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RAM SSDs versus flash SSDs? - (aspects of asymmetries in SSD design)

by Zsolt Kerekes, editor, April 24, 2012


Nowadays over 97% of high speed enterprise SSD capacity is flash based.

The original popular classic article below (which had contributions from 10 of the leading SSD companies at the time) was published in August 2007 at a time when the enterprise rackmount SSD market was still dominated by RAM SSDs and there were many questions about how the market was going to change. To put this article into context - with the benefits of 20 20 hindsight.

It was 2 years before SandForce emerged from stealth mode and reset the 2.5" SSD market.

It was the same month that the 1st enterprise rackmount array of flash SSDs appeared from EasyCo with a flash management scheme which reduced write amplification in notebook SSDs and boosted throughput. In those days - you could burn out most notebook SSDs in a few months if you placed them into a high IOPs system. Converting unreliable flash into reliable SSDs remains a design challenge today - because flash is getting intrinsically worse as geometries shrink - while markets demand increasing SSD performance.

It was the month before Fusion-io's first PCIe-SSD shipped. (It was great on performance - but still didn't include all of the fault tolerance features which were introduced in later models.)

It was the month before Texas Memory Systems launched the RamSan-500 - a 2TB flash SSD rackmount with 100K / 10K R/W IOPS. (Symmetric R/W IOPs performance in flash SSDs came a year later in 2008.

Despite advances in flash SSDs - and their market dominance today (in 2012 - year of the enterprise SSD goldrush) there are still niche roles for RAM SSDs in very low latency applications. And there are good reasons why RAM SSDs will remain 1 of the 7 SSD types in the pure SSD datacenter market of the future too.

The conceptual comparisons in the article below - are still valid. But the numbers have changed.


RAM SSDs versus Flash SSDs - which is Best? - War for the datacenter core

10 leading SSD experts discuss the state of the market - intro by Zsolt Kerekes, editor (August 2007)

New product architectures have been closing the performance gap between RAM and flash based SSDs.

Meanwhile improved flash technology and media management controllers have blown away the reliability limitations which made earlier generations of flash SSDs unsuitable for datacenter applications (regardless of speed).

There is no single "right answer" to the question - which technology is best. It depends on the business application and the performance / cost trade offs of reaching higher server performance.

Revenues for companies in both SSD market segments (RAM and flash) have been growing fast in recent years. That may change for individual companies due to lower average selling prices and competitive alternatives. Lower ASPs will come from falling memory prices (over the long term) and shorter term from aggressive competition from new SSD market entrants who want to buy market share. Despite the inevitable shakeout of many SSD oems - I expect the number of SSD vendors to continue growing for the forseeable futures and overall revenue for both SSD technology segments to continue high double digit growth.

The market opportunity represented by the enterprise server SSD market will be worth billions of dollars.

Which is best for you? Where is the market going?

I asked leading companies to contribute to this article. This is what they said.
....
Since this article was published - other ways of looking at the RAM to flash question in SSD apps which are discussed in other articles are:-
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Violin Memory
In August 2007 Violin Memory launched the world's fastest 2U SSD and the densest SSD using RAM..

It was the first oem to publicly announce that it is developing SSD products with both RAM and flash technologies.
DRAM vs. Flash SSDs

A Perspective from Morgan Littlewood, VP Marketing and Business Development at Violin Memory

It seems unlikely that a single technology will ever excel across all key dimensions of a storage system, including performance, capacity and price. The obvious analogy is that while the world's goods are transported via planes, trucks, trains and ships, each of these markets continues to grow as trade increases. Performance, capacity and price are also important attributes of a transport system.

Each core technology has its own limitations. Extracting more IOPS from disks is increasingly difficult and, hence, disk developments focus on capacity and bandwidth enhancements. Flash developments focus on capacity and cost effectiveness, but as a result, performance and reliability are being negatively impacted. DRAM developments focus on cost and performance improvements, but are constrained by reliability requirements.

The table below of storage technologies illustrates the unique attributes of each technology.

Capacity hard disks have a cost/GB which is significantly better than solid state.

Flash SSDs are very well suited to read-only applications (and mobility applications).

DRAM SSDs have costs per IOPS which are much better than Performance hard disks or Flash SSDs.

