I've been asked where I think the flash SSD market is heading in terms of performance. Although the article RAM SSDs versus Flash SSDs - which is Best? looked at the state of the art (at the time) it didn't include a performance roadmap. This article does. This preliminary version (published April 17, 2008) is based on my own reading, interpretation and predictions for the relevant technologies.
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the Flash SSD Performance Roadmap

Predicting Key Trends in Future Flash SSD Performance

by Zsolt Kerekes, Editor STORAGEsearch.com

My thoughts re a flash SSD performance roadmap are summarised in this article which contains 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.

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  • 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 read to write IOPs will improve to 5 to 1 by 2010 - but not progress beyond 4 to 1. (This ratio was 10 to 1 in the fastest devices in 2007.)

    The analysis for this appears in an article that's not yet published (Demystifying SSD IOPS). Until then - I've just extracted some conclusions from it below.

    In (typical) database applications with Read:Write ratios of 4:1, an ideal flash SSD with 10:1 R/W IOPS is approximately 3x (2.8x to be exact) slower in overall applications performance than an ideal RAM SSD with similar MB/s throughput.

    When flash SSDs improve to 5:1 R/W IOPS - the overall applications ratio will be about 2x slower than RAM SSDs (1.8x to be exact).

    It's unlikely that the flash SSD market will try to develop better than a 4 to 1 R/W IOPS ratio - for the reasons below.

    1 - endurance will once again become worryingly significant for flash SSDs with this type of architecture - even if they are SLC. And if they are MLC - they will burn out faster than incandescent light bulbs.

    2 - if the ratio gets much closer to 1:1 then what we are talking about is some form of RAM - or something that's a different technology to flash anyway.

    3 - the response time (or latency) for reading small blocks of data will become a more significant limiting factor than IOPS asymmetry - for the reasons below.
  • 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.
  • SSD Market History - lists key market, business and technology milestones in the 30 year history of the SSD market.
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