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some twisty turny stories of DWPD

Editor:- this somewhat messy but serious article below - which I've added to over a period of 4-5 years - started as a simple question about the write limits of enterprise flash accelerators (PCIe SSDs) used as caches and virtual memory compared to SAS / SATA SSDs in storage arrays. Then to my surprise it twisted and turned as DWPD discovered roles in other SSD markets (industrial, consumer, cloud) before the narrative bounced back to generate "no limits" petabyte AFA good denizens. But the story of DWPD in the storage news archives didn't stop there and grew in the telling. And DWPD didn't stay rooted in the flash world either and became quoted for other non volatile memories too - and in this article there has even been talk of DWPD being a useful way to talk about hard drives too.

what's the state of DWPD?

endurance in industry leading enterprise SSDs

(and also DWPD in consumer SSDs, industrial SSDs and military SSDs too)

by Zsolt Kerekes, editor -
DWPD (Diskful Writes Per Day) for 5 years - has become an established part of SSD jargon in the writings of enterprise SSD makers.

This article began in 2014 when I thought it curious that I hadn't seen an endurance figure expressed in this way (as DWPD) from Fusion-io for its marketing leading PCIe SSD accelerators.. Then a blog - written in March 2014 by Chris McCall gave me the answer for the ioDrive2 . It was 4.4.

You may ask... Is that good or bad?

In the number score wars - there are competing enterprise flash SSD drives which are better. And some that are worse.

I realized that DWPD numbers were popping up all over the place. And sometimes the DWPD rating was conspicuous by its absence - a supporting clue in technically lightweight enterprise SSD press releases - that the related product launch signified a new model line designation rather than a fully characterized and shippable SSD.

When I saw how popular my initial brief roundup of DWPD examples had become - I decided to update the list from time to time with interesting examples as you can see in the lists below.

uses and limits of DWPD

DWPD was created to mitigate the fear that SSD integrators could choose the wrong type of flash SSD for the anticipated prohect workload. It provided a simple to understand "goodness" metric by which competing SSD manufacturers were able to signal the endurance of the SSD which integrated all the combined effects of the internal memories, interface speeds, controllers and architecture. Before DWPD it was harder to do this because SSD datasheets were specified in different ways.

The convenience of DWPD as a way of selecting SSDs for application roles meant it quickly gained widespread adoption in the enterprise and cloud. Indeed it was so useful that within 3 years it got picked up as a specification seen in most of the other markets in which SSDs were used too including:- consumer, industrial and military drives. That's also why just looking at the single DWPD rating for an SSD doesn't tell you whether it's a good or bad SSD. You need to know the context of the application.

no DWPD? - detects vaporware

DWPD has also been a useful way to recognize products which have been"announced" far in advance of their actual sampling and characterization.

The missing DWPD ratings in vendor press releases about new products were a good clue to products which you were not likely to see for many quarters (if indeed ever).

dwpd - let's calm it down a bit

Now before this starts to sound too much like a DWPD fan club I'd like to say that - like all technical specifications - DWPD has its uses but also has its limitations too.


Created for traditional storage roles it's questionable how useful a DWPD rating is when it's provided in the context of an NVDIMM which is intended for use as a memory (tiered flash emulating RAM). That's because the R/W and caching characteristics of flash deployed as RAM are installation and applications dependent. And there haven't been enough examples in the market to determine what are the useful specification shortcuts for choosing between competing devices. If there are any I suspect that DWPD won't be top of the list and even IOPS are dubious in this context. Having said that - the DWPD ratings which have appeared in various RAM roled SCM products do still provide an indication that the designers are trying to allay fears about wear-out.

no-limits DWPD

Meanwhile - slower down the throughput curve but higher up the drive capacity - in Q3 2017 we started seeing the emergence of SSDs with "unlimited DWPD" - in the shape of a high capacity SAS SSD platform from Nimbus which has no write limits for 5 years.