The success of solid state will undoubtedly have most impact on the performance HDD market.
Table of Storage Technologies and Key Metrics - source Violin Memory
Technologies Capacity (GB) Latency (µS) IOPs Cost / IOPs ($) Cost / GB ($)
Capacity HDDs 2,500 12,000 600 13.3 3
Performance HDDs 700 7000 1,200 16.6 28
Flash SSDs 700 200 500 140 100
Flash SSDs (read only) 700 45 50,000 1.4 100
DRAM SSDs 250 3 200,000 0.5 400
Notes

1 - Metrics are all normalized as typical rackmount system statistics per U of height (1.75")

2 - Latency and IOPs estimates assume same numbers of random reads and writes

3 - Latency and IOPs estimates assume 4K block sizes
Within a single Enterprise, there may be thousands of applications and each of these has its own performance, cost and capacity issues. Even a common software application (e.g., database) can have a wide variety of requirements depending on the size of the databases, their update rates and the rates and complexity of queries the users require. Solid state solutions must be able to scale to cover this wide range of requirements.

The scalability (capacity, density and performance) of solid state solutions will determine the size of the market opportunity. Improved density has the secondary effect of driving down both capital costs and the operating costs associated with power and cooling. Scalable memory solutions increase the affordability of the technology to a wider range of applications and will drive market growth.

DRAM and Flash technologies will co-exist with disk systems and address the performance and reliability constraints of rotational media. The unanswered question is: "How will each of these technologies best be used?" Is Disk emulation the only solution? Why connect to DRAM via a slow disk attachment interface like Fiber Channel? DRAM has read and write latencies so low that it can be used as an extension of server memory. Flash reduces the need for duplicate data copies by dramatically accelerating read-only storage. It is not clear whether emulating the 4K blocks of hard disks is the most efficient long term use of solid state technologies.

Violin has designed memory appliances to maximize the technical benefits of DRAM and Flash technologies either separately or in combination. DRAM and Flash are installed as high-density plug-in modules. Both disk emulation and other APIs are supported as configuration options. The objective is to enable the enterprise with optimal solutions that meet their specific needs, for a broad range of applications.

In summary, "Solid State Drives" is too limiting a term for the market segment. Emulating a spinning disk is a not a great aspiration for solid state technologies, which are radically higher performing. Then again, car engines are still measured in horse power!
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Solid Data Systems logo - click to see their home page
Solid Data Systems has 14 years experience supplying RAM SSDs for enterprise speedups. In 2007 they joined the terabyte SSD club and announced lower price / capacity products.
Hard / Flash / RAM Disk Performance in Databases

by Wade Tuma, founder and CEO of Solid Data Systems

Database transactions are by nature random and usually come with small block sizes. This white paper analyzes and tabulates cost and performance tradeoffs between hard drives, flash-based solid state disks and DRAM-based SSDs.

It discusses considerations in choosing the right technology and provides a viewpoint on performance comparison using industry-standard benchmarks and handy graphical guides for estimating read and write database performance improvements.
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Solid Access Technologies
Solid Access Technologies shipped the industry's first serial SCSI compatible SSD in April 2005. They also make parallel SCSI and FC models. That gives them a unique insight into a wide range of enterprise customer speedup applications.
"RAM versus Flash SSDs - which is Best?"

by Tomas Havrda, Solid Access Technologies

HDD vs. SSD, Flash SSD vs. RAM SSD, RAM SSD vs. Ultra fast RAM SSD, mice vs. frogs whatever it takes to build on the unquestionable excitement surrounding SSD in 2007, I'm for it.

Regardless of which type is purchased, every SSD sale helps to accelerate the SSD solution concept into the mainstream user's mind-set. Flash SSDs have received a lot of media buzz, and for some customers it is the perfect solution. However, they simply won't cut it in demanding enterprise environments because of their limitations relative to write cycles durability, write performance, connectivity and device leveraging (multipathing), not to mention multi-bit error correction capability. Fortunately, SSD customers have plenty of options.

As the producer of the ultra-fast RAM SSD Solid Access USSD 200, we know that our USSD 200 solution is, for many applications, the most effective solution. It can process, via just one single FC port, 95,000 IOPS (read or write, random or sequential) and 55,000 IOPS via a single wide SAS port. For more than 2 years, it has been the only SAS-interface SSD available.

