Storage considerations for industrial applications

By Steve Gudknecht ACT/Technico

Many storage solutions are designed for the enterprise markets. Selecting the right storage solution for industrial applications requires careful scrutiny of a range of factors. Steve outlines considerations in environment, data rate and capacity, network connectivity, life cycle, and security as points to consider.

Like most aspects of system design, storage requirements for today’s industrial embedded systems are driven by application type, environment, increased performance expectations, and the availability of new and innovative products. It is vitally important that system designers correctly establish expectations for the operational and environmental characteristics of storage for industrial systems; this will ensure proper form, fit, and function of the chosen solution.

An important aspect of choosing storage for embedded industrial applications is the consideration of a wide set of variables. These include environmental demands, capacity and data rate needs, reliability requirements, physical space, life cycle, and form factor. Medical applications that require high reliability do not necessarily demand the same environmental armor as pipeline inspection systems. The same holds true for capacity and data rate considerations, where machine recipes for automatic equipment contrast the larger capacity requirements of data logging, surface inspection, or process monitoring applications.

In addition, every system must be as reliable as possible in the context of the application, with the only difference being the cost of life, limb, and dollars in unreliability. As business ekes out every last expense in the relentless drive to improve efficiency and reduce manufacturing costs such as floor space, it drives smaller and lighter form factors. This can be seen particularly in the highly competitive and cost-sensitive semiconductor manufacturing segment, where per-unit cost savings are measured in the fractions of a cent.

Regardless of the application, storage usage falls into just a handful of categories such as:

• Initial boot
• Application load
• Application data storage and retrieval
• Data backup

Understanding the intended use of storage within the application will help determine the characteristics required of the solution. At the design stage, tailoring the solution to the intended use saves both time and money.

Storage has generally been considered the weakest link in embedded systems design, especially in the case of rotating media. Therefore, careful consideration must be given to environmental factors when reviewing storage product selections to help bolster system reliability.

Environmental considerations
The durability of any storage solution is a measure of how well it stands up to its intended application environment. Given the wide range of industrial application environments, no single storage solution can always be considered ideal. As with all other components of embedded industrial equipment, the storage solution must be selected with an eye toward its operating characteristics and performance within the target environment.

Environmental extremes, especially in operating temperature and shock and vibration, will have a significant impact on operating life as well as Mean Time Between Failure (MTBF). MTBF is the average estimated time in power-on hours before a failure occurs in a component for a given environment. Environmental factors in storage device operability include:

• Shock and vibration
• Operating and nonoperating temperatures
• Humidity, condensation, and dust

Most industrial storage applications are beset by at least two of these three extreme environmental factors. For rotating-media, high-end Hard Disk Drive (HDD) models targeted for industrial usage top out at about 500 g shock, and 1 g at 10 Hz to 500 Hz for vibration, with smaller form factors generally being more robust. Recently introduced extended temperature drives are capable of performing at temperatures reaching +85 °C, but beware of MTBF trade-offs.

The MTBF of any electromechanical device declines quickly with extremes in temperature, vibration, and shock, even in cases where the component specifications have not been violated. Often, the stated MTBF applies to a relatively benign environment and not to the stated specification extremes. This is especially true in temperature extremes. Stated MTBFs for HDDs today frequently top 1 million hours, but this typically applies to controlled environments where ambient temperatures are in the 30 °C to 40 °C range.

Most HDD and Solid-State Drive (SSD) manufacturers will supply more detailed MTBF information on request. In any event, the system designer is wise to rely on empirical methods to determine the reliability of a component under load in the application environment.

By applying careful execution of highly accelerated life testing, the MTBF of a storage device in the application may be predicted. Software tools can predict MTBF using guidelines and environmental factors spelled out in MIL-HDBK-217F and the various Bellcore test methods. These methods, however, use theoretical models and should not be viewed as substitutes for empirical testing under full application load. Frequently, the decision regarding the suitability of the solution comes down to whether to choose solid state or rotating media, as the application demands straddle the line between the two.

Data rate and capacity
Surface inspection applications demand high data rates as well as high-capacity requirements. In one application, six cameras used to inspect the surface of industrial wrapping paper can produce data at the rate of up to 120 MBps. The streaming data and sequential access attributes of this application result in a sustained bandwidth for write operations of up to 12 MBps per HDD.

Just like MTBF, however, data rates are highly application dependent and can vary dramatically. Data rates in more common random access applications can be as low as 10 percent of the quoted theoretical sustained data rates for HDDs. No such trade-off exists regarding SSDs; data transfer rates are virtually the same for both sequential and random access applications because SSDs have no moving parts. Access times in SSDs are measured in fractions of a millisecond, whereas HDD access times can reach 20 milliseconds.

Pipeline inspection involves a similar process with the added requirement that the equipment must endure extremes in shock, vibration, temperature, and humidity. Equipment passes through the pipeline as it records defects that can lead to leakage or rupture of the line. Pipeline diameter and length, as well as the frequency at which the data is reviewed and refreshed, determine the storage capacity required. Here, the harsh environment demands SSDs.

