David McIntyre Altera Corporation

Increased data processing and storage demands are challenging traditional ways of maintaining data integrity in data centers. Increasing performance demands from applications in enterprise finance and Web 2.0 coupled with green environmental requirements are causing data center designers to look for more cost-effective, alternative approaches to today's battery-backed data recovery systems.

RAID adapter cards, a critical component of data center servers and storage complexes, traditionally rely on battery charged recovery circuitry to preserve the integrity of data during server power outages. Burdened by hazardous disposal and total cost of ownership (TCO) issues like field maintenance, system designers are looking for battery-free RAID adapter solutions that are more power efficient without impacting performance and data integrity specifications.

A battery-free RAID adapter design approach for protecting and restoring data during such power conditions is discussed in this article. Advancements in FPGA, flash memory, and capacitor technologies have enabled a lower power, nonvolatile memory backup solution that supports a battery-free environment and the benefits thereof. This article provides an overview of the supporting component technologies that can support such an environmentally-friendly data recovery solution.

Green Trends for Enterprise Data Centers

Increasing data center performance demands under constrained power budgets are creating management issues in power management, system cooling costs, and preservation of data integrity. These factors further contribute to the TCO of running these data centers. In addition, regulatory agencies are mandating green standards that stipulate data center power reductions and strict rules for battery materials disposal.

Power outages across server farms can result in data loss or even worse, undetected error conditions within the host application. To remedy these problems, companies have developed RAID adapter cards that provide a data recovery mechanism during such power outages. The issue with current RAID adapter products is that they are powered by batteries. The table below addresses many of the constraints associated with battery back up systems.

Power Constraints Associated with Battery Back Up Systems Click on image to enlarge.

Convergence of technologies

The challenges for developing a more power-efficient data recovery design lies within the principal circuit components, namely the field programmable gate array (FPGA) for circuit management, data control, and the flash memory for data storage and retrieval upon power recovery. Each of these components must support a power budget specified by the data recovery and restore circuit of the RAID adapter card.

Determining the FPGA and flash memory specifications is a function of the DRAM density that is backed up during a power outage (e.g. 2-8 GB) and the time available to store this data into flash memory, where the time constraint is driven by the available energy from either a battery or alternate green energy source like ultra capacitors.

With lithium ion battery backed designs, data can be stored over a period of 48-72 hours. This approach requires field resource maintenance to address the power outage within this timeframe without losing the data that was stored in flash memory by the battery backed RAID adapter backup circuit. Furthermore, battery disposal must be factored into the cost of supporting this type of design.

Power efficiency and performance advancements in FPGA and flash memory technology now make it possible to develop effective data recovery circuits in RAID adapter cards that can be charged from alternative charging sources like ultra capacitors.

FPGA Technology

The process technology used in Altera FPGAs has moved downward from the 65nm node to the 40nm node. Sharing the cusp of the technology curve with standard cell ASICs and full custom designs in microprocessor and associated chipsets, FPGAs are now being used in volume production computer and storage applications like high performance computing (HPC), I/O virtualization, high speed serial interface bridging, and memory backup/restore functions.

In addition, the latest advancements in FPGA architecture design and supporting power simulation tools have allowed increasing FPGA logic densities to be offered without the penalty of increased power consumption.

FPGA Architecture of the Cyclone III and its' low power attributes Click on image to enlarge.

Embedded processing, once limited to ASIC implementations, is now available as an option in production FPGAs. As an example, Altera offers the Nios II configurable processor core that supports up to 340 DMIPS with a single instantiation. The power saving benefit of a soft-core processor over external microprocessors and microcontrollers further contributes to a more power-efficient data recovery circuit.

In a RAID adapter memory backup circuit (the figure below), the Nios II embedded processor controls the operating state of the DIMM and data movement between the DRAM blocks and flash memory.

Embedded Processing Consolidation with a Nios II Processor Subsystem Click on image to enlarge.

Flash memory

Flash memory technology has accelerated past Moore's Law by greater than doubling the density/performance every 18 months. Exponential demand for increased flash memory capacity driven by new consumer all-in-one phones is expected to move flash technology to the 20nm node by late 2010 (the figure below). NAND flash provides read/write characteristics similar to hard disk drive sectors with lower power, lower cost($/GB), and environmental robustness.

Process Technology Node Projections for Flash Memory. (Source: Wikipedia Flash Memory)

Furthermore the ONFI 2.0 FLASH interface, supporting bandwidths up to 200MB/s provides an impressive step in data link speeds thereby enabling RAID adapter memory recovery for DRAM capacities up to 8 GBytes using ultra capacitor charging circuitry.