All of our customers could have deployed flash SSDs. Why didn't they? Because a thorough evaluation of their application issues and potential solutions led them to USSD 200. They did what every customer should do they paired a specific problem with the right solution.

SSD OEMs naturally showcase the highest IOPS numbers reached with the shortest data blocks. However, because applications utilizing SSD typically use 4Kb, 8Kb , 16Kb block sizes, the combination of IOPS and data throughput is a more relevant measure of the overall application performance boost. For example, absolute IOPS numbers for USSD 200 FC are higher than for SAS, but the USSD 200 SAS offers twice as much data throughput for read or write (almost 800MB/s sustained via each wide port) than Fibre Channel, making SAS the fastest SSD connectivity out there. SAS is also the fastest growing; by the end of 2008, Gartner predicts data storage interface representation of 41.2% SAS, 22.9% FC, and 7.3% SCSI. On top of that, SAS HBAs cost 4x less than FC HBAs.

Assertions of superiority of one technology over another actually miss the point. The real issue here is transparency. Performance and pricing data should be clearly and accurately presented to the consumer; then, let the customer decide which technology best suits their needs. At Solid Access, we publish both our price list and our benchmarks. Furthermore, if a potential customer sends us an IOmeter script based on their app patterns, we will run it on USSD 200 FC, SAS or SCSI and generate a test result report for them. Potential customers may also test the SSD solution before committing to buy.

In short, we believe that an unambiguous presentation of pricing and performance data places the customer in the best position to make an informed, sound decision regarding their SSD needs.
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Objective Analysis
Editor:- in August 2008 - Objective Analysis published a report (price $5,000) called - "Solid State Drives in the Enterprise".

That report covered flash SSDs. So I asked the author, Jim Handy to share some of his thoughts about the RAM SSD market.
Are RAM SSDs Threatened by Enterprise flash SSDs?

by Jim Handy, Objective Analysis

With all the buzz that is surrounding flash-based enterprise SSDs these days it would be natural to assume that the DRAM SSD's days are numbered.

Quite the opposite is the case.

Enterprise flash SSDs offer very fast read speeds, but since they are based on NAND flash they cannot match the speed of a DRAM device. DRAMs have truly random access, where NAND flash has a relatively long latency before bursting out data. Although NAND's 25 microsecond latency is 80x as fast as the 2 millisecond average latency of a 15K HDD, it still is over 1,000x slower than the 20 nanosecond maximum access of a typical DRAM.

DRAMs also have significantly faster write speeds than those of NAND, and are byte-writeable: Any byte of the DRAM can be written to at any time at about the same speed as its read speed. A NAND chip requires an entire 2K-byte page to be written at any one time, and before that happens, a 256K-byte block must first be erased, taking up to 10 milliseconds. To this end enterprise-class flash SSDs use a small DRAM array to accelerate writes.

A flash-based enterprise SSD is really good at accelerating an enterprise system at a modest price ($5-10,000) with few changes to the system's software or hardware. The degree of acceleration depends upon the system's configuration before the SSD was added. Systems that taxed the ingenuity of the IT team - ones that use RAID or short stroking to coax more bandwidth out of enterprise-class HDDs - will see the greatest benefit. The less such techniques are used, the less the flash SSDs will be able to help.

DRAM SSDs have always been for the most extreme cases. If a flash SSD can be likened to using dynamite, then a DRAM SSD can be compared to a nuclear bomb. The system is physically larger than a flash SSD, using a rack or cabinet rather than an HDD's form factor, and the price may be a cool half million or so, but the cost/benefit tradeoff can be compelling for those few who need everything and more.

While a flash SSD can reduce the number of enterprise HDDs a system requires, DRAM SSD makers tell us that the addition of one of their systems can eliminate the need for a ten or more entire servers.

So, are DRAM SSDs threatened by flash SSDs? Not at all! As a matter of fact DRAM SSD makers have told us that the heightened awareness of SSDs in general has brought prospective customers to them who may otherwise have remained aware that these extraordinary devices exist.

see also:- Jim Handy's article Flash vs DRAM Price Projections - for SSD Buyers
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Memoright flash SSDs
Memoright makes the industry's fastest shipping 2.5" SSDs (October 2007). They are targeting the hard disk replacement market - but their products can also be useful in the server acceleration market.
Introducing the Fastest 2.5" Flash SSD

by Jasmine Hong, Sales Manager, Memoright Memoritech

With pricing trends stable /increasing in Nand Flash post the release of the Apple Iphone, it appears that the concerns over SSD encroachment have eased and we could see significant SSD penetration in the notebook market over the next few years. More and more manufacturers have already begun to introduce SSDs with varying densities and transfer interfaces, while many kinds of Brand SSD have entered the market in succession. However, there is a big difference even among similar looking SSDs in the market.

