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Can you trust flash SSD specs & benchmarks?

by Zsolt Kerekes, editor - July 9, 2008

I warned you first! - you can see updates at the end of this article
One of the things I've noticed is that the published specs of flash SSDs change a lot -from the time products are first announced, then when they're being sampled, and later again when they are in volume production. And flash SSD performance in real applications can be considerably lower than predicted from published benchmarks due to common"halo" errors which occur when using test suites designed for HDDs / RAM SSDs without understanding and changing the implicit assumptions which have been built into the legacy test set ups.

Sometimes the headline numbers get better, sometimes they get worse. There are many good reasons for this, because unlike memory devices, SSDs are complex systems in which software and controller hardware can all play a part in shaping the characteristics of a device. Tweaks in the controller algorithms can have any of the following major effects:- If you're a systems integrator / oem who has designed flash SSDs into a particular application - then it's unlikely that you chose your favored models at random. It's much more likely that you carefully evaluated products against a wish list of characteristics which include:-
  • performance,
  • environmental tolerance,
  • power consumption and
  • longevity.
These factors play as big a role as the obvious ones:-
  • capacity,
  • form factor,
  • host interface,
  • price and
  • security of supply.
But the flash SSD market is very volatile.

How can you be sure that the products which are going into your production systems are the same (or similar enough) to what you tested?

It's possible that after you did all your qualification testing that the original SSD oem did some "improvements" which they may not necessarily tell you about - because they make the product "better". But what if the new SSD controller chip does work faster - but puts more stress on your limited battery budget? Or what if the SSD oem has switched suppliers of memory or power management chips and the overall product fails to operate reliably over the full range of temperature you need?

Worse still - maybe you can't get the original product at all. To keep your production line going you have to stuff slots with products that your distributor suggests from companies you've never heard of before.

Many of these problems have been around in the electronics industry in past decades. But in 2008, 2009 and maybe 2010, the unique characteristics of the flash SSD market mean that the risks are a lot worse.

Many SSD oems haven't been in the market very long. But because they make attractive products you can't afford to ignore them. Although some oems have been in the industrial or military markets for years - and do test their products and do inform you when those specs change - when their demand surges and products go on allocation - you still face the risks of switching suppliers to guarantee your own product's continuity.

And here's another thing to worry about.

Can you be sure - for example that the flash SSDs your buyer has bought at such a good price from an alternate source really are SLC? - and not MLC or SLC/hybrids. This is such a new market that you can't be sure that the supplier's SSD product manager (who may have been in the flash SSD business for less than 2 months) understands the intricate concerns you have - or what your questions mean.

One solution to protect yourself - may be to do much more sample testing of incoming product. Or if your volumes aren't high enough to justify the capital expense - another option might be to ask your distributor to do the testing for you.

The flash SSD market opens up tremendous opportunities for new products and systems which leverage that technology. But due to the diversity of products in the market and lack of industry standards - it's got tremendous risks as well.

Paying proper attention to compliance testing and quality assurance will make the difference between the market success or failure of many new SSD based products.

...Later:- Sometimes the "specs" don't tell you anything meaningful about the product's performance at all - particularly in the case of write IOPS tests for flash SSDs.

I've seen several published documents in which the measurements of such parameters appear to have been done incorrectly.

Because there isn't widespread market experience of flash SSDs - it's easy to fall into the trap of running tests which were originally designed for hard disks or RAM SSDs and not realise that some of the inbuilt assumptions or test parameters may be inappropriate.

It makes little or no difference to a small block size random write test on an HDD or RAM SSD whether the media being written to already contains data.

The performance impact of fragmentation on HDD performance is well known. Flash SSDs don't suffer from a dropoff in performance due to fragmentation - but there can be a similar performance droppoff over time due to the lower availability of pre-erased blocks. So when benchmarking a new flash SSD (which is initially erased) it's important to take this factor into account.

Some high performance lash SSDs have a background process which manages and tries to maximize the availability of pre-erased blocks. How well that works determines the sustained random write IOPS figure in 24x7 enterprise applications.

That's why when testing a flash SSD for 24x7 enterprise applications you must make sure that when the benchmark window begins - the disk is already full. Otherwise you get a "halo" effect (similar to that caused by cache hits in traditional server benchmark tests) in which the pool of pre-erased flash storage makes the SSD "appear" to operate much faster than it will be in a real application after it has been running for days, weeks or months.
SSD testing and benchmark news

flash SSD performance characteristics and limitations
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SNIA publishes draft SSD performance testing doc

Editor:- July 12, 2010 - SNIA today announced the availability of its Solid State Storage Performance Test Specification (version 0.9) for public review.

A typical flash SSD taken "fresh out of the box" and exposed to a workload, experiences a brief period of elevated performance, followed by a period of transition to an eventual performance Steady State. The new SNIA methodology will close the gap between performance measurements in the lab and in normal working life and make competitive vendor comparisons more useful.

This is exactly the same point as I made 2 years earlier in the article here on the left.

"That's why when testing a flash SSD for 24x7 enterprise applications you must make sure that when the benchmark window begins - the disk is already full. Otherwise you get a "halo" effect (similar to that caused by cache hits in traditional server benchmark tests) in which the pool of pre-erased flash storage makes the SSD "appear" to operate much faster than it will be in a real application after it has been running for days, weeks or months."
is the SSD Performance Deterministic?

