Tuesday, February 26, 2008

Serial ATA - A Brief About the Storage Medium

What is SATA

SATA - Serial ATA is an evolutionary replacement for the Parallel ATA physical storage interface.

SATA International Organization
The Serial ATA International Organization (SATA-IO) is the group responsible for developing, managing and driving adoption of the Serial ATA specifications. Users of the Serial ATA interface benefit from greater speed, simpler upgradeable storage devices and easier configuration. Serial ATA is the next -generation internal storage interconnect, designed to replace parallel ATA technology. Serial ATA is the proactive evolution of the ATA interface from a parallel bus to a serial bus architecture. This architecture overcomes the electrical constraints that are increasing the difficulty of continued speed enhancements for the classic parallel ATA bus. Serial ATA will be introduced at 150Mbytes/sec, with a roadmap already planned to 600Mbytes/sec, supporting up to 10 years of storage evolution based on historical trends. Though Serial ATA will not be able to directly interface with legacy Ultra ATA hardware, it is fully compliant with the ATA protocol and thus is software compatible.

To further understand the Serial ATA architecture, click here.

The SATA-IO’s goal is to drive industry adoption by defining, developing and delivering standardized specifications for the Serial ATA interface.The Serial ATA International Organization (SATA-IO) is an independent, non-profit organization developed by and for leading industry companies. Officially formed in July 2004 by incorporating the previous Serial ATA Working Group, the SATA-IO provides the industry with guidance and support for implementing the SATA specification. The standardized SATA specification replaces a 15 year old technology with a high speed serial bus supporting up to 10 years of expected future. Members of the SATA-IO have the ability to influence, or directly contribute to the development of the SATA specifications.

SATA Anatomy

It’s best to start first with a quick history lesson on the SATA design, as it hasn’t been static through the years. It has gone through some important revisions - some visible and some not – that have shaped the SATA connector in to what we see today. We’d also like to thank Knut Grimsrud, chairman of the SATA International

Organization and Intel Fellow in their Technology & Manufacturing group for helping us out with this article and answering some of our questions.

First and foremost, the SATA-IO identified issues early on with the design of the SATA connector, and has revised it several times. This is most obvious in the move from the SATA 1 motherboard connector to the SATA 2 motherboard connector, where the SATA 1 motherboard connector was far more exposed than the SATA 2 connector. This switchover was particularly evident on boards using both native and external SATA controllers, where one would be SATA 2 compliant and the other SATA 1, requiring the different connectors. The SATA 2 motherboard connector added shrouding around the connector, which significantly improved the durability of the connector as the shrouding prevented cables from easily bending the wafer part of the connector. Unfortunately this kind of shrouding couldn’t be added to SATA devices, because the standard was designed for use in small devices that don’t have clearance to fit a full shroud. This results in the modern half-shroud design for devices, where the top-half of the connector is shrouded by the device but not the bottom half. The other significant changes in the standard since its inception have been latching and chamfering. Most SATA devices now have recesses in their top side shroud that allow barbs on SATA cables to latch in to in order to provide downward stability since the underside is the unshrouded side. Chamfering has been added to the square edges of the tongue, and while not a visibly dramatic change, we’ve been told this is one of the primary improvements in strengthening the SATA connector. Yet in spite of these changes we’ve still managed to do something wrong and break the connector on our hard drive. For the reason why, we’ll start with case designs and cable designs. Part of our problem can be attributed to the amount of space we have to work in with the case involved in this investigation, the Antec P182. The P182, like many other cases, doesn’t leave a lot of space behind the primary hard drive bay. For most cases this is because there are motherboard components and cards in the way, while with the P182 it’s a matter of a 120mm fan being located behind the hard drive bay, which helps pull air through the lower chamber. Either way, with this specific case, we measure that we have about 2” between the rear end of a hard drive and the fan, which is very little space to work in. We then come to the cable, in this case it was a bog-standard cable that came with one of the products we’ve used over the years. As we mentioned earlier, SATA cables aren’t quite as flexible as PATA cables when it comes to longitudinal bending, and while we can technically bend a cable completely over at any location, this isn’t great because it causes the cable to pull back in to a more relaxed position. From the tip of the connector, the cable we measured needs about 2.5” of space to bend comfortably. With the amount of clearance we have being less than the amount of space to need to ideally bend the cable, it becomes obvious that this will quickly become tricky. We need to bend the cable at a sharper angle so that it stays well clear of the fan, which means it’s going to be exerting some force on our hard drive, a generally acceptable but not ideal situation. And then finally there is the layout of the P182’s hard drive bay. The P182 requires the hard drives to be installed in to the cage at an angle, such that they are on their sides with the bottom pointing towards the bottom of the case, which you can see here. Keep in mind the fact that the top hard drive is on the opposite side of the case from where the motherboard is. In this combination, we have the anatomy of our failure. Our cable is putting force on the SATA data connector towards the bottom side, which as we covered earlier is the weaker direction to go because it’s not shrouded like the top side is. Furthermore we’re using a SATA-1 style cable without a latch, which means we don’t have said latch to reinforce the connection. It turns out that this is enough force to break the SATA data connector on our hard drive, and when the drive cage was being secured one day the connector broke.