In part this is the consequence of using a comparatively slow - 6Gbps - interface and connecting it to enough flash chips in a 3.5" form factor that you would struggle to get high drive writes per day even if your tried hard - and the rest is the use of high quality flash and good controller management. But for the intended roles of such SSDs - which are high capacity storage drives - the assertion of "unlimited DWPD" shows that every specification has its uses and limits.

DWPD - a list of products from the news archive (2014 to 2017)
  • from Seagate - the 1200 SSD (12Gb/s, eMLC SAS SSD) is rated from 10 to 25 DWPD - depends on model - for 5 years
  • from Intel - the P4800X (a 3DXpoint / Optane memory PCIe SSD) rated at 30 DWPD for 3 years - which is 18 DWPD for a 5 year comparison basis.
  • from Intel - the DC S3700 (1.8" and 2.5" 25nm "older technology" - my words / "high endurance technology" - Intel's words - SATA 3 SSD) is rated at 10 DWPD for 5 years
  • from Mangstor - the MX6300 - a fast NVME PCIe SSD FHHL gen3 x 8 which uses eMLC and 100% overprovisioning and has a proprietary internal 100 core controller - is rated at 7 DWPD for 5 years.
  • from Virtium - the "XE models" in its StorFly (form factors from 2.5" down to M.2 range of iMLC rugged industrial SSDs) are rated at 7 DWPD for 5 years.
  • from Solidata - the Rana (a military grade, industrial temperature 2.5" SATA 3 MLC SSD) is rated at 5 DWPD for 5 years.
  • from Micron - the 5100 MAX (a 2.5" SATA SSD using 3D TLC) is rated at 5 DWPD and in March 2017 Micron claimed this was one of the fastest SATA enterprise SSDs on the market with steady state random writes (up to 74,000 IOPS - 4KB).
  • from TCS - the Triton 2 (a military grade, industrial temperature 2.5" SATA 3 MLC SSD) rated at 1.4 DWPD for 5 years.
The real answer to - what's good enough? - when it comes to DWPD - depends on the application.

PS - before you get too hung up on a particular value of DWPD...

A reader told me before I published the original version of the above article - that he was interested in knowing DWPD at different operating temperatures - within the industrial and military range. (The reason being that in his projects it would be known in advance - at what temperature the SSDs would spend most of their operational lives. He didn't have much confidence in the numbers which various vendors had given him. This doubt was fueled by having being told widely different ratings for SSDs which had apparently identical controllers and memory.

I added to that skepticism when I said that I wouldn't be surprised to see DWPD ratings drift in a more favorable direction even in the case of the same model SSD - even when nothing had actually changed in the BOM.

My reasoning was that DWPD - like all endurance figures - is the output of negotiations between engineers and marketers in the same company. With new products and scant hours of validation - it's natural for engineers to want to specify a cautious figure - which tends to pull the rating down. But as time goes on - and more evidence is gathered - and firmware changes get done - and the manufacturing tolerances in the memory wafer fab becomes more tightly managed - it may be technically justified to publish a much improved DWPD rating compared to the original version - even when it looks like all the components are exactly the same as they were before.

Competitive pressures from the market play a big a part in the DWPD numbers you see being published in the enterprise. It's a fashion statement tied in closely to design wins. If you've got the wrong number - the customer design team is not going to look at your SSD. DWPD is a slippery relativity thing rather than a hard and fast technical absolute.

You might think that marketers always want the highest rating which engineering will allow them - so as to justify a higher ASP - but marketers may also push to offer good models with a much lower rating so as to position the product with a lower price for a different application segment. In the latter case - the lower DWPD rating isn't a guarantee that the product you buy today will be substantially worse in real life. A low rating can be applied to a higher rated product to temporarily test the market reaction for a cheaper (gap filler) product - in a way which protects the brand investment in the higher rated models.
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metascale advised, life-cycle fitted

the new virtually hardened flash endurance

another plot twist in the continuing elasticity of flash's DWPD story

Editor:- April 28, 2016 - There was a time when you could write a simple list of nand flash memory types which you'd be likely to encounter in the market and alongside the expected endurance.

