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sugaring flash for the enterprise

10 years of enterprise flash

how the market changed from 2004 to 2013

and how it will change again

S / e / brand x / T / M /LC...

...and other aliases for enterprise nand flash


by Zsolt Kerekes, editor, StorageSearch.com


Unlike the Cola Wars - you can't afford the risk of a bad enterprise MLC SSD taste test.

MLC and other flash in enterprise SSDs - past, present and future

If you're unclear about the differences between MLC and SLC - see SSD jargon.

The use of flash SSDs in enterprise server acceleration has been hotly and seriously debated here in the pages of StorageSearch.com since about 2004. The notes below summarize what those past technical issues were and how things look today.

In 2004 the typical endurance of the flash memory used inside a 2.5" SLC flash SSD was 100K write cycles. Today the typical flash memory inside a 2.5" MLC SSD is rated at 3,000 cycles (30x worse). And yet in the same period (2004 to 2013) the sustained R/W speeds of the fastest 2.5" SSDs have gotten 45x faster (from 40MB/s 2.5" PATA to 1,800MB/s 2,5" PCIe) putting 1,350x more pressure on managing already strained endurance (assuming the same number of flash chips in the SSD).

The detail of the debates surrounding enterprise flash SSD has changed over the years. But all the arguments revolve around the question of - is this SSD going to be reliable enough in my application?

Back in 2003 all enterprise acceleration SSDs were RAM SSDs.

In 2004 - a customer case study by BitMicro - showed that a single 3.5" flash SSD could provide useful speedup in a 25,000 user server - compared to hard disk based RAID. But flash SSD makers weren't active in the enterprise market in those days. They sold to oems and systems designers. It wasn't economic for flash SSD oems to educate end users about SSDs, understand the complexities of user applications and configure the hot spots in a user server - simply to sell a single SSD.

In 2006 - SSD makers started shipping small form factor flash SSDs in volume aimed at the notebook market.

In 2007 - many SSD makers started shipping rackmount products and small form factor SSDs specifically into the server acceleration market. Those early SSDs were SLC - which typically had endurance of 100,000 write cycles per block. By selecting memory chips and processes - some SSD makers claimed their SLC SSDs could last 10x longer.

In 2008 - there was a tempation for systems integrators to deploy low cost consumer MLC flash SSDs in enterprise applications. But MLC endurance was 10x worse than SLC - and consumer SSD controllers couldn't manage MLC reliably in high IOPS environments. Some customers found out the hard way when their flash arrays burned out. The conventional wisdom at the time was - don't use consumer MLC SSDs in caching / accelerator environments.

In 2009 - a new wave of SSD companies including Fusion-io and SandForce made waves in the market with fast high IOPS enterprise SSDs which used consumer grade MLC flash inside. They said - what made the difference - was the intelligent management of flash risks inside their architectures.

In 2010 - some leading flash memory chipmakers started marketing so called "enterprise grade" MLC flash. This was a formal productizing of high endurance MLC - achieved by factory processes - to achieve similar ends which some SSD makers had been doing since 2004 with SLC - that is to say is selecting the best of breed flash to cream off batches with 10x better than average endurance.

In 2011 - you can find at least 3 different types of flash memory (SLC, consumer grade MLC and enterprise grade MLC) inside fast enterprise SSDs such as PCIe SSDs. And the situation could get even more confusing in future with x3 MLC and other nv memory types possibly appearing in enterprise 2.5" SSDs in the next year or so too.

The argument is shifting from which type of flash memory is best? - to whose SSD controller and flash management scheme do you believe is best?

That makes it harder to evaluate competing products and make decisons which are safe without paying more than you need to.

For more on what's happening today - see these articles:-
SSDserver rank is a latency based configuration metric - proposed by StorageSearch.com - which can tersely classify any enterprise server - as seen from an SSD software perspective - by a single lean number rating from 0 to 7.
what's in a number?
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"...The debate about using MLC flash in enterprise SSDs - aka "eMLC" - has moved on to a new level. The argument is no longer - can MLC can be made to work reliably? Or how many lifetime writes are good enough? It's - who's way of doing - so called - enterprise flash (of any kind) tastes best?"
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Quality Comparisons of SLC, MLC and eMLC (pdf)
This white paper - by C.C. Wu - Director - InnoDisk compares the data integrity of several generations and brands of SLC flash. and confirms what some SSD makers had been telling me since the early 2000s (as reported in my SSD endurance article) - which is to say that SLC in reality has often been 5x to 20x better than specified in the original memory chip makers' datasheets.

The same is not true for MLC - however - where the margins are closer to 2x (consumer MLC) and 3x (eMLC). This means that experience of what works with real world SLC SSD controllers and flash management - cannot be carried through safely to MLC.

MLC also presents greater risks of data loss when power is suddenly turned off - which in turn requires better architecture to recover and automatcially recover and heal the data.
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Are MLC SSDs Ever Safe in Enterprise Apps?

SLC versus MLC in Enterprise SSD arrays

looking at the risks posed by a new generation of MLC Nand Flash SSDs.

classic article - by Zsolt Kerekes, editor, June 2008

The original purpose of my SSD Myths article was to show that you needn't worry about wear-out if you use "best of breed" flash SSDs with write-endurance on the order of 1 million cycles and above.

