Optimize Your Investment with Serial Attached SCSI

Author: Alvin Ooi, Marketing Manager,

Since the market emergence of Serial Attached SCSI two years ago, many high-end storage solution providers and OEMs have recognized the advantages of SAS over its parallel predecessor. Vertical markets that have since adopted SAS technology are the entertainment industry, medical imaging, financial institutions and research and government institutions, which typically need performance and/or capacity. SAS can offer good value, performance, and scalability.

SAS Advantages

In general, everyone recognizes the following advantages SAS offers:

  • Superior aggregated performance over 4-lane wide ports — providing throughput of up to 12Gb/s SAS
  • Dual-ported design with fail-over capabilities
  • Support for both SAS and Serial ATA (SATA) HDD in a single enclosure to eliminate redundant qualification and certification efforts, as well as the ability to achieve cost-effective storage
  • Available edge and fan-out expander technologies for easier expansion and unmatched scalability
  • Longer interconnect distance allowed compared to SATA for low-cost expansion enclosure solutions

Although overall market share is still relatively small compared to SATA, parallel SCSI and even Fiber-Channel, the majority of the next generation solutions currently being designed by OEMs are based on SAS technology. But demand for SAS is high and increasing.

New SAS/SATA Products

During 2007, AIC/Xtore introduced three important additions to their SAS product offering, all of which take full advantage of SAS/SATA compatibility:

  • XC-23D-SA10-0-R SAS/SATA II hard drive canister
  • XD 2000™ series enterprise-class RAID interleave-based Fibre Channel and SAS/SATA RAID enclosure
  • XJ 100™ series SAS/SATA JBOD tower for video, graphics, and small and medium business (SMB) applications.

The SAS/SATA II hard drive canister, the XC-23D-SA10-0-R, supports three 3.5-inch SAS/SATA II drives in two drive spaces on tower and rack-mount form factors. High performance SAS drives and low-cost SATA drives can be populated into this SAS/SATA II HDD canister for both mission-critical and near-line storage applications. This advanced system enclosure can sustain up to 2.2 TB of storage capacity in a single canister with up to 900MB/s bandwidth. In addition, the XC-23D-SA10-0-R provides hard drive activity/failure LED notification, fan speed control and over-temperature alarm. The user-friendly and easy-maintenance design operates with HBA/RAID controllers and disk drives to insure storage system functionality and reliability.

The XD 2000™ series SAS/SATA RAID enterprise-class enclosures deliver various interconnect options for a wide range of storage applications. The XD 2000 provides dual-active controller blades. Each blade supports dual 4Gb Fibre Channel or SAS 4x host channels and SAS 4x capacity expansion for high scalability. Its advanced web GUI management allows users to easily maintain the configurations and usage log/report. The cable-less design and hot-swapping features enable ease of management and maximum reliability.

The XJ 100™ series SAS/SATA JBOD tower is a small footprint storage box with the unprecedented mini-SAS wide port that supports 1200MB/s storage throughput and up to 5TB high capacity storage. Users can choose to use high performance SAS drives as well as low-cost, high-capacity SATA drives. The XJ 100ution for high definition video storage and many other desktop storage applications. The SAS JBOD tower also provides an expansion port which allows cascading for more capacity. It can support up to 120 drives from a single SAS initiator. This subsystem also supports SCSI Enclosure Service (SES) for status monitoring and in-band firmware updates. Its cable-less enclosure and hot-swap drive tray design put system configuration and maintenance at ease.


Figure 1: XJ 100™ series SAS/SATA JBOD tower for video, graphics, and SMB applications


SAS Expanders

Since SAS does not use a parallel bus to attach multiple devices to the same data path, the only way to share the bandwidth would be to design a bridge device to connect multiple devices to the available bandwidth. This bridge device is a SAS expander.

A SAS Expander is basically designed to allow the maximization of available bandwidth utilization. In other words, it maximizes the value on investment.


Figure 2: A typical 8-port SATA RAID controller can offer a total bandwidth of 24 Gb/s (3Gb/s x8)
See Drawing B


However, one can only connect eight HDDs to this controller, the average throughput on a SATA 3G HDD is no more than 60 MB/s under the best case scenario in a 100% sequential read. This is only 20% of 300MB/s which each SATA port is designed to support. It is not a good way to connect more HDDs to each SATA port unless a SATA II port multiplier is used. Since SATA II port multipliers are not widely supported by many manufacturers, 80% of the total available bandwidth is wasted.

Where large numbers of low-cost SATA disk drives are desired for applications such as Disk-to-Disk Backup or Virtual Disk Library, a large port-count SATA controller or multiple smaller port-count controllers would be required.


Figure 3: A typical system with 16 SATA hard disk drives


Installation Scenario 1:
Two PCI-E 8x lane slots, totaling 40Gb/s bandwidth, are on the server board to support 16 hard disks. Each HDD is allocated around 60MB/s of bandwidth to use, even though actual throughput from one HDD is not close to that.

Cost for such a solution can add up quickly as more controllers are needed to support the increasing drive capacity the system is calling for. Also, when using multiple controllers that do not support RAID-spanning, the use of software RAID striping implemented across multiple RAID arrays is required to bring all the disks under one volume. Besides adding more cost, this function would be handled by the server CPU, hampering the overall performance.


Figure 4: SAS expander-based installation


Installation Scenario 2:
With a 24-port SAS expander, only 4x lanes (one cable) from an 8-port RAID controller is required, leaving the other 4x lanes for future expansion using external enclosures. Although this controller is using a PCI-E 8x slot, it is using only half of the total bandwidth of 20Gb/s to support 16 hard disks. This is equivalent to 1GB/s of data path (60+ MB/s per HDD allocated), a much better utilization of available bandwidth. Should actual performance exceed 1 GB/s (aggregating the performance of 16 hard disks), a second SAS RAID controller with 8x internal ports can be used instead to double the bandwidth, provided the controller supports link aggregation.

Cost Comparison

For the overall cost comparison, a SAS expander-based server chassis with a single 24-port expander costs about $2,200. Together with an 8-port SAS RAID controller, priced around $1,000 retail, total cost is $3,200, without MB and HDD. Although this solution is $500 more than using a pure SATA solution, it leaves a precious PCI-E 8x slot open on the MB for other add-on card use. When compared to a SAS/SATA solution using two SAS RAID controllers and a non-expander based enclosure, the net cost of this solution is actually less expensive ($3,200 versus $3,500). More importantly, the ability to expand from the same 8-port controller externally, coupled with and the better utilization of total bandwidth brings much better value to the solution.


Top View of Storage Server


One last important advantage of a SAS expander is the ability to provide external expansion solutions using direct SAS links. Until this was available, users were compelled to use FC-SATA or SCSI-SATA solutions, which can be expensive, and in some cases, they are not daisy-chainable, or have scale or ID limitations. For disk-based archive solutions, where throughput is not a concern, the ability to daisy-chain from enclosure to enclosure using one single SAS RAID controller adds even more value to SAS. Despite claims to the infinite scalability of SAS on a single port, there is a physical drive limitation in practical terms to 120-128 drives per port. That computes to seven 16-bay JBODs behind each port of the controller. With drive capacities at 1TB, one controller can manage 128TB from a single port or expander.

By taking advantage of SAS technology, a storage system architect can minimize the Total Cost of Ownership and maximize value on investment.


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