We know how these lists look - because we've seen them so often.

Often the lists are presented as graphs instead - because they brighten up a web page or powerpoint.

The list goes something like:- SLC, MLC, TLC, then for each of those a note saying if it's 2D or 3D, along with an entry for each nanometer cell geometry.

Then alongside that list you have the range of likely endurance (P/E cycles).

There's widespread agreement about what these numbers should be. And if we see the wrong number in the wrong place we know that this was probably a simple transcription error.

These hard endurance expectations - when applied to an SSD content - got a bit of a reality check back in 2011 when - as I commented in my article the new SSD uncertainty principle - we started to learn more about some of the stretching effects of endurance which were being enabled by a mix of technology tricks which were later called adaptive R/W and DSP ECC IP. It became clear that this pioneering approach would become the standard way that many controllers used.

Recently the SSD market has become accustomed to compartmentalizing the raw endurance figures attached to memory chips (on the one hand) and the endurance magnification effects of adaptive controller techniques (on the other).

But I learned more recently about another new way of introducing more entries in the list of memory types when seen from the endurance point of view.

There isn't a word for describing this - so let's just call it - software hardened virtual endurance.

And the unexpected thing is that after this software hardened virtual endurance figure you can still apply over and above that another improvement ratio due to other methods.

This means that SSD designers have another set of memories they can play around with to fit their DWPD pricing curves. Because for nearly every real physical memory type they're considering there's an associated and software spawned hardened endurance which can be upto 10x better.

Where does this come from? And who's been working on it?

Several companies have been known to be working on similar techniques. Most notable among these now being NVMdurance whose technology story is long and convoluted.

I chatted with their CEO about how their technology fits into the trends of managing SSD endurance and wrote about it in here - relating NVMdurance's machine learning to manual tuning (July 2016).
what about DWPD in consumer SSDs? - part 1 of 2 - guessing games
SSDs designed for the consumer market typically don't come with 5 year DWPD ratings - partly because that's not the key characteristic in their selection - but also because almost none of the companies who design them would be foolish enough to offer a 5 year write intensive related warranty.

But we can infer a ballpark "consumer DWPD" rating from published data.

This makes for a cautionary tale - which can be interpreted in 2 completely different ways.
  • it provides some good arguments for avoiding consumer SSDs in write intensive enterprise arrays
  • it shows that consumer SSDs (in 2.5" form factors) are better than you might have expected - but are still not nearly as good as low end enterprise SSDs - designed for read intensive cloud applications
Where does the data come from?

A longtitudinal study based on a set 6 consumer 240GB (SATA) SSDs - from 5 leading companies - called the the SSD Endurance Experiment - published by the Tech Report.

I chose as a convenient reference point the December 2014 update - at which stage the test had been running for about 14 months - interspersed with short periods in the powered down state to check data rention.

By this time 4 of the 6 drives had failed.

If we take the average for the failed drives - and assume instead they had received the identical terabyte amount of writes - but spread out at a slower write rate per month - this group of consumer SSDs failed at about 2 DWPD for 5 years.

This is not the same as a 2 DWPD rating - because the point of such a rating is to signal to the designer that most SSDs used in this way would survive - instead of fail.

So you'd have to downgrade to a lower figure.

But it does give a comparison point.

Does it matter that 2 of the 6 drives may be better - and were still operational when this interim update was published?

No - because as a systems specifier - there's enough data already for you to decide how you should regard this type of drive.

And endurance is only part of the story.

Although having said that - reports from the SSD data recovery market at the end of 2014 - suggested that endurance related failure (in consumer and notebook SSDs) accounted for significantly more failure incidents than all the other component failure causes combined.