SSD's storage media is generally based on RAM and Flash. Historically RAM based SSDs were faster than flash based products - but a RAM based SSD is much expensive than Flash based SSD and is not usable in all the same applications for example notebooks.

Memoright's SSD is composed of an independent intellectual property core controller and flash chip.

(Many flash SSD oems use licensed host interface cores which were originally designed for slower hard disk drives - Editor.)

Memoright's patented technology (which also includes balanced multi-channel read/write) enables their SSD to deliver 100M Bytes/sec sustained read and write transfer rates - which is the fastest throughput in a currently available 2.5" flash SSD, while its random access performance is typically 5x faster than a hard disk.
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BiTMICRO Networks
BiTMICRO Networks has been a pioneer and evangelist for the use of flash SSDs in the enterprise market.

In 2004 BiTMICRO shipped the world's first Ultra320 SCSI flash SSD (a 3.5" unit with 155GB capacity and 68 MB/sec sustained R/W rate.)

Driven by a strong belief that the enterprise storage market will go solid state, BiTMICRO promises to launch more enterprise-class SSDs in the long term.
DRAM vs. Flash SSDs

by Joanne De Peralta, Marketing Communications, BiTMICRO Networks, Inc.

Among the inherent advantages of Flash SSDs over DRAM are lower power consumption, higher density in smaller footprints, ruggedness of solution, security (securErase), non-volatility and price.

As far as applications are concerned, each application would have different requirements in terms of performance, ruggedness, security, and footprint. Flash SSDs would inherently be strong in military/industrial apps and DRAM SSDs may have an edge in enterprise applications due to some performance advantages. However, DRAM SSDs are really expensive compared to flash SSDs, which offer significant performance improvements (vs HDDS) that meet the requirements of business enterprises.

The random I/O performance of DRAM is superior but the device is more expensive. Flash is way cheaper than DRAM and can also provide better random performance than HDDs. Price of flash memory is also going down, making flash SSDs even more palatable to the enterprise market.

Applications such as OLTP, databases, would require high random performance and these are the types of applications that DRAM or flash would be very strong in. However, flash SSDs gets the plus points with regard to non-volatility (they don't require batteries for backup), capacity (denser memory chips due to R&D advances) and form factor (drop-in replacements for HDD).
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Attorn
I asked Attorn for a contribution to this article because its HyperDrive4 (available in 5.25" CD drive form factor) stands at the threshold of entry level RAM SSDs.

In August 2007 the company launched a rackmount HyperDrive4, which the company said (at $250 per gigabyte) was the lowest price per gigabyte for a RAM based solid state drive.
"RAM versus Flash SSDs - which is Best?"

by Jan Wels, Attorn

Our RAM SSD products based on the HyperDrive4 are able to solve various application issues that were not previously being solved by alternative methods of archival and retrieval. This is due to the constant available high IOPS and STR regardless of the setting in which it is applied, be it in web or file servers, databases or as replacement of in-server memory systems.

There is no other technology that can match the performance of a RAM SSD, especially in an environment where there are a significant number of write operations.

Of course for many applications the performance will not be so critical that the investment cost of a RAM SSD is warranted and a Flash SSD will offer a good alternative.

For the foreseeable future we see a market where there is room for both solutions alongside the normal hard disk. With HyperDrive4-array sales and placements in multi billion dollar corporations around the globe our RAM SSD is gaining market respect with each installation. The fact that this new product leads the market from a pricing perspective has not disappointed customers from a quality, reliability and most importantly performance perspective.

The expectations for the fourth quarter of 2007 look good and we expect the HyperDrive4 to gain further momentum in 2008. Development in the next 2 to 3 years of our RAM SSD's will result in a further increase of the performance and we expect to reach 1,000,000 IOPS per drive and a STR of 300MB/s.

