Anatomy of a Serial ATA storage enclosure

Computer Technology Review, April, 2004 by J. Peter Herz

As a company's demand for on-line and near-line data storage grows, the job of balancing the cost of equipment against performance and reliability becomes more and more challenging. New storage products based on Serial ATA architecture are promising lower costs while still providing new levels of high performance. Of course, as with the other storage systems, all SATA-based storage enclosures are not alike. Ideally, if all these systems were built the same, the selection process would be greatly simplified and the acquisition could be based on cost and features. Unfortunately, this is not the case.

The design and construction of these storage enclosures are as varied as the number of manufacturers that produce them. Some of these manufacturers design storage solutions with great care and intricacies while others produce products that are no more than sheet metal boxes haphazardly loaded with a bunch of inexpensive disk drives. It is important for those tasked with acquiring data storage systems to understand that a properly constructed storage enclosure needs to meet certain levels of structural, electrical, and environmental integrity. If these levels are not met, the resulting consequences can appear in a number of unappealing, poor performing, and short-lived systems. By having a good, fundamental understanding of what constitutes a properly built storage enclosure, a lot of these issues can be avoided and the expectations of performance, reliability, and low system cost can be attained.

Serial ATA storage enclosures are built in variety of sizes and shapes, depending on the application. Some are designed for desktop or deskside applications while others are designed to be mounted into racks. The physical bits and pieces that constitute a SATA enclosure are referred to as the enclosure's mechanicals and have a major influence on a product's cost, performance, and reliability. There are a number of items that should be checked and verified when evaluating the mechanicals of a storage system. Some of the more important and relevant of these include:

* Verifying that the overall structure of the enclosure is sufficiently rigid and soundly constructed

* For rack-mounted configurations, confirming that the enclosure dimensions are within the parameters specified by EIA standard, RS-310-D

* Ensuring that the hard disk drive (HDD) carrier is built robustly with good isolation for the disk drive and that its mechanical interface to the storage enclosure is not loose or sloppy

* Confirming that any removable disk drive carrier(s) or customer replaceable component(s) function properly for mechanical swap

* Verifying that all circuit board modules fit correctly within the storage enclosure and are properly grounded

* Confirming that electromagnetic interference (EMI) shielding is properly installed and making contact with mating surfaces

* Ensuring that all assembled components are properly secured into the enclosure and that there are no loose or ill-fitting parts.

Although the reasons for examining these points seem fairly obvious, it is worth expanding on them to better understand their relevance.

[FIGURE 1 OMITTED]

Enclosure Structure

There are a number of reasons why having a rigid, soundly constructed enclosure is important. One is that a properly built enclosure tends to prevent or restrict the generation of disruptive noises and vibrations. If not properly confined, vibrations from internal sources like cooling fans can migrate into sensitive areas such as the disk drive bays. If these vibrations are significant enough they will affect a drive's ability to accurately position its read/write heads over a requested disk location. Depending on the frequency and magnitude of the vibration, the resulting problems from this can range from mild system performance degradation to catastrophic loss of data. It is worth noting that the disk drives themselves can also be a source of unwanted vibration. The rotary actuators used to position the heads over the disks tend to be the culprits in this situation. In a heavy I/O environment this can be the source of some considerably large vibrations. This phenomenon is referred to in the storage industry as Rotational Vibration (RV) and the effects can be just as disastrous as those previously mentioned.

In addition to better controlling vibration, a well-constructed, rigidly built storage enclosure will also ensure minimal body flexing at key interface points. This is particularly important when rack mounting a system. All enclosures sag from their own weight when mounted into a rack. A rigidly constructed system will have minimal sag with little or no impact to the operation of the enclosure. A poorly or improperly built storage enclosure will have a large amount of sag that will adversely impact the fit and operation of the system in a number of areas. One of the areas most affected by a large amount of sag is the fit of disk drive carriers and other customer removable components. If not properly supported, the bays that these items fit into become distorted making insertion and extraction of the devices difficult. This distortion can also adversely affect the EMI sealing of these removable components, which in turn can produce significant leak points for EMI emissions. Since rack-mounted systems must fit within a specified vertical envelope, having a large amount of sag can also cause the enclosure to breach this envelope and affect adjacent systems.