...Later:- the endurance SSD "experiment" data mentioned above above was based on older SSDs. And as we know from flash memory basics - older (in 2D) equates to larger cells and better intrinsic endurance.

In 2016 - the consumer market moved in a direction where you didn't have to play guessing games any more as you can see in the story below.
what about DWPD in consumer SSDs? - part 2 - end of guessing games
Editor:- February 3, 2016 - TLC was originally intended as a consumer SSD technology (not that you'd realize this from reading about all the enterprise arrays which have assimilated it).

OCZ recently announced availability of a 15nm TLC based consumer range of 2.5" SATA SSDs - the Trion 150.

One of the interesting things about how the marketing of consumer SSDs has evolved is that these new SSDs come with DWPD guidance ratings which are 0.25 DWPD.

Be aware, however, when comparing DWPD ratings for consumer, enterprise and industrial SSDs that the warranty periods for these different classes of drives - are different.

The Trion 150 warranty is 3 years - which is typical for client SSDs - rather than 5 years (as for enterprise drives).

Endurance related marketing messages have come a long way in the past 12 years or so.

In October 2014 IBM said (in effect) "You don't need to worry about the endurance of our FlashSystems." That was my summary of an IBM blog at the time.

Nowadays OCZ says this about their Trion SSDs...

"Never Fear, OCZ Endurance is Here."

In one way I've got to admire the reckless implied simplicity of OCZ's endurance message. But I also groan in anticipation of how other vendors will retaliate with similar endurance messages of their own.

I think OCZ's "never fear" tagline may have been around since last summer (for the earlier Trion 100 - which OCZ says "quickly became a top seller for us") but as I don't visit consumer SSD pages any more than I have to (even my own) I didn't see it until today.

See also:- Branding Strategies in the SSD Market
SSD news
SSD Reliability
the Top SSD Companies
SSD endurance - forever war
can you trust SSD market data?
bad block management in flash SSDs
how fast can your SSD run backwards?
meet Ken - and the enterprise SSD software event horizon
why is 3D nand flash endurance better at the same geometry than 2D?

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endurance needs are application dependent

SSD news
SSD history
storage market research
the problem with Write IOPS - in flash SSDs
the enterprise flash story... could it have been simplified?
After 40 years thinking about it
this is my last article on endurance.
No more. Ever.
I promise.
wrapping up SSD endurance
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How can a flash SSD have endurance enough to emulate DRAM in a DIMM?
SSD aspects of Diablo's Memory1

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an SSD way of looking at hard drives
Editor:- May 4, 2016 - In an ironic twist of fate - it looks as though hard drive vendors may find it useful to characterize some aspects of HDDs in a way which can be easily related to value judgement numbers created for SSDs.

hard disk drives news and articles
.. hard drives
A recent article - when did HDDs get SSD-style DWPD ratings? - in The Register brings to our attention that hard drrives are now being specified with write limits and the author Chris Evans (who also blogs as Architecting IT ) conveniently provides readers with a list of HDD models along with their DWPD equivalent ratings. the article
toughening up DWPD
Editor:- October 28, 2015 - DWPD ratings have become a useful shortcut to filter enterprise SSDs because there's an industry-wide consensus that the number should somehow map into recognizable application zones and price bands.

This shows how optimistic the SSD market mood still is today - when you factor in the jitter level uncertainty of exactly how elastic that DWPD drive number really is (in the minds of its creators) and how much it will get twisted around, modified and stretched when it meets up with the (is it really) DRAM, software and SSD array cousins with which it will cohabit life in the box.

All well and good - and my reference article on DWPD examples in the market was already becoming quite popular earlier this year (which was no surprise - as it's simply another way of talking about endurance) when I got a wake up email (in April 2015) about a new military grade industrial SSD which had a DWPD rating.

I didn't write about it at the time because even though the product had been stealthily working its way into designs it hadn't been publicly launched. Nevertheless I kept my eyes open for signs that others might also be doing similar things.

