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the Flash SSD Performance Roadmap

Predicting and Reporting Key Trends in Flash SSD Performance - upto 2012

by Zsolt Kerekes, editor, April 2008
I'm often asked where I think the flash SSD market is heading in terms of performance.

Although my earlier article RAM SSDs versus Flash SSDs - which is Best? looked at the state of the art (in 2007) it didn't include a performance roadmap.

This article does. And it has a bunch of simple to remember predictions or "laws". For convenience you can think of them as "Zsolt's Laws". Or maybe "Z's Laws" is easier to spell. The first law is the only one you need to remember.

Overall this article tells you the market model assumptions I would work to if I were setting up a new SSD company to design the fastest flash SSDs.

Obviously I'm not doing that - because otherwise the many people I talk to in SSD companies around the world wouldn't be talking to me.

I've got a good crystal ball and the lucky thing about my many past predictions related to the storage market - is that if they weren't going to be true at first - then suggesting to companies that they ought to spend more time looking at things in a particular kind of way becomes a self fulfilling prophecy. My safety net is that as many designers in the industry also read these articles - if I say something that's too outrageously wrong - they correct me - before I look too ridiculous. A quick delete, cut and paste, and it's even more accurate than it was before.

The rate of speedup predicted is much faster than would be predicted by Moore's Laws - and that's the significant point. Some explanations why - are included at the foot of the article.

  • Prediction 1 - Flash SSD throughput and IOPs (in traditional HDD form factors) will more than double every year in the period from 2007 to 2012.

    This predicts (in effect) that in 2011 a single 3.5" form factor flash SSD will be able to deliver similar throughput to some of the fastest RAM SSDs available in 2007, with over 2,000MB/s sustainable reads and writes.
The single most useful thing to take away from this article is the assertion above. But if you want to have some more to think about read on.
  • Prediction 2 - Rackmount flash SSD throughput and IOPS performance will be a multiple of the performance for a single disk. These factors have already been shown to be scalable in SSD RAID arrays.

    This needs little explanation as some of the results are intuitive and we've already published plenty of articles on this subject. However, some of the architectural features which are now used in SSD RAID systems - such as MFT technology - can also be designed into individual SSD disk modules.

  • Prediction 3 - The asymmetry of sustained read to write IOPs will improve from 10 to 1 (the fastest devices available in 2007) - but will never achieve parity (1 to 1).

    As this change occurs in the market flash SSD arrays will become viable choices in many enterprise server speedup applications which hitherto had been the exclusive domain of RAM SSDs.

    In (typical) database applications with Read:Write ratios of 4:1, an ideal flash SSD with 10:1 R/W IOPS is approximately 3x slower in overall applications performance than an ideal RAM SSD with similar MB/s throughput.

    ...Later:- I was wrong about this one.

    Violin Memory designed a flash SSD with balanced random R/W IOPS in Q4 2008 using a combination of factors.
    • over-provisioning and fast garbage collection ensured a constant flow of pre-erased flash.
    • their non blocking RAID RAM cache architecture enables a read to be done from a virtual flash block while an erase-write is still in operation on the same flash block.

    And in April 2009 - SandForce published - Fact or Fiction - all flash SSDs have unbalanced R/W performance . This is one of a set of articles which discuss design factors inside their small form factor SSD controller.

  • Prediction 4 - Latency in flash SSDs will not scale in the same way as throughput, and will always be significantly worse than that in ideal RAM SSDs.

    The ratio of read access times for RAM SSDs compared to flash SSDs may improve for a few years (as the gap gets smaller) but then it will hit a brick wall - and may in fact get worse again.

    The reason is - that flash SSDs have not yet been optimized for latency - so there is some scope to reduce the latency gap with RAM systems (which have already been highly optimized).

    But in future product generations as flash SSDs increase in density - a read or write cycle becomes an increasingly complicated on-chip process - which includes calibration, error correction and address translation all being done by controllers between the memory array and the host interface controller or card data bus.

    This series of steps (to do a simple read) will diverge from what happens in a typical RAM to the point where flash and RAM look like completely different species. That's unlike earlier generations of flash in which the read cycle looked the same as a static RAM - but simply took longer.

