An NVDIMM (non-volatile dual in-line memory module) is hybrid computer memory that retains data during a service outage. NVDIMMs integrate non-volatile NAND flash memory with dynamic random access memory (DRAM) and dedicated backup power on a single memory subsystem.
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NVDIMMs operate at or near the speed of the memory bus, but are not native plug-and-play devices. Manufacturers must modify server motherboards and BIOS/UEFI drivers to recognize the discrete memory types presented by an NVDIMM.
There is some debate whether the NVDIMM acronym is misleading, since the devices primarily use volatile memory to process normal procedures and only invoke the flash element via a backup battery or capacitor to preserve data.
DIMMs and NVDIMMs
An NVDIMM is one of several types of hybrid DIMMs on the market. Server manufacturers historically have designed system memory with either registered DIMMs (RDIMMs) or unregistered DIMMs (UDIMMs).
RDIMMs use an onboard register to electrically isolate the server's memory module from other circuitry on the motherboard. Conversely, a UDIMM is an RDIMM that lacks an onboard buffer. UDIMMs normally are used in servers that require only modest amounts of memory with minimal latency.
Load-reduced DIMMs (LRDIMMs) refer to a newer form factor that also uses a buffer to reduce the electricity load, but without risking power and memory performance.
Advantages of NVDIMMs in servers
NVDIMM technology is emerging as an alternate to DRAM because it is an order of magnitude faster than writing data to 4K block storage. NVDIMMs can be useful for accelerating workloads on next-generation server and data storage platforms.
NVDIMMs provide high-speed DRAM performance coupled with flash-backed persistent storage. Aside from providing an additional memory tier in servers, NVDIMM persistence allows applications to continue processing I/O traffic during planned or unexpected system failures.
The onboard DRAM controller triggers a backup process if it senses a power failure is imminent. Copy data is immediately written to the onboard flash.
NVDIMMs provide an additional enterprise option for server-side flash storage. They can be an addition or an alternative to traditional solid-state drives (SSDs) in servers, add-in Peripheral Component Interconnect Express SSDs, internally mounted M.2 SSDs or mSATA storage devices.
They boost resiliency by retaining data in the event of an electrical power loss or system crash. An enterprise data center relies on an uninterruptible power supply to safeguard data. In this scenario, rebuild time depends on how long it takes to write critical data to a storage area network or network-attached storage system.
Integrating NVDIMMs could help mitigate data loss and downtime associated with writing backup copies.
Workloads for nonvolatile DIMMs
As enterprise data sets become increasingly complex, NVDIMMs have emerged as one possible way to bridge the gap in existing scale-out storage architectures. Their speed and performance advantages gear NVDIMMs toward hot data, warm storage, cache acceleration and in-memory database processing associated with real-time big data analytics.
NVDIMM-N vs. NVDIMM-F
Access to an NVDIMM-N device occurs at or near the speed of DRAM, with the goal of fast data transfer. The DRAM is mapped to the system memory. Applications typically see an NVDIMM-N as a block device. Each NVDIMM-N uses approximately 16 GB of DRAM, matched with an equal share of NAND capacity, to create devices that currently have a top capacity of 32 gigabyte (GB).
The DRAM handles routine data transfers. For persistent storage, NVDIMM-N saves data to the onboard NAND flash element during an interrupt, and then restores it to DRAM when power is resumed. Offloading data protection to flash cuts I/O latency to nanoseconds, although current DRAM power and packaging limitations restrict NVDIMM capacities to the gigabyte range.
NVDIMM-F is an all-flash DIMM of memory-mapped NAND that can operate in two modes:
- Storage mode. NVDIMM-F behaves in similar fashion to a disk drive with block I/O. The NVDIMM-F flash element is accessed in native block format via a shared command buffer such as a mounted drive.
- Memory mode. NVDIMM-F operates alongside DRAM DIMMs for byte-addressable storage, scaling to terabytes of application memory in a single system.
A NVDIMM-F device has slower performance than a NVDIMM-N device.
Emerging NVDIMM hardware, software standards
The evolution of NVDIMMs is tied to advances in double data rate synchronous DRAM (DDR SDRAM) memory chips. As the name implies, DDR SDRAM memory captures data at twice the frequency of the clock speed of a computer processor. Data is transferred in DDR SDRAM twice per clock cycle: once at the falling edge of the clock, and once at the rising edge.
Updates to DDR include versions DDR2 and DDR3. Double data rate fourth-generation (DDR4) is the most recent version. Specifications for DDR4-compliant hybrid NVDIMMs were published in 2015 by the JEDEC Solid State Technology Association, a standards body for the microelectronics industry. The JEDEC standards encompass support for both NVDIMM-F and NVDIMM-N.
JEDEC and the Storage Networking Industry Association are reviewing proposals to standardize NVDIMM-P devices that would map DRAM and NAND in the same memory space. As proposed, NVDIMM-P would provide both byte- and block-oriented drive access.
Leading NVDIMM vendors
NVDIMMs are available from variety of vendors, either directly or through resellers. Flash DIMM vendors include AgigA Tech Inc., Micron Technology Inc., NetList Inc. (via development deals with Samsung Electronics Co. Ltd. and Supermicro), SK hynix Inc., Smart Modular Technologies and Viking Technology.
Intel Corp., through a development partnership with Micron, has announced forthcoming NVDIMMs based on its 3D XPoint storage technology. The checkerboard-like architecture houses two columns that sit atop a composite-based switching mechanism that manages reads and writes by varying the voltage across hundreds of billions of memory cells. Billed as a bridge between DRAM and NAND storage, the vendors claim 3D XPoint will be much faster, have greater endurance and provide higher storage density than NAND.
Capacities of NVDIMM devices are expected to increase as enterprise server and storage makers start delivering their own branded devices. For example, Hewlett-Packard Enterprise (HPE) in 2016 introduced an 8 GB NVDIMM for use with its HPE Persistent Memory storage system, based on HPE ProLiant Gen 9 servers.
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