The similar things being:- the growing use of enterprise architecture in mobile datacenters and portable and remote rugged systems.

SSDs have been used in such systems for over 25 years - but often this was essentially a repackaging exercise to place a rack of industry standard stuff into a dustproof, ant-fungus treated, drop resistant box with an invertor so it could run off batteries while keeping the weight and size down so it could be lifted onto a truck or plane and survive long enough to do useful data-capture and analysis in the field.

The modern aspirations of these engineering systems are to do more of the same old things in less space but also to do entirely new things in widgets which you'll probably see in season 20 of NCIS.

That's why you're going to see more military grade, secure, rugged, industrial SSDs coming onto the market with full fledged DWPD ratings.

It's no longer just an enterprise market parameter. DWPD rated SSDs are getting tougher.
SAS SSD shipments beefed up by DWPD-lite models
Editor:- June 2, 2015 - Commenting on the SAS SSD market - Don Jeanette, VP - TrendFocus says in his new blog - SAS SSDs continue to show strength in an ever increasing competitive market - "SAS SSDs are not getting squeezed out by the incursion of SATA on the low end and PCIe on the high end as many have thought would happen."

SAS SSD growth Don explains that SAS SSD makers have populated their product lines with value models which have much lower DWPD ratings than the headline performers - which can go some way towards cost competing with SATA SSDs while at the same time validating the higher prices of 25 DWPD etc models. the article

Editor's comments:- Rackmount SSD users don't have to compromise reliability when they choose SATA SSDs in their value engineered arrays.

In a conversation with Andy Lee, Marketing Director - EchoStreams (a whitebox storage company) - in January 2015 - I learned that one of the design elements in their 20 bay 1U and 48 bay 2U systems - for their customers who want to build high density flash arrays using COTS SSDs - is that EchoStreams have deployed interposers on their drive bays which allow COTS SATA SSDs to be used in the same way as dual ported SAS but at lower system cost.
re MLC

how does NV become V?

NV + 0.4 DWPD @ 85C = V
Editor:- October 24, 2014 - Even a modest amount of drive writes per day can render modern day MLC flash incapable of retaining data for long in the unpowered state - depending on the temperature in the rack where those writes took place. This effectively means that the flash inside the SSD is no longer "non volatile".

The physics behind this are revealed in a blog by Virtium - a company which operates in the industrial market - and which does a lot of work characterizing memories for use in SSDs and other memory systems. They can leverage that knowledge for customers by adjusting controller and firmware characteristics to optimize the memory's life and data integrity - particularly if it is known in advance what proportion of time the embedded SSD is likely to be operating at particular temperatures.

Virtium's paper - temperature considerations in SSDs (pdf) includes some stark graphs and observations about data retention - which you should be aware of - even if you're not in the industrial market.

Virtium's paper says - "This shows the dramatic effects that temperature has on data retention for given workloads.

"For the same 750 full drive writes (0.4 DWPD drive writes per day for 5 years), SSDs operated and stored at 85C will only have 2 days of data retention, whereas those drives at 40C will have 1 year and those at room temperature 25C will exhibit characteristics of nearly 8 years of data retention." the article (pdf)

Adaptive R/W flash care & DSP ECC in SSDs

Efficiency - making the same SSD - with less flash

How will the hard drive market fare... in an SSD world?

flash wars in the enterprise - SLC vs eMLC vs MLC vs TLC

The sages of flash now agree
That if you fall out of a tree
To increase your chances
Grab hold at five branches
And swing around ju
life sliced endurance budgets and some limericks

10 years ago there weren't so many people who worried enough about what happens inside SSDs to scrutinize the subject of reliability at this level of detail.

I checked our stats archive and in in 2005 only 7,500 people read the SSD wear leveling article here on (That was 1% of our readers at that time).

In those days if an SSD maker said their SSD was reliable - you believed it. And there weren't so many alternatives to choose from.
razzle dazzling flash SSD cell care and retirement plans