hybrid DIMMs, NV DIMMs, flash backed DRAM DIMMs etc



This resource page includes notes and articles about DRAM modules which automatically save their contents when electrical power drops to an integrated non volatile memory from which the data is reloaded after normal power is restored.

This scope includes but is not limited to:- flash backed DRAM DIMMs, hybrid DIMMs and NV DIMMs - in which the primary resource seen by the host is a random access low latency, infinite endurance memory - which from the R/W point performance of view is identical to a DRAM - but whose contents are transparently saved to an integrated non volatile memory (such as flash) in the event of power disturbance - and restored when normal line power resumes its ready state.

The scope excludes:- memory channel SSDs - which are low latency flash SSDs which are in a DRAM form factor and which use the native DRAM interface as the primary data transfer route.


DRAM with integrated backup and restore in a DIMM socket

by Zsolt Kerekes, editor - StorageSearch.com- October 21, 2014 (updated in 2017)

historical context and evolution

Almost as soon as the first bytewide CMOS static RAMs became available in the late 1970s - electronics designers realized that with the addition of a small lithium battery (which could maintain the RAM's contents in the unpowered state for 5 to 10 years) they could easily create a unique type of storage device which was easier and cheaper to integrate into small physical spaces than the other options then available.

This enabled many useful functions to be included in products which would - less than a decade later - be mostly replaced by flash.

For applications which required more than a single NV RAM memory device - the economics of system level integration were different. 

And for applications which used large numbers of RAM chips in a single non volatile array configuration - the early history of non volatile RAM is really the early history of RAM SSDs - as cards, drives or rackmount products - rather than as specialized components.

how flash entered the scene of keeping DRAM content alive

As RAM SSD capacities grew in the 1990s - the physical space occupied in the racks by batteries became a significant percentage of the SSD itself. 

And to reduce the risk of data loss - the battery systems themselves started to get more complex in order to prevent the loss of all data due to a single battery failure.

So by the early 2000s - the leading RAM SSD makers optimized the battery footprint problem by including a hard drive in the SSD rack to which data was backed up when the incoming power went down. That also meant the batteries only had to maintain the DRAM array in a powered state long enough to support the backup and restore operations.

By 2007 - the gap between the growing size of enterprise SSD capacity and the poor random IOPS performance of a single backup hard drive - resulted in cold boot restore times for high capacity RAM SSDs which could take from 30 minutes upto over an hour. 

That disparate asymmetry between R/W and power down/up characteristics - compared to the needs of the big server users who formed the vanguard of enterprise SSD adoption was an uncomfortable truth.

enter flash

In July 2008 - a new solution to the power up restore ready time for large DRAM arrays was illustrated by Texas Memory Systems - who included a RAIDed flash array in their RamSan-440 (a 512GB 600K IOPS FC compatible RAM SSD). 

And although - at 4U rack space and $290K price - it was a long way from the flash backed DRAM DIMMs in the market which would follow in later years - in another way it was the ancestor of them all - inasmuch as it demonstrated that viable enterprise storage products could be engineered by intertwining the most desirable characteristics of each type of memory - DRAM and flash - into a useful new type of hybrid.

enter flash backed DRAM in modern DIMM form factors

As has oftentimes happened in SSD history - any good architectural ideas which first prove their worth in system level rackmount SSDs soon trickle down and reappear at component levels. And so it was with the flash backed RAM concept. 

The main market events (as they appeared from an SSD news point of view) being represented in the following timeline.

In May 2009 - AgigA Tech began sampling the AGIGARAM - a DDR2 compatible module which implemented upto 2GB of non volatile SDRAM (with integrated flash backup and restore) which plugged into standard JEDEC DIMMs. This required a companion module to be located somewhere close to the memory - which routed power to a bank of supercaps and power management controller.

AgigA Tech explained in a related architecture paper (pdf) that the supercaps were only required to support the backup and restore operations to and from the onboard flash. The backup took about 9 seconds for each gigabyte of RAM.

In October 2011 - Viking - entered the flash backed DRAM DIMM market with the launch of the ArxCis-NV - which plugged into DDR3 RAM sockets and provided uptto 8GB RAM backed up to SLC flash in the event of a power failure. As was the case with the earlier products from AgigA Tech - the Viking ArxCis-NV required a separate 'tethered' supercap module to implement a functioning system.

In October 2013 - SMART announced that its DDR3 Non-Volatile DIMMs (NVDIMMs) were fully compatible with and automatically recognized by the BIOS on new Supermicro X9DRH-iF-NV server boards.

In January 2014 - SNIA formed the NVDIMM Special Interest Group (ORG) - which at the time of launch included 12 companies:- AgigA Tech, IDT, Inphi, Intel, Micron, Microsoft, Pericom, Samsung, SK Hynix, SMART and Viking.

In August 2014 - AgigA Tech announced that it was sampling the industrys first DDR4 Nonvolatile DIMM (NVDIMM) to key OEMs and development partners. 

In November 2014 - Netlist's CEO Chuck Hong - said in an earnings conference call - 'We have shipped almost 0.5 million units of NVDIMM over the past 5 years. This is over 20x more than the total shipments from all of our competitors combined.'

In August 2015 - Diablo, Intel and Micron unveiled high capacity RAM-like products which would compete alongside and take business away from the flash backed DRAM DIMM market. See DIMM wars opening salvo for more about this.

In December 2015 - 'retiring and retiering enterprise DRAM' was named as one of the the big SSD ideas to have emerged 2015.

In September 2016 - Netlist announced the issuance of U.S. Patent No. 9,436,600 (the '600 patent) relating to a multi-channel hybrid memory architecture that uses non-volatile memory to protect critical data in the event of power loss.