Here are some other preliminary notes of explanation.

Why Z Predicts a Faster Roadmap Acceleration than Moore's Laws

The main reason that the flash SSD market will deliver faster products much sooner than predicted by Moore's Law is that 2 additive factors are at work in this phase of the market:- architecture and semiconductor process technology.

In the past there was little point in manufacturers integrating very fast architectural features into flash SSDs - because they added to the cost - and there wasn't a big enough established market to buy them.

The architectural technologies that can speed up performance and IOPS were originally independently developed by various oems to suit particular products or markets. Until the SSD market reached a critical mass where enough users signalled they would buy faster products if they were available - there was no point in developing them.

the key architectural features which will increase throughput and shrink the asymmetry gap in read / write IOPS are:-
  • parallelization of the internal media arrays
  • improved media management technology.

    MFT from EasyCo is a software solution which has already been mentioned above. But the same algorithms could be run in the SSD hardware.

    Another licensable solution - which is already being developed at the chip level is IOP Buster architecture from Link_A_Media Devices
  • faster media controllers
  • faster host interface controllers (and faster interfaces driven by the needs of the SSD market rather than adapted from the HDD market)
  • hybridizing on board memory technologies - for example using faster RAM-like non volatile memory in some parts of the device and slower flash-like memory in the bulk storage arrays
Scaling any one of the factors above requires significant investment in IP. There is also significant risk that the overall balance of the product specs which results doesn't match the market's expectations for price and performance at the time.

A lot of trial and error will be involved as oems throw products at the market which tweak the technologies they understand best - and see which products stick.

Underlying all the architectural improvements - there will also be evolutionary and revolutionary changes in semiconductor processes occurring at the same time. Some of these will enhance currently known architectures, while others may make some architectural features obsolete.

By around 2013 - the flash SSD tornado should have reached a point where the architecture of an ideal SSD is well established - and the ongoing developments will be drive more by process changes than anything else.

We're going to publish hundreds of articles about this subject as the roadmap takes shape. So don't expect to see all the answers in a simple note like this. But I'll add more notes and links to this article as time goes on.

Here are some related articles.
  • the Fastest SSDs - updated regulalry - this gives the state of the art in popular form factors.
  • Understanding Flash SSD Performance (pdf) - by Douglas Dumitru, CTO EasyCo LLC is a reference for systems engineers who want to understand the dynamics of flash versus hard disk performance running real applications.
  • Flash in the Enterprise (pdf) by Jamon Bowen, Texas Memory Systems describes some the properties of Flash memory and then explains how their RamSan-500 product leverages its strengths and compensates for its weaknesses to offer the fastest enterprise ready rackmount Flash SSD system.
  • Design Tradeoffs for (SLC flash) SSD Performance (pdf) by Nitin Agrawal, Vijayan Prabhakaran, Ted Wobber, John D. Davis, Mark Manasse and Rina Panigrahy looks in detail at the internal architecture of a typical commercially available SLC flash SSD and the various points where choice of software algorithms can impact performance.
  • Why I Tire of - "Tier Zero Storage" - You don't need to waste any of your precious brain cells by investing "tier 0 storage" with an importance this travesty of storage jargon really doesn't deserve.
  • SSD Market History - lists key market, business and technology milestones in the 30 year history of the SSD market.

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the 3 fastest flash PCIe SSDs?
Are you tied up in knots trying to shortlist flash SSD accelerators ranked according to published comparative benchmarks?

You know the sort of thing I mean - where a magazine compares 10 SSDs or a blogger compares 2 SSDs against each other. It would be nice to have a shortlist so that you don't have to waste too much of your own valuable time testing unsuitable candidates wouldn't it?

StorageSearch's long running fastest SSDs directory typically indicates 1 main product in each form factor category but those examples may not be compatible with your own ecosystem.