For the longer term we think that two factors will determine whether or not there is room in the market for both RAM and Flash SSD's. First if the Flash SSD's will be able to bridge the performance gap with RAM SSD's, and secondly the development of the price difference between RAM and Flash memory.
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EasyCo
In August 2007 EasyCo launched its revolutionary "Managed Flash Technology."

Building arrays from commodity flash SSDs (but addressing them in a completely new way) the technology results in system write performance that is typically 100x faster than the bare solid state flash drive.

MFT, which is shipping today in Linux servers, changes the boundaries of flash SSD system performance.
Understanding Flash SSD Performance

by Douglas Dumitru, CTO EasyCo LLC

Editor's intro:- Douglas Dumitru has written a white paper which will become the classic reference for systems engineers who want to understand the dynamics of flash versus hard disk performance running real applications.

It begins by comparing in detail the performance achieved by commodity flash SSDs for various read / write size blocks to that of high performance hard drives. The dropoff in overall performance for various ratios of reads to writes is also documented.

This article also briefly notes and analyzes the performance of high end RAM SSDs and the fastest commercially available flash SSDs.

Understanding these real world constraints, the article leads on to an explanation of EasyCo's patent pending Managed Flash Technology which delivers random write performance in a flash SSD array which is almost 500x the performance of the bare drives running RAID-5. This changes the mapping scheme in the flash memory dynamically to work around contention caused by block erases. This revolutionary approach works with commercially available flash SSDs and is scalable. ...read the article (pdf)
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Texas Memory Systems
Conspicuous by its absence from the first published version of this article, the reason for the company's reticence on the subject of flash versus RAM SSDs became clear in September 2007 - when Texas Memory Systems launched the RamSan-500.

The RamSan-500 is 4U rackmount flash SSD with 100,000 IOPS sustained random read, 10,000 IOPS sustained random write and 2G bytes / sec sustainable throughput from fibre-channel hosts to internal flash storage.

At first glance it looks as if the RamSan-500 takes a similar approach to flash write management as EasyCo's MFT (see above) but TMS uses different design decisions - and implements the process transparently within the hardware of the SSD instead of needing an OS dependent driver.
Flash in the Enterprise

by Jamon Bowen, Texas Memory Systems

Flash memory is poised to become a major player in datacenter storage systems. It provides an all electronic method of storing data in silicon, removing the only component in modern data processing systems that still relies on moving parts in the data path: the spinning hard disk. Though Flash memory offers some compelling advantages over disk-based storage, it is not well understood in the storage industry. This paper describes some the properties of Flash memory and then explains how the RamSan-500, produced by Texas Memory Systems, leverages its strengths and compensates for its weaknesses to offer the first enterprise ready Flash system.

Article extract - Write Performance

"The RamSan-500, due to its design, achieves a much higher level of Flash write performance, making the technology enterprise-ready. The large battery backed DDR RAM cache is used to buffer write operations (random or sequential) so host processing can continue after only the cache write. The back end Flash controller takes the buffered writes and maps them to a new physical location every time they are written, ensuring that new writes are always written sequentially to locations in the Flash and thus avoiding the random write Flash penalty."
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Editor's footnotes

2007 was a Year of Revolutions in the SSD Market.

The snapshot given in this article is far from the last word on innovation in the "RAM versus Flash SSD War".

Let's not forget that a general factor which is increasing the level of random write performance in flash SSDs is increased parallelization within the SSD itself.

STEC (which unfortunately was unable to meet our publication deadline) has "bet the company" on its ability to deliver high capacity, high IOPs flash SSDs with its Zeus IOPs family. In some applications STEC's products will compete for rack space with many of the alternative solutions from the vendors above (and others). On the other hand EasyCo's MFT (which the company says it is offering to license to other oems) may work equally well as a speedup - using intrinsically faster disks. That remains to be seen.

Also let's not forget that in May 2007 MOSAID unveiled its HyperLink NAND Flash memory architecture which (the company claims) can increase write performance by 10x using current levels of technology. That could push the performance of hard disk form factor flash SSDs to new heights.

In April 2008 - we published Predicting Future Flash SSD Performance - a timeline and notes predicting flash SSD performance upto 2012.

