What is SATA II? How is it different from SATA "one"? Should I care if my system supports SATA II?

First there was a Serial ATA organization working group that authored the SATA 1.0a specification, which supports a first generation (Gen 1) PHY at 1.5Gbps. After this Gen 1 effort, the second generation (Gen 2) effort defined a whole series of optional features to enhance and improve the first SATA specification. One of these enhanced optional features was the Gen 2 PHY, which has double the speed of first generation and is 3Gbps.

This Gen 2 PHY began to be called SATA II by the press in the market. Some of the other key enhanced SATA features were published by the SATA II working group in several specifications and included Native Command Queuing (NCQ), Port Multiplier, Staggered Spin Up, Port Selector, External SATA, and Hot Plug. The group has reformed and is now called the SATA IO (international organization). Some of the new enhanced optional features also began to be called SATA II by the press, even if the product had a Gen 1 PHY. Currently there is confusion in the market as to what is meant by SATA II. So I will go through the key enhanced optional SATA features, one at a time, and address the real question: "Do I need the new enhanced optional features of SATA or would SATA 1.0a be good enough for the needs of my system?"

[FIGURE 1 OMITTED]

Do I Need a Port Multiplier?

When a port multiplier is added to a single SATA port, a single host port can access up to 15 drives. Many host ports do not support the port packet steering control bits necessary to make the port multiplier work. Once you have verified that your host controller supports port packet steering, then you might want to test for the next feature called bandwidth aggregation.

The average drive supports about 50-60MBbps, and a single SATA Gen 1 port is capable of 150MB/s peak bandwidth. Based on these numbers, it would take about three drives to effectively utilize a single SATA Gen 1 port. So, if your host controller supports only port packet steering and does not support bandwidth aggregation, then the port multiplier acts like a JBOD interface without any performance aggregation. But, if your host controller supports port packet steering and supports bandwidth aggregation, then the port multiplier acts like a performance striped array and can effectively utilize the SATA host port bandwidth.

[FIGURE 2 OMITTED]

In summary, if you want to fully utilize your SATA host port, then a port multiplier can provide expanded storage capacity and enhanced performance. It is possible to put a RAID controller behind a SATA host port and provide protected performance striping as well.

Can I use a 3Gbps, Gen 2 PHY?

Most system host motherboards do not support a Gen 2 PHY, and a separate card is needed to upgrade the system. The Gen 2 PHY can deliver 300MB/s peak bandwidth, and this means it would take a device that supports port packet steering, bandwidth aggregation and at least six drives to fully utilize a Gen 2 PHY host SATA port.

In summary, if you want to fully utilize your SATA Gen 2 host port, then a port multiplier or another similar device can provide expanded storage capacity and enhanced performance.

Does NCQ Provide a Real Advantage?

NCQ stands for native command queuing and is designed to improve storage performance and reliability. The drive can optimize (re-order) the completion of multiple commands to reduce mechanical workload and improve performance. If your host controller supports the NCQ protocol and the drive supports it as well, then your drive performance and reliability can be increased. If you look at the SATA website www.sataio.org/featureofmonth.asp, there is an article discussing the NCQ feature. This article has some performance data that states a performance improvement of 9-12% over a non-NCQ drive. As for reliability, the drive moves less to deliver the same data request, and this will produce less heat and less mechanical wear and tear. So, the reliability improvement is a result of a more optimized system operation--very logical, but I have not seen any measurement data as of yet.

There is a flash demo on the SATA IO site that reads:

"The hard drive on the left, with NCQ, can execute four commands (A, B, C, and D) with one and a quarter complete rotations due to proper ordering of the operations. The hard drive on the right, without NCQ, requires two and three quarters complete rotations to execute the same four commands (A, B, C, and D) as a result of poor ordered operations."

"NCQ can also improve your overall system performance from booting your system to file copying."

In summary, if you need more performance and improved reliability, a NCQ system design is your ticket.

Hot Plug?

Hot plug requires a focus on upgraded cables and improved signaling from the original SATA 1.0a specification. This feature allows for drives to be added or removed without any downtime for your system design.

In summary, if you need to replace or add drives while your system is operating, use the Hot Plug feature.

Staggered Spin-Up?

The load on the power supply is one issue with system design when using multiple drives. One of the peak loads happens at spin-up and can worsen if all the drives spin-up at the same time. This SATA feature staggers the spin-up to lower the load on the power supply.

In summary, if you are concerned with the power issue, use this feature.

When Do I Need a Port Selector?

The port selector is the flip side to the port multiplier--instead of supporting multiple drives, the port selector supports multiple host controllers for controller redundancy. If one host controller fails, the other one can take over without missing a step. This feature is for advanced system design where a protected performance array is not enough reliability.

In summary, if you need ultra reliability and cost is not the issue, then a port selector is just what you need.

Should I Design in an eSATA port?

The expanded electrical specification for the Serial ATA physical layer (PHY) was written to improve on first generation PHY designs and lengthen the cable distance between transmitter and receiver. The original 1.5Gb/s Serial ATA 1.0a, or Gen li, has a maximum length of 1 meter internal cable, and so does the second generation 3.0Gbp/s or Gen2i. The expanded specifications, called Gen1m and Gen2m, are targeted at "short" backplane and External Desktop Applications. These "m" level PHYs have modified transmitter and receiver differential swing specifications. Minimum transmitter amplitudes are increased and minimum receive amplitudes are decreased in order to accommodate additional cable lengths of approximately 2 meters with backward compatibility for Gen li devices. Most motherboards do not support this expanded PHY level and an aspter card will be needed to add a port for this purpose. The SATA IO group calls the m PHY "eSATA," which means an external SATA port.

Today, the only methods for attaching a computer or consumer electronics device to an External Serial ATA storage device is via the Firewire (1394), or USB interface, using bridge ASICs. The problem with this approach is that the Serial ATA signals need to be converted or encapsulated within the USB or Firewire (1394) protocols, resulting in unnecessary cost and reduced performance. Removing signal conversions for External Serial ATA provides the lowest cost and best performing solution to connect to an External Serial ATA storage device. Beyond lower price and higher performance, the average workstation, desktop pc, or consumer device, do not provide sufficient drive bays, large enough power supplies, or adequate cooling to support a robust internal storage system. The only answer is to use external storage upgrade box(es), typically connected by USB or 1394. These older connection devices are as much as 6 times slower than an external SATA box.

In summary, this direct connection provides the opportunity for Desktop, Workstation, or Consumer Devices to add Gen 1m ports on the external skin of every PC, allowing for low-cost, high-performance, easy upgrades. See the SATA IO site for more on this: www.sata-io.org/featureofmonth.asp.

Can I Utilize an xSATA port?

The final part of the expanded electrical specification, Gen 1x and Gen 2x, offers extended length (approximately 8 meters) based on the Serial Attached SCSI Electrical specifications (SAS). This level of PHY signal is not backward compatible, and careful attention to drive and host signals are needed.

In summary, if you need the extra signal length for a high-speed storage array, then the xSATA or i level PHY is what you need.

Final Recommendation

If you want expanded performance over parallel ATA drives, if you want higher reliability, if you want high speed upgrades using an external cable, if you want to upgrade without downtime, or just optimize a single drives transfer rate, you should add SATA to your next storage architecture. It's the right time to use SATA for your next project.

Conrad A. Maxwell is CEO of Maxwell Consulting Associates (Corona, CA)

www.maxwellca.com