If so a new article - the 3 fastest PCIe SSDs list (or is it really lists?) may help you cut that Gordian knot. Hmm... you may be thinking that StorageSearch's editor never gives easy answers to SSD questions if more complicated ones are available.
the 3 fastest  PCIe SSDs  - click to read article But in this case you'd be wrong. (I didn't say you'd like the answers, though.) the article
a new way to classify enterprise SSDs
Editor:- September 29, 2010 - published a new article called - Legacy versus New Dynasty - a new way of looking at the Enterprise SSD market.
Legacy vs New Dynasty - the new way of looking at Enterprise SSDs It introduces a new conceptual framework which cuts across technology boundaries and will help you understand where all new enterprise SSD products fit in the scheme of things - and which ones you can safely ignore. the article
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Why should you care what happens in an SSD when the power goes down?

This important design feature - which barely rates a mention in most SSD datasheets and press releases - has a strong impact on SSD data integrity and operational reliability.

This article will help you understand why some SSDs which (work perfectly well in one type of application) might fail in others... even when the changes in the operational environment appear to be negligible.
image shows Megabyte's hot air balloon - click to read the article SSD power down architectures and acharacteristics If you thought endurance was the end of the SSD reliability story - think again. the article
RAM Cache Ratios in flash SSDs
Editor:- July 28, 2009 - today published an article - which proposes new terms to describe and differentiate products in the flash SSD market.
read the article RAM Cache Ratios in flash SSDs It is hoped that the new classification jargon will be useful to users who have to evaluate lots of products, and will be useful to vendors as a shorthand when communicating about different segments within their flash SSD product lines. the article
SandForce Unveils New flash SSD Controller
Editor:- April 13, 2009 - SandForce today emerged from stealth mode and unveiled its SF-1000 family of SSD Processors - aimed at oems building SATA flash SSDs.

Its 2.5" SSD reference design kit is the fastest 2.5" SATA flash SSD on the market - with 250MB/s symmetric R/W throughput and 30,000 R/W IOPS.

Leading OEMs are expected to release both SLC and MLC flash-based SSDs using SandForce single-chip SSD Processors later this year

"With a deep understanding of both system- and silicon-level issues, we've integrated the right balance of reliability, performance, power, cost, and time-to-market in our SSD Processors while supporting multiple flash vendors' technology," said Alex Naqvi, President and CEO of SandForce. "Our products combine key processing elements with hardware automation to efficiently address the traditional shortcomings of flash memory. This allows OEMs to provide enterprise-class SSDs to the mass-market using both SLC and lower-cost MLC flash devices while delivering peak read and write performance throughout the drive's lifecycle."

Editor's comments:- I asked SandForce's President & CEO, Alex Naqvi, for more details about the various package of technologies which are bundled in the company's "DuraClass Technology" - which achieves impressively high IOPS without relying on over-provisioning or large external RAM caches. In particular I wondered what part, if any its choice of processor SoC (from Tensilica) had to play.

Alex Naqvi explained - DuraClass performance doesn't come from the choice of processor - but in the way that they have integrated various design techniques with very fast hardware (proprietary chips) which the company has designed to accelerate the core bottleneck functions of a flash SSD controller.
SSD SoCs controllers In concert with other techniques, such as the ability to reorder data before it is written to flash (thereby attenuating write endurance by 2 orders of magnitude), RAID like internal protection and very fast garbage collection SandForce's DuraClass Technology results in small form factor enterprise class flash SSDs which have no daily write limits for MLC flash and symmetric R/W IOPS.
the Problem with Write IOPS in flash SSDs
the "play it again Sam" syndrome

Flash SSD "random write IOPS" are now similar to "read IOPS" in many of the fastest SSDs.

So why are they 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 similar.
the problem with flash SSD  write IOPS This article tells you why the specs got faster - but the applications didn't. And why competing SSDs with apparently identical benchmark results can perform completely differently. the article
Can You Trust Your Flash SSD's Specs?
Editor:- I've noticed is that the published specs of flash SSDs change a lot -from the time a product they are first announced, then when they're being sampled, and later again when they are in volume production.

Sometimes the headline numbers get better, sometimes they get worse. There are many good reasons for this.

The product which you carefully qualified may not be identical to the one that's going into your production line for a variety of reasons...
ssd specs article And here's another thing to worry about...

The enterprise flash SSDs which you benchmarked yourself - may surprise you by running much slower when deployed in your own applications due to common "halo" errors which are implicit in the set ups of many performance test suites which were originally designed for HDDs. the article