Semiconductor manufacturing equipment such as automatic wire bonders have light data rate and capacity requirements by comparison. An HDD or SSD is used to store the various wire-bonding programs that perform the interconnect operation on the product. Multiple so-called recipes may be stored on the drive – one for each product type to be processed through the equipment. Lower-capacity storage requirements such as < 5 GB typically move this application into SSD rather than HDD even if the environmental requirements allow for either due to the availability of lower capacities in solid state versus rotating media.

The economics of HDDs are such that as the cost per megabyte falls, manufacturers produce higher-volume drives. Consequently, drive costs remain relatively stable, but drive capacity is always on the increase. This may result in difficulty finding a 2.5" HDD with a capacity of less than 30 GB, for example.

Data accumulation – the net effect
Advances in networking capability increase operating efficiency in industrial settings by creating more opportunities for process monitoring and data collection. Using a system of remote sensors and software, real-world data including vibration, temperature, motion, throughput, downtime, machine utilization, operator identification, and a host of other measures can be recorded for real-time or post-shift analysis. As the amount of data grows in volume, so too does its value to the organization. Long-term data collection and trend analysis provide engineers and managers the tools they need to meet economic and safety goals. Thus, solutions are needed to safeguard and preserve data against accidental loss.

New developments in Network Attached Storage (NAS) present a reliable method of providing redundant storage with the capacities needed for today’s applications. Blade-level NAS in a single-slot 6U board with two 2.5" IDE drives and configurable RAID technology can provide for intra- and inter-blade redundancy using the current max drive capacities of 64 GB in SSD and 100 GB in HDD. These small-footprint RAID solutions provide remote access capability to multiple hosts with data duplication and synchronization and no bandwidth penalty at the application level. The RAIDStor (Figure 1, right) from ACT/Technico includes two drives per blade in a RAID 1 (mirrored) configuration and is the first NAS RAID solution for the embedded computing industry in a 6U single-slot form factor.

The dreaded End-Of-Life (EOL) notice
The EOL acronym strikes fear in even the most seasoned engineers in the embedded computing business. Managing media obsolescence is a key concern among industrial equipment manufacturers. It is not unusual for the usable life of capital equipment to reach 20 years, while storage technology can change 15 times or disappear in the same time frame. For example, across all types of storage, lower-capacity models are routinely dropped in favor of the next higher capacity as the available range constantly slides higher. Capacity changes can drive a requalification effort that always falls at the wrong time. Feature enhancements that improve on product capabilities in the long run may also force device driver changes. This can lead to a potentially long and costly requalification effort depending on the level of change introduced.

An EOL situation can be addressed in several ways, the most common and most painless being a last-time buy. Unfortunately, last-time buys are also temporary BAND-AIDs that only serve to stave off the inevitable – requalification when supplies run dry. The risk in predicting future demand and the ramifications of carrying excess inventory or worse yet, being caught without a timely replacement, is always a concern. After-market and remanufactured drives are often utilized, but prompt concerns about reliability, nonexistent or limited warranty programs, and poor post-sale support.

With the help of industry guidelines and specifications for both SCSI and IDE/ATA devices, however, the extent of the requalification effort can be minimized. End users should work closely with drive manufacturers whose industry-standard drives are developed with backward compatibility to legacy systems.

Safe and sound
Another method of data preservation is write protection. Embedded board suppliers are developing small form factor board-level solutions that bring write protection to embedded applications. New products in the familiar IEEE 1386 PMC form factor use solid-state media to produce a robust secure storage solution in convection- and conduction-cooled versions. The media is mounted to the card in one of two ways, either soldered directly to the PC board in an array or as a socketed unit using the popular CF-1/CF-II SSD form factor. The array-based solution is suitable for applications needing higher capacities, though the soldered media cannot be upgraded or replaced. CF-based versions support prequalified  drives in a fully tested and integrated package where the drive  can be upgraded or replaced. Currently CF form factor capacities max out at 16 GB. Figure 2 pictures ACT/Technico’s Secure PMCStor.

Figure 2

Embedded system designers benefit from the wide array of storage devices on the market today, and more choices emerge as the competition heats up between solid-state and rotating storage. Board manufacturers continue to leverage these new developments into form factors ready to meet the demands of the industrial storage marketplace. A complete understanding of the available options will ensure the solution meets the project objectives.

Steve Gudknecht is product manager for ACT/Technico. He has held positions as a field applications engineer and marketing program manager in the semiconductor equipment and embedded computing industry for more than 25 years. Steve’s responsibilities include many phases of product development as well as sales support, with a special focus on storage and networking products.

To learn more, contact Steve at:

ACT/Technico
760 Veterans Circle
Warminster, PA 18974
Tel: 215-956-1200
E-mail: steveng@acttechnico.com