In May 2008 - we published Calling for an End to Unrealistic SSD vs HDD IOPs Comparisons

In June 2008 - the article Is the SSD Market Recession-Proof? paints a different picture for the RAM and flash SSD markets. Many oems have told me that greater awareness of SSDs (due to flash SSD marketing) is making it easier to sell RAM SSDs too.

also in June 2008 - Solid Access Technologies published a powerpoint presentation explaining the advantages of its fast RAM SSDs. Of particular interest are slide 5 (which lists traditional methods for reducing I/O bottlenecks) and slide 6 (which uses a mail delivery analogy to position various types of storage).

In October 2008 - Solidata published an article - Flash-based SSDs vs. DRAM-based SSDs - which points out the similarity of the SSD market today with the NAS market about 10 years ago - and the difficulties facing customers. Do they risk more by buying from more expensive old style vendors? - or from newer companies selling at a lower price?

For updates on this subject check the main SSD page.

These articles may also be interesting:-

principles of bad block management in flash SSDs
Flash Memory vs. Hard Disks - Which Will Win?
How Bad is - Choosing the Wrong SSD Supplier?
Using SSDs to Boost Legacy RAID Performance
3.5" Terabyte SSDs with Gigabyte / S Performance
Hybrid Storage Drives - winners, losers and maybes
SSDs, tonic medicine and the next flash / flu pandemic
Flash vs DRAM Price Projections - for SSD Buyers
War of the Disks: Hard Disk Drives vs. Flash SSDs
Why Consumers Can Expect More Flaky Flash SSDs!
Clarifying SSD Pricing - where does all the money go?
Can you believe the word "reliability" in a 2.5" SSD ad?
Fast Purge flash SSDs - when "Rugged SSDs" won't do the job
Calling for an End to Unrealistic SSD vs HDD IOPS Comparisons
an Introduction to SSD Data Recovery Concepts and Technologies
the Most Popular Products on StorageSearch.com - (2007 to 2010)
Legacy versus New Dynasty - a new way of looking at Enterprise SSDs
solid state disks - home page for SSDs since 1998
SSD news on StorageSearch.com
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how fast can your SSD run backwards?
Ratios of SSD capacity - server vs SAN
Adaptive R/W and DSP ECC in flash SSD IP
Efficiency - making the same SSD - with less chips
how will Memory Channel SSDs impact PCIe SSDs?
...
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The single big idea about SSD acceleration is that it can give you the same performance increase as doubling or trebling your processor clock speed!

In datacenters that means faster applications and budget saving by deploying less enterprise servers.

In notebooks it means better performance and longer unplugged..
from the ...SSD Market Adoption model (2005)
...
the Problem with Write IOPS in flash SSDs
Random "write IOPS" in many of the fastest flash SSDs are now similar to "read IOPS" - implying a performance symmetry which was once believed to be impossible.

So why are flash SSD IOPS such a poor predictor of application performance? And why are users still buying RAM SSDs which cost an order of magnitude more than SLC? (let alone MLC) - even when the IOPS specs look superficially similar?

This article tells you why the specs got faster - but the applications didn't.
the problem with flash SSD  write IOPS And why competing SSDs with apparently identical benchmark results can perform completely differently. ...read the article
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RAM vs flash SSDs tipping point
Editor:- in December 2010 - I interviewed Jamon Bowen, Director of Sales Engineering for Texas Memory Systems and asked him about the use of SSDs in financial applications like banks and traders - a market which he said accounts for most of their RAM SSD sales.

The company which started in RAM SSDs over 30 years ago - now sells more flash SSDs than RAM SSDs (even though the product brand for both types of SSD is confusingly called RamSan.) Bowen said that flash is 70% of their business.

Jamon Bowen said that in many bank applications RAM SSDs are actually cheaper than flash - because of the small size of the data. TMS still sell a lot of 16GB RAM SSDs.

Production bank systems are typically shared by many hosts and get a lot of write IOPS. To achieve the same reliability and latency with flash would require over provisioning which would drive the cost up.

He suggested a simple rule of thumb for intensive IOPS bank SSDs on the SAN
  • < 128GB capacity - RAM SSDs cheaper
  • 128GB to 4TB capacity - middle ground could be either - or determined by other constraints
  • > 4TB - flash SSDs cheaper
Jamon Bowen said that the analysis side of operations in banks is different. That tends to have much larger data sets and is more read than write intensive. In these apps - flash SSDs are usually more economic.
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