Battery backup solutions for saving cache data during unplanned shutdowns are long-established and proven, but have disadvantages. The major issues with batteries include replacement, service calls, disposal costs, system space limitations and “green” content requirements. Sometime around 2004, some RAID storage system designers began observing “wouldn’t it be great if we could get rid of the batteries needed to power the DRAMs for emergency 72-hour cache backup and switch to a flash-based solution?” It was referred to as a “self-backing DIMM” and other similar names. Today, the somewhat intangible costs of using battery-backed solutions are helping to drive the transition to battery-free RAID backup solutions that make use of non-volatile registered DIMMs (NVDIMMs), which are rolling out in various densities and form factors.
Power efficiency and performance advancements in FPGA and flash memory technologies now make it possible to develop effective data-recovery circuits in RAID back-up solutions that can be charged from alternative charging sources, such as ultra capacitors. For example, the ONFI 2.0 FLASH interface enables very fast RAID memory backup and recovery times – writing from DRAM to flash for backup and then back to DRAM for recovery. This architecture uses an NVDIMM, which is an ONFI flash-based registered DDR3 DIMM that transfers data in the DRAM array to non-volatile flash memory in the event of a system power loss. Only the DIMM needs power until the data within the DRAM devices are written to flash memory. Once the transfer is complete, the DRAM array can be powered off until system power is restored. After that, the data can be restored to the DRAM array from the flash memory. Reducing the backup time interval allows the power source to be replaced by a supercap card. (The typical backup and restore times average less than 10 seconds per gigabyte for each function.)
The innovation known as supercaps or ultracaps can supply enough power to move data from the DRAM array to the flash memory. Consisting mainly of carbon and aluminum and containing no heavy metals, supercaps are environmentally friendly with no hazardous disposal issues. They also have a longer shelf life than batteries, making them particularly well-suited for RAID back-up applications where power outages may be infrequent. They can be recharged in a fraction of the time required by batteries, and they retain data for years. While supercaps cannot supply power as long as rechargeable batteries, they can supply enough to power a DIMM for a few minutes. Once the backup is complete, the DIMM turns off until the next power cycle. Densities up to 8GB and 16GB can be achieved per module by combining ONFI flash and DDR3 DRAM that are then powered by the supercap cards.
NVDIMMs combine advancements in flash, DRAM, FPGA, and supercap technologies, and they are helping to drive industry adoption in the RAID storage market. IDC projects that storage capacity is growing at an average rate of 52% per year. The capacity trend for emergency cache backup storage is also growing, and it is now averaging between 2GB to 8GB per processor. Today’s commonly used backup solutions of either RAID adaptor cards or battery-backed portions of main memory are being stretched to the limit. As a result, NVDIMMs and Mini-NVDIMMs are poised to gain significant traction over the next two years. According to IDC, the forecast for external storage systems which includes RAID adaptors is expected to grow from $22B in 2010 to approximately $26B in 2013. With NVDIMMs, the need for RAID adaptors becomes redundant (because the NVDIMMs can function as main memory as well as be used for cache backup), and there is no longer a need to create a separate memory subsystem for emergency cache backup. A growing percentage of the external storage system market will transition to include NVDIMMs.
SMART Modular Technologies and other vendors are offering various types of NVDIMM solutions. Their evolution continues as industry standardization efforts and work between memory vendors and processor suppliers are underway. These efforts will accelerate once there are a more common set of cache backup solutions available to storage system OEMs and a more common set of NVDIMM system integration schemes. However, despite these challenges, NVDIMM solutions are available now, and the industry expects that they will shortly become a plug-and-play solution.
The NVDIMM is powered by supercapacitors long enough for the DRAM contents to be copied to NAND flash. After the copy to NAND flash is completed, the NVDIMM no longer needs power and the original volatile DRAM contents are saved to the non-volatile NAND flash.