A non-volatile dual in-line memory module is a type of random-access memory for computers. Non-volatile memory is memory that retains its contents even when electrical power is removed, for example from an unexpected power loss, system crash, or normal shutdown. "Dual in-line" identifies the memory as using the DIMM package. NVDIMMs improve application performance and system crash recovery time. They enhance solid-state drive endurance and reliability. "Non-volatile" products may use volatile memory during normal operation and dump the contents into non-volatile memory if the power fails, using an on-board backup power source. Volatile memory is faster than non-volatile; it is byte-addressable; and it can be written to arbitrarily, without concerns about wear and device lifespan. However, including a second memory to achieve non-volatility increases the product cost compared to volatile memory.
Types
There are three types of NVDIMM implementations by JEDEC Standards org:
NVDIMM-F: DIMM with flash storage. System users will need to pair the storage DIMM alongside a traditional DRAM DIMM. NVDIMM-F has been available since 2014.
NVDIMM-N DIMM with flash storage and traditional DRAM on the same module. The computer accesses the traditional DRAM directly. In the event of a power failure, the module copies the data from the volatile traditional DRAM to the persistent flash storage, and copies it back when power is restored. It uses a small backup power source for this.
NVDIMM-P specifications will be released by JEDEC in 1H2020. It will enable computer main memory to be persistent, using Persistent memory technology and can share the DDR4 or DDR5 DIMM interconnect with DRAM DIMMs.
Non-Standard NVDIMM implementations:
NVDIMM-X: DDR4 DIMM with NAND Flash storage and volatile DRAM on the same module, developed by Xitore.
As of November 2012, most NVDIMMs used NAND flash as the non-volatile memory. Emerging memory technologies aim to achieve NVDIMM without a cache or two separate memories. Intel and Micron have announced use of the 3D XPointPCM technology in NVDIMM-F. Sony and Viking Technology have announced an NVDIMM-N product based on the ReRAM technology. In 2015, Samsung and Netlist announced a NVDIMM-P product, possibly based on Z-nand.
Backup power
NVDIMMs evolved from the BBU DIMM, which used a backup battery to sustain power to a volatile memory for up to 72 hours. However, batteries are disfavored in computer components because they have a limited lifespan, they may be regarded as hazardous waste, and may contain heavy metals which violate RoHS compliance. When the module includes non-volatile memory, backup power is required for only a short time after the computer's main power fails, while the module copies the data from volatile to non-volatile memory. Therefore, modern NVDIMMs use on-board supercapacitors to store energy.
Interface
A few server vendors, still make products using the DDR3 interface to the computer, but standardization work in 2014 and 2015, such as at JEDEC and ACPI, was based on the DDR4 interface.
Uses
The BBU DIMM was originally designed for use as the cache of RAID HBAs or systems, to enable data in the cache to survive a power failure. NVDIMMs have moved beyond RAID applications into fast storage appliances or in-memory processing for the data center and cloud computing.