A Lesson in User Failure: Investigating the Serial ATA Connector

Every once in a while though, we will break something in a process that's genuinely interesting. Failure is its own reward, it teaches us how to not do something or do something better than we did before. And in those handful of cases, we like to get to the bottom of what went wrong, what we did wrong, and what can be done to avoid the issue in the future.

Regarding Hard-disk SATA cables and connectors aren't quite as robust as the old PATA design. PATA cables could be worked in to rather impossible situations as the connector was extremely snug fitting, and the cable itself was extremely flexible when it needed to be folded longitudinally; it was hard to set up but also hard to break. We'll still take a SATA setup any day of the week, but we've come to the realization we can't abuse SATA setups like we could PATA setups.

Some Updates and Jurgons

SATA 3Gb/s Features
SATA 3Gb/s interface speed enables up to 300MB/s data transfer rates.
SATA 3Gb/s enables the highest level of performance, while maintaining desktop cost structures.
SATA 3Gb/s facilitates bandwidth aggregation for multiple devices, enabling max throughput as well as, higher cache through put performance in single drive configurations.
100% backward compatible with 1.5Gb/s SATA.
No software, driver or cable upgrades required.

What is SATA 3Gb/s?

SATA 3Gb/s is the next generation of Serial ATA interface speed. This advanced optional feature is one of 8 specifications published by the previous Serial ATA Working Group II. Since the optional specifications are advanced additions to the core specification (Serial ATA 1.0a) and authored by the 2nd Serial ATA working group they were quickly nick-named SATA II. The most important fact that system builders and consumers must keep in mind is that SATA II is not synonymous with 3Gb/s. Since the advanced features are optional and referred to as

SATA II by the industry, SATA II could mean any or all of these specifications..
SATA 3Gb/s is double the speed of the current SATA interface of 1.5Gb/s. SATA 3Gb/s enables the highest level of performance while maintaining desktop cost structures.

The Serial ATA bus & bandwidth design
In contrast to Ultra ATA’s parallel bus design, Serial ATA uses a single signal path to transmit data serially, or bit by bit, and a second serial path to return receipt acknowledgements to the sender. Because each signal path is a 2-wire differential pair, the Serial ATA bus consists of 4 signal lines per channel. The 16-bit wide parallel Ultra ATA bus is capable of transmitting two bytes of data per clock. Though Serial ATA transmits only a single bit per clock, the serial bus may be run at a much higher speed to compensate for the loss of parallelism. Serial ATA was introduced with a bandwidth of 1500Mbits/sec, or 1.5Gbits/sec. Because data is encoded using 8b/10b encoding (an 80% efficient encoding used with digital differential signaling to maintain a constant average “DC” bias point), the effective maximum throughput is 150Mbytes/sec.