Editor:- March 6, 2009 - if you know that enough readers are confident about SSD jargon - it paves the way to discuss weighty concepts.

Pliant Technology has neatly brought together the themes of SSD jargon and benchmark reliability in a blog which asks - is the performance deterministic? - among many other important questions.
EDN Publishes Benchmarks on Intel's Fast Flash SSD

Editor:- November 17, 2008 - EDN has published benchmarks comparing Intel's new fast SLC flash SSDs with various other SATA flash SSDs and HDDs.

The author, EDN Senior Technical Editor Brian Dipert, has also published useful raw results on R/W IOPS and throughput.

However Dipert voices some concerns about the validity of these tests on flash SSDs.

"Since the data sets I wrote to the drives were fairly small in comparison to the SSDs' total available capacities, and since the drives were freshly formatted at the beginning of the tests, I suspect that the file patterns I wrote to the SSDs didn't trigger much if any media management. But sooner or later under normal usage conditions, the drive controller will need to begin tackling..."

The halo effect caused by invalid hard disk benchmark assumption were discussed in my article Can you trust your flash SSD's specs?
Another Publications Reports Unrepeatable Numbers from Repeated SSD Benchmarks

February 13, 2009 - an article published on reported declining performance in successive benchmarks of Intel's fast SSD.

The article mistakenly (in my view) attributes this to fragmentation. But the more likely cause is halo effects and poor garbage collection performance.
Benchmark Reviews Confirms flash SSD Performance Uncertainty Principle

Editor:- January 4, 2009 - Benchmark Reviews has recently published an article - "SSD Benchmark Performance Testing."

This provides experimental confirmation of something I warned about in an earlier article - Can you trust flash SSD specs & benchmarks? - namely that hard disk based performance software provides unreliable results for SSDs.

The author, Executive Editor, Olin Coles (who has published many disk benchmarks) concludes "I warn readers to regard SSD reviews with a high degree of caution".

He says he's going to stop using certain test suites, and the discovery that he can't trust all the results which he has worked hard to collect and publish made him feel sick. Coles is a very experienced benchmark tester - and his article makes interesting reading. the article
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Benchmarking enterprise SSDs - some later articles

In 2009 - STEC published this useful white paper - Benchmarking Enterprise SSDs (pdf). It reiterates many of the points originally raised in this article (above).

In 2011 - Running Consecutive Microsoft Jetstress Performance Tests? - Prepare for Wake Turbulence written by Allon Cohen at OCZ warned about the interdependence of running consecutive tests - and the halo effect of invisible caches which can make the benchmarks results look much better than what you would see in real-life.

In 2012 - Modern Methodologies for Benchmarking Enterprise SSDs - written by Shridar Subramanian at Virident Systems - reviewed the problematic history of enterprise SSD performance evaluation which he said has gone through 4 different phases. The company also this year published a set of software benchmarking tools which can be used to simulate enterprise workloads. The tools can be used to evaluate any brand of SSD - not just those from Virident.

In March 2013 - factors which influence and limit flash SSD performance characteristics - which was the home page blog on - provided a single toolkit overview of many design factors in SSDs which had been discussed by the editor Zsolt Kerekes in earlier separate articles.

In August 2013 - EDN published a introductory article on the subject of measuring enterprise SSD performance - SSD performance measurement: Best practices - written by Doug Rollins, Senior Applications Engineer at Micron - which expounded some of the basic assumptions and jargon.

In August 2013 - LSI published a paper - Don't Let Your Favorite Benchmarks Lie to You (pdf) - which showed how and why many commonly used so-called "benchmark programs" will deliver conpletely different results for the same SSD - depending on the setup before the test aka the "preconditioning".
the Fastest SSDs
flash SSD Jargon Explained
RAM Cache Ratios in flash SSDs
Surviving SSD sudden power loss
Adaptive R/W and DSP in flash SSD IP
how fast can your SSD run backwards?
the Problem with Write IOPS - in flash SSDs
Efficiency - making the same SSD - with less flash
Why Consumers Can Expect More Flaky Flash SSDs!
Clarifying SSD Pricing - where does all the money go?
SLC to XLC - flash in the enterprise - from 2004 to 2016
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how fast can your SSD run backwards?
Editor:- April 20, 2012 - today published a new article which looks at the 11 key symmetries in SSD design.

SSDs are complex devices and there's a lot of mysterious behavior which isn't fully revealed by benchmarks and vendor's product datasheets and whitepapers. Underlying all the important aspects of SSD behavior are asymmetries which arise from the intrinsic technologies and architecture inside the SSD.

Which symmetries are most important in an SSD?

That depends on your application. But knowing that these symmetries exist, what they are, and judging how your selected SSD compares will give you new insights into SSD performance, cost and reliability.

There's no such thing as - the perfect SSD - existing in the market today - but the SSD symmetry list helps you to understand where any SSD in any memory technology stands relative to the ideal. And it explains why deviations from the ideal can matter.
SSD symmetries article This is the most important article about SSDs that I've written in the past few years. I couldn't have written it before. I hope you like it. to read the article
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the 3 fastest flash PCIe SSDs list (or is it lists?)
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
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. the article
flash SSD capacity - the iceberg syndrome
Have you ever wondered how the amount of flash inside a flash SSD compares to the capacity shown on the invoice?

What you see isn't always what you get.
nothing surprised the penguins - click to read  the article There can be huge variations in different designs as vendors leverage invisible internal capacity to tweak key performance and reliability parameters. the article
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