These advances in flash memory technology directly support the need of a RAID adapter recovery circuit that needs to protect increasing DRAM densities while supporting minimal power requirements thus enabling alternate green energy sources like ultra capacitors.

Charging circuit options

Batteries

Lithium ion batteries are the predominant charging source for today's memory backup products. The single advantage of continuing to use batteries for RAID adapter cards is the maturity and wide adaptation of this technology mitigates reliability concerns using newer, alternative technologies like ultra capacitors.

However, the disadvantages are significant. Companies do not want to incur the overhead costs with hazardous waste battery disposal nor do they want to staff field teams to address server data recovery prior to the energy depletion from the battery.

Ultra Capacitors

Ultra capacitors (also known as electric double layer capacitors) were once only applicable to industrial applications with short burst energy requirements like engine control and turbine generators. However, recent technology advances have supported smaller form factors and densities enabling higher volume applications like RAID adapter cards.

In the past, a one Farad capacitor would be a housed in a formidable container certainly not suited for space constrained environments like RAID adapters. Today's ultra capacitors can supply sufficient short-term energy bursts in smaller packages.

Ultra Capacitors (Photo courtesy of Maxwell Technologies) Click on image to enlarge.

Ultra capacitors are environmentally friendly. They consist mainly of carbon and aluminium and contain no heavy metals. There are no hazardous disposal issues associated with ultra capacitors as with batteries. Ultra capacitors also have a longer shelf life. This is important to RAID adapter applications where power outages may be infrequent. In addition, ultra capacitor recharging can be done in approximately 20% less time.

The following Ragone Chart (the figure below) shows the suitability of ultra capacitors for applications that require quick delivery of their rated energy density. There are trade-offs using traditional lithium batteries for higher energy density requirements versus using capacitors that offer lower energy densities, but compensate with higher power density attributes. Ultra capacitors offer a compromise between these two technologies with an increased energy density over conventional capacitors, while still delivering greater power densities than lithium batteries.

RAID adapter card specifications require short bursts of energy to provide sufficient power density for backing up data during power down conditions. Capacitor technology supports these energy requirements through the physical law of Energy (Joules) = 1/2*Capacitance*Voltage^2. Ultra capacitors suppliers, (i.e. Maxwell Technologies) can support power densities greater than 1kW/Kg in compact packages (see the figure above).

Ragone Chart Comparing Battery and Ultra Capacitor Technologies Click on image to enlarge.

There are some concerns that RAID adapter card designers should be aware of when employing ultra capacitors. Life expectancy will degrade as a function of voltage and temperature. The figure below shows how capacitor life expectancy is reduced by 50% for each 10° C rise in temperature. Therefore, it is prudent to remove the ultra capacitor module from any intense source of heat, such as a microprocessor and supporting power circuitry.

Ultra Capacitor Life Expectancy (Graphic courtesy of Maxwell Technologies) Click on image to enlarge.

Green memory backup solution example

The following section provides an overview of a RAID adapter memory recovery design. As described above, power efficiency improvements in FPGA, flash memory, and ultra capacitor technologies have enabled a more power-efficient design compared to present day battery backed products.

System overview

The figure below illustrates the example memory backup circuit. During a power outage, the microprocessor will send a power down signal to a low power Cyclone III FPGA that initiates and manages the recovery circuitry. The FPGA functions as a management controller with an advanced embedded processor, a custom DDR3 controller with custom ONFI host interface, and an I2C bus.

The primary benefits of the FPGA are derived from a higher performance, lower power architecture versus external processor based designs. The power savings benefit is achieved through a variety of techniques that are commonly used to reduce device power, including multiple device threshold voltages and variable channel lengths in the gates that allow slower circuits to consume less power. Banks of nonvolatile flash memory perform the data storage function to recover the DRAM array data during a power outage. The following table describes the functions and benefits of the primary components in this design.

RAID Adapter Memory Recovery Design Click on image to enlarge.

Primary Components of Memory BackUp Design Click on image to enlarge.

Summary

RAID adapter cards are critical data center subsystem components that ensure data storage and recovery during power outages. Current designs are battery backed that create green issues around hazardous waste disposal as well as shelf life and maintenance issues. Recent advances in FPGA and flash memory technologies support lower power memory backup designs that are powered by batteries or ultra capacitors. Ultra capacitors provide an environmentally green alternative to battery usage, which is an advantage for data center designers that are being challenged to find lower power, green alternatives without sacrificing data integrity and performance.

Biography

David McIntyre manages the Computer and Storage Business Unit at Altera Corporation. His responsibilities include driving top tier customer growth with initiatives and solutions. With 20 years of experience at leading semiconductor and systems companies, he has held various engineering and marketing management positions including a Director of Strategic Marketing post for the IBM Storage Systems Division.