Ultra ATA/100

25MHz strobe

x 2 for double data rate clocking

x 16 for bits per edge

/ 8 bits per byte

= 100 Mbytes/sec

SATA 1.5Gb/s

1500MHz embedded clock

x 1 bit per clock

x 80% for 8b10b encoding

/ 8 bits per byte

= 150 Mbytes/sec

SATA 3Gb/s

3000MHz embedded clock

x 1 bit per clock

x 80% for 8b10b encoding

/ 8 bits per byte

= 300 Mbytes/sec

Storage Arrays love SATA 3Gb/s interface speed, for host interfaces, the 3Gbits/sec speed is necessary to meet the increasing performance requirements for bandwidth-intensive applications such as disk to disk backup, video editing, medical imaging, research and near-line data storage as the amount of data that companies need to store, manage and keep readily available continues to increase. Serial ATA can help meet rising data throughput needs via the introduction of SATA 3Gb/s, enabling the transfer of more than the current 1.5 gigabits of data in aggregated arrays and other multi-drive configurations. From the host side, SATA 3Gbits/sec essentially provides a larger pipe to move data faster. Even though the interface speed itself is a small portion of the overall drive performance and single drive performance increases may not be as noticeable with typical office applications. 3-Gbits/sec interface on the disc drive can increase system performance when the application takes advantage of the drive’s cache burst ability. In the case of writes, if the drive’s write cache capability is enabled, any write that fits in the buffer will be transferred at the full 3 Gbits/sec into the buffer. In the case of reads, caching helps when the data requested is small enough to fit into the drive cache and is present in the cache at the time of the request. That data will be transferred to the host at the full 3-Gbits/sec rate. In either case, the data must be in the form of small, sequential or near-sequential files, such as those found in some video editing applications. The reason: small sequential or near-sequential file transfers require less mechanical movement or seek overhead and maximize caching capabilities of the drive. In these types of applications, 3-Gbits/sec drive interface rates can greatly increase performance. This cache performance increase may enable larger cache sizes in the future.

Serial ATA 3Gb/s Value

As always the case, starting with the earlier times of the Parallel ATA interface, the industry attempts to avoid the limiting factor for the speed at which data are transferred to and from the disc drive. If the interface pipe is not fast enough for the disk drive’s data rate the system becomes bogged down by the interface bottleneck causing performance degradation. To stay ahead of the data rate the interface rate typically leads by at least one year. In the case of SATA 3Gb/s, customers can now experience greater than 150MB/s data rates in aggregated bandwidth and increased cache burst rate performance with single drive configurations.

External SATA

The official eSATA logoStandardized in mid-2004, eSATA defined separate cables, connectors, and revised electrical requirements for external applications:

Minimum transmit potential increased: Range is 500–600 mV instead of 400–600 mV.
Minimum receive potential decreased: Range is 240–600 mV instead of 325–600 mV.
Identical protocol and logical signaling (link/transport-layer and above), allowing native SATA devices to be deployed in external enclosures with minimal modification
Maximum cable length of 2 m (USB and FireWire allow longer distances.)
The external cable connector is a shielded version of the connector specified in SATA 1.0a with these basic differences:
The External connector has no “L” shaped key, and the guide features are vertically offset and reduced in size. This prevents the use of unshielded internal cables in external applications.
To prevent ESD damage, the insertion depth is increased from 5mm to 6.6mm and the contacts are mounted further back in both the receptacle and plug.
To provide EMI protection and meet FCC and CE emission requirements, the cable has an extra layer of shielding, and the connectors have metal contact points.
There are springs as retention features built into the connector shield on both the top and bottom surfaces.
The external connector and cable are designed for over five thousand insertions and removals while the internal connector is only specified to withstand five.

As of 2007, an eSATA external drive enclosure will typically ship with a passive eSATA-to-SATA bracket/cable-adapter to install on desktops that lack an eSATA port or that need another. Desktops can also be upgraded with the installation of an eSATA host bus adapter (HBA), while notebooks can be upgraded with Cardbus or ExpressCardversions of an eSATA HBA. With passive-adapters, the maximum cable length is reduced to 1 meter, due to the absence of compliant eSATA signal levels. Full SATA speed for external disks (115 MB/s) have been measured with external RAID enclosures.

From the second half of 2008, SATA-IO expects eSATA to provide power to eSATA devices without the need for a separate power connection. In a news release from 2008-01-14, SATA-IO calls it the "Power Over eSATA initiative."

Because of its hotplug capability and consumer-level price-point combination eSATA may be of interest to the enterprise and server market, which has already standardized on the separately-developed Serial Attached SCSI (SAS) interface.

Hotplug - Conceptually hotplug is a means to add or remove a component while the system is running without requiring a reboot. Hotplug is a combination of both hot-add and hot-remove. In the Linux implementation of this concept, hotplug is a facility that supports dynamic (re)configuration of GNU/Linux distributions by kernel reports to user mode "agent" software.


1. SATA International Organization website - http://www.sata-io.org
2. Anandtech Hardware Forum - http://www.anandtech.com/storage/showdoc.aspx?i=3185&p=3
3. Wikipedia.org - http://en.wikipedia.org/wiki/SATA

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