A Non-Volatile Dual In-line Memory Module (NVDIMM) is a computer memory RAM DIMM that retains data even when electrical power is removed either from an unexpected power loss, system crash or from a normal system shutdown. NVDIMMs can be used to improve application performance, data security, system crash recovery time and to enhance SSD endurance and reliability. Some consider having "non-volatile" in the name misleading as it is still uses volatile memory during normal operation, it just has a dedicated power source to allow the DIMM to dump memory into on board non-volatile memory in case of system powerloss.
There are two main types of NVDIMM cards: NVDIMM-F and NVDIMM-N. NVDIMM-F is a Flash-only device that resides on the memory interconnect. NVDIMM-F can be operated in two modes. In "storage" mode, it is accessed via a block driver and has NAND Flash attributes such as high capacity, but slower access times (when missing the small front-end cache). In "Memory" mode, it is deployed in conjunction with DRAM DIMMs making it byte-addressable and expanding application memory to multiple terabytes in a single system. NVDIMM-F has been available since 2014.
NVDIMM-N is a byte-addressable memory-mapped device, which is typically accessed at memory or near-memory speeds. DDR4 NVDIMM-N cards are being sold by multiple hardware vendors and support is getting wider.[non-primary source needed] These NVDIMM-N devices utilize DRAM for performance and endurance, though are limited in capacity and result in higher costs due to the use of both DRAM and flash memory.
NVDIMMs have evolved from the BBU (battery backed up) DIMM which used a backup battery to sustain power to the volatile DIMM for up to 72 hours; now, to a complete subsystem that includes multiple memory technologies and uses supercapacitors as a power source. The BBU DIMM was originally designed for RAID HBAs (host bus adapters) or systems in order to preserve the contents of the volatile DRAM memory used for cache. However, in computer based applications, batteries have several maintenance and environmental concerns because of their limited life span and hazardous disposal. Furthermore, batteries contain heavy metals which violate RoHS compliance. The NVDIMM was conceived to eliminate the problems associated with batteries in BBU DRAM memory modules. As of November 2012, most NVDIMMs use NAND flash to preserve data, and have moved beyond RAID applications into fast storage appliances or in-memory processing for the data center and cloud computing.
DDR3 NVDIMM cards are supported by few server vendors (e.g. Supermicro), but most of the standardization effort in 2014 and 2015 is focused on DDR4-based system. Such standardization efforts include JEDEC and ACPI.
Current NVDIMM-N and NVDIMM-F technologies are seen by some[who?] as the stepping stone towards emerging non-volatile memory technologies. These families of technologies are being matured, as can be learnt from recent (Jul-Aug 2015) announcements. Intel and Micron announcements on 3D XPoint based NVDIMMs. Sony and Viking Technology announcement on ReRAM based NVDIMM-N.
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