Beat the heat - designing SVDR systems for optimal thermal performance

September 2006 CCTV, Surveillance & Remote Monitoring

While the popularity of digital video surveillance systems continues to soar (JP Freeman estimates worldwide annual growth at approximately 42%), there have been challenges along the way.

Surveillance digital video recording (SDVR) system reliability has proven challenging for some, particularly in terms of the hard disk drive (HDD). This is primarily due to the harsh environmental and operational factors (poor ventilation, 24x7 write workloads) typical of SDVR disk drive applications.

Fortunately, following a few simple guidelines for optimal SDVR system design and deployment will substantially enhance drive reliability. The single greatest threat to disk drive longevity is heat. Not only is heat a leading cause of outright component failure, it can also degrade system performance and stability. To understand why heat management is so critical to disk drive reliability, it is useful to briefly review the fundamentals of drive operation.

Disk drive basics

HDD storage relies on magnetism to store and retrieve data on the drive's platter(s). Tiny independent magnetic cells in the platter's coating are magnetically reoriented into a specific pattern by the drive's read/write head to write data, and those cells' magnetic orientation can be subsequently detected by that same head to read back that data.

This arrangement is made possible by the extraordinary proximity of the read/write head to the platter surface. Riding a scant few nanometers above the rapidly spinning platter, the complex read/write head system has been likened to a Boeing 747 flying at 960 kilometres per hour - 15 cm off the ground.

The strength of any object's magnetic field rapidly falls off as distance from it increases; this principle applies to both the magnetism of the read/write head and the magnetic cells on the disk surface. Thus, to reliably write and read information, the head must remain a very close and consistent distance from the platter. Any disruption of the physical or magnetic relationship between head and platter surface can jeopardise the integrity of the drive's data.

Why heat is a killer

Excessive heat can undermine this sensitive relationship in a number of ways. The read/write head rides on a cushion of air pressure above the platter surface; should the drive case become too hot (over 35°C), the air inside expands so much that it affects the flying height of the read/write head. While drives are designed to compensate for changes in head/platter spacing, such compensation degrades signal integrity, alters the media's magnetic properties and, in extreme cases, can lead to data loss.

Internal chassis volume, fan output (measured in cubic feet per minute, or CFM) and fan count all influence the airflow rate of the chassis. The larger the chassis, the more air must be moved to attain a given flow rate (CFM) through the chassis. Smaller fans are sometimes used to generate high flow rates in specific areas of the enclosure, while larger high-capacity fans handle overall airflow through the enclosure.

Matching fan capacity and speed to the internal volume and shape of the chassis is key to achieving optimal cooling, lower power consumption and reduced acoustics.

Size, shape, number of heat sinks

Heatsink efficiency is affected by the sink's mass, surface area and fin configuration. Heavier heatsinks can absorb more thermal energy, while more square inches of surface area speeds transfer of that energy to the surrounding air. Heatsinks with fins perpendicular to the floor tend to be more effective because they enhance convection cooling.

Adding more heatsinks can theoretically yield more cooling capacity, but heatsinks are relatively expensive and the law of diminishing returns soon comes into play. When properly configured and located, only a few sinks can greatly increase system cooling.

Power profile

The energy requirements of the system's internal components significantly impact its power profile, and in turn its heat production.

Power supply efficiency

An SDVR system's power supply converts AC line voltage (nominally 120 or 220 volts) to an appropriate DC voltage level (typically 12 volts or less). Different types of power supply designs, such as switched-mode vs linear mode, can consume significantly different quantities of AC power to deliver the same DC voltage. More efficient power supplies require less AC input for a given DC output, which in turn means less energy is dissipated as heat. Furthermore, such high-efficiency power supplies are typically smaller and thus take up less chassis space, enhancing system airflow.

Power consumption of system components

An SDVR system's CPU, video subsystem (including MPEG encoder and decoder) and HDD storage all contribute to the system's total power consumption. While the CPU handles overall system functionality, the MPEG encoder works in concert with the CPU, converting analog video signals into digital data that is written onto HDD storage.

Depending on the number of cameras attached to the system and the desired frame rate (frames per second) of the incoming video streams, the ensuing workload can place significant demands on both the encoder and CPU, forcing them to work harder and thus produce more heat. Employing multiple encoders to handle additional video streams exacerbates the thermal challenges.

Careful planning of video surveillance zones can trim both the number of cameras and minimum frame rates required for optimum video coverage, reducing hardware costs and cutting the workload (and thus heat output) of the SDVR system's CPU and video encoder.

Choosing HDD storage that combines maximum capacity-per-disk with sophisticated power management capabilities can also significantly contribute to lower system power consumption (see below).

Quantity of components

A typical SDVR system uses only one CPU, but the number of HDDs it employs can range from one to several or more. Not only does each disk drive impose its power needs on the system's power supply, it also introduces another potential point of failure into the system.

Minimising the number of HDDs used to meet an application's capacity requirement is one of the simplest and most effective ways to cut system power consumption and heat production. And by occupying fewer drive slots in the system chassis, greater airflow is also promoted.

The growing availability of high-capacity (250, 400 and 500 GB), purpose-built SDVR HDDs enables systems to deliver enormous capacity with a minimal number of drives. These new drives also incorporate sophisticated power management features (see below) to enable even greater system power economy and thermal efficiency.

Operational practices

HDD power management features will play an increasingly important role in minimising SDVR system heat challenges. Leveraging the ATA-7 command set enables drives to efficiently address both video- and data-specific tasks.

HDD spin down

At any given time, only one HDD in a typical SDVR system is actively writing video images. Though the rest of the drives in the system are idle, they are nevertheless kept spinning; this needlessly consumes power, produces more heat and entails additional wear on the drives.

Specifically designed for SDVR duty, a new generation of drives incorporates intelligent power management features that enable the host to spin down idle drives into sleep mode. The power savings are significant; while an idle drive may consume roughly 8 watts, a sleeping drive needs only 0,25 watts of power. Reducing power consumption by up to 96%, these new drives deliver greater energy efficiency, cooler running and longer drive life.

Reduced current draw at HDD startup

A conventional disk drive requires significantly more power at system startup or when awakened than when simply spinning. While bringing a single drive's platter(s) up to speed and energising the various components within demands substantial power, the load on the system's power supply is particularly severe at startup, when multiple drives are simultaneously powered on.

This increased current draw demands larger (and more expensive) power supplies, whose higher-capacity output also entails more heat. Purpose-built SDVR drives require less startup current (under 2,0 amps), which enables the use of smaller, less costly power supplies. Once again the result is greater efficiency and cooler running.

Optimise read/write strategies for data activity

SDVR systems focus on video streaming capabilities, but they must also be able to read and write traditional data structures used to manage video databases and related applications. Optimal efficiency can only be achieved by matching the HDD's read/write strategies to the type of data being used.

Purpose-built SDVR drives support the ATA-7 command set, enabling the drive's read/write profile to be specifically tuned for video or data. This ensures video read/writes stream more reliably, while data read/writes are optimised for data integrity. By curtailing extraneous disk activity, this approach also reduces power consumption and operating temperatures.

Conclusion

Optimising the thermal performance of your SDVR system can pay a host of dividends: higher HDD reliability and greater data integrity, longer component life, plus improved system performance and stability. In addition, the steps outlined above for lowering system temperatures can also significantly enhance the energy efficiency (and thus cost effectiveness) of your SDVR system.

For more information contact Mark Campbell, Storgate Africa, +27 (0) 11 695 1600, Mark@storgate.co.za, www.storgateafrica.com




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