers leverage expander technology to enable greater scalability. Instead of being limited to a maximum of four drives on a four-port controller or eight drives on an eight-port controller, a single SAS controller enables the connection of up to 128 drives.

Typically, expander technology is embedded in the backplane or enclosure, and allows SAS controllers to support multiple simultaneous active host connections, increasing the effective throughput. Cascading expanders enable JBODs to be daisy-chained for maximum scalability. As such, SAS expander technology not only increases effective performance; it also reduces the net cost per drive.

The use of SAS controllers and SAS drives together provides the maximum performance benefits, taking advantage of their higher rotational speeds (up to 15K), larger caches, deeper queue depth, and lower command latency to deliver better I/O throughput.

SAS technology was designed for rigorous 24/7 high-availability enterprise duty. SAS drives have specifically been engineered for these mission-critical environments, boasting features such as 10K and 15K rotational speeds, dual ports that provide failover capability, and duplex operation that increases throughput.

But the added performance and capability of SAS drives comes at a premium cost particularly if you consider their relatively smaller capacities. SAS capacities typically top out at 300GB, compared to 750GB for SATA drives. This has discouraged their adoption for anything but the highest performance applications, including mission-critical and transaction-based data. For most storage applications, including nearline, secondary and other reference storage applications, SATA remains the most popular choice. And with new and improved RAID technology such as RAID 6, business data stored on SATA drives stays protected even in the rare case of a dual disk drive failure.

One of the most important benefits of SAS technology for developers is the ability of the SAS infrastructure to support both SATA and SAS drives. This Unified Serial Architecture enables the creation of solutions more finely tuned to customer performance, capacity and budget needs than stand-alone SAS and SATA environments. A SAS controller can issue SATA commands as effectively as a SATA controller and when used with SATA drives offers some of the benefits of SAS, such as broad scalability to connect up to128 drives or storage arrays.

Simply put, a SAS controller can be used with SAS drives for the ultimate performance solution, or it can be used with SATA drives for a cost-effective, high-capacity solution that can be extended to a full SAS solution at a future date.

In short, the ability to freely mix SAS and SATA drives allows you to create a single storage infrastructure that can seamlessly support both primary and secondary storage needs from the same controller.

There are also software advantages for the developer. One driver set can be used for both SAS and SATA technologies because the controller can perform the conversion from SCSI commands to SATA commands. This SCSI-to-SATA translation within SAS controllers has been standardized by the T10 STA committee.

Storage management applications are also simplified. A single management utility can be used for both SAS and SATA within the storage system. One utility can monitor and manage all the controllers and disks in your storage space from a single location, receive event notification, group disks into logical drives, and build-in redundancy to protect data and improve system performance.

Tiered storage provides an excellent example of how SAS and SATA can be integrated into a single solution that meets a variety of workload, performance and capacity requirements. Primary storage requires high performance and high availability for mission-critical data, while secondary storage benefits from higher-capacity/lower-cost disks. In the past, this typically required separate storage platforms, but since the advent of SAS you can solve this problem with a single infrastructure.

Figure 1 illustrates how to build tiered storage on a SAS backplane, the controller implementation providing the storage platform with all aspects of primary storage using SAS disks and secondary storage using SATA disks.


Figure 1 – Tiered storage example using SAS and SATA disks.


This solution is also easy to expand as storage needs grow. With its 100% SAS infrastructure, both primary and secondary storage benefit from the ability to scale up to over 128 storage devices.

The ability to integrate SATA or SAS drives into a single infrastructure provides a number of benefits to developers. Now with a single infrastructure, driver set, and management application, a wide variety of solutions can be created that are better tuned to customers’ performance, capacity and cost requirements. A solution can be developed so that both meet the customers’ storage needs now and have the flexibility to meet the needs in the future.

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