When it was first published (in March 2007) all flash SSDs in traditional hard disk form factors used SLC.

But in the year following publication many leading SSD oems (including Samsung, Mtron and STEC ) have also introduced MLC products too.

To confuse things even more - in June 2008 - Silicon Motion announced a new family of flash SSD controllers which enable oems to mix and match MLC and SLC chips in the same drive - creating in effect SLC-MLC hybrid SSDs.

MLC doubles the capacity of flash memory by interpreting 4 digital states in the signal stored in a single cell - instead of the traditional (binary) 2 digital states.

This technique has been commercialized and proven over many years in hundreds of millions of cell phones and MP3 / iPod music players - where the theoretical consequence of data corruption (if anything went wrong with this risky "new" storage technology) was no more serious than an inaudible sub millisecond sound blip or invisible pixel splat.

In the SSD market MLC yields much lower cost storage than SLC with read / write speeds which are nearly as fast as the best SLC devices.

The manufacturers of first generation "hard disk replacement" MLC flash SSDs have responsibly classified them as aimed at the "notebook market" and by subtle wording differentiated them from their more pricey "enterprise" products. In the low duty cycle world of a notebook these MLC SSDs should give a good operating life - typically similar to the hard disks they replace. (Most SSD marketers would claim their MTBFs are even better than HDDs).

But there's no way to tell the difference between SLC and MLC SSDs externally (apart from the model numbers). Put them in a rackmount system in a datacenter with fast processors which can pump them continuously close to the maximum speed and what happens?
It's a simple matter to plug new data for MLCs into the calculation I did for the worst case wear-out process for flash SSDs - which I called the Rogue Data Recorder.

Instead of the 64GB example I used then, I'll assume the MLC SSD has 128GB capacity. MLC SSDs have more capacity than SLC. And more capacity means longer operating life - before cells wear out.

I'll still use the 80M bytes / sec sustained write speed - because the fastest MLC products (in Feb 2008) can already do that. (Meanwhile the fastest SLC products have moved up in the world and are about 50% faster.)

The next factor is where we hit the big problem... Instead of a write endurance rating of 2 million cycles (for the best SLC) - I can only use a figure of 10,000 for MLC. MLC has a much lower rating due to the complex interaction of discriminating multiple logic levels reliably coupled with the intrinsic failure mechanism of wear-out.

Plugging these numbers in the same calculation gives an estimated MLC flash SSD operating life (at max write throughput) which is 6 months! (instead of 51 years for a 64GB SLC SSD).

That's not good enough for a data driven enterprise. There isn't a wide enough safety margin.

Proponents of MLC might say - can't you batch select MLC chips for better write endurance in the same way that some oems do for SLC wear out? - Couldn't that give a figure that is 10x better?

There's not enough data to give a definitive answer - but I suspect the answer would be no!

The reason is that you would be selecting for the mutual inclusion of a single chip being inside 2 different probability curves for what are already secondary characteristics. (Like looking for the ideal man in Sex and the City.) Even in the unlikely event that you could find some devices with the magic properties to do this - the yield would be small - pushing the cost up and eliminating the main reason for using MLC.

That's where I thought this "SLC versus MLC in enterprise SSDs" discussion would end. But then another factor appeared out of the blue.

Sam Anderson at EasyCo pointed out to me that one side effect of their patent pending Managed Flash Technology is that their software "effectively erases erase blocks 10 to 100 times less frequently than drives doing traditional random writes" because it writes address blocks monotonically.

EasyCo's MFT was originally designed to give much faster system IOPS in flash SSD arrays by using patent pending write algorithms which manage arrays of standard SSDs in a way which reduces the probability of successive writes to an address block which is already busy in a time consuming erase/write cycle.

This new (to me) attribute of MFT opens up the possibility of yet another generation of high speed rackmount SSDs with new price points which could be 50x lower than RAM SSDs while being only 3x slower overall in typical applications.

Some of the papers listed in the footnotes below cover topics such as Data Retention (which in gets worse for blocks which have been more frequently erased), and Disturbances (caused by adjacent R/W operations) - all of which are much more significant issues for MLC compared to SLC.

Conclusion?

I can't give a definitive answer to the question - Are MLC SSDs Ever Safe in Enterprise Apps?

With the current state of technology in 2008 - it depends on the application and the consequences of data corruption.

I wouldn't risk it if I were a bank - but I might not mind if my own bank risked it and changed some pluses to minuses...

Seriously though I hope this article has shown that there are serious risks inherent in using MLC flash SSDs if they are not applied correctly.

Some of these risks can be managed by choosing an SSD array supplier who has qualified and tested their racks with products from a single known source (because every make of MLC flash SSD has its own unique failure profile).

I know that despite my warnings - MLC flash SSDs will get used in some enterprise apps - because the cost difference (compared to other options) is very attractive.

In my view using an MLC flash SSD array for an enterprise application without at least using the (claimed) wear-out mitigating effects of a technology like Easyco's MFT is like jumping out of a plane without a parachute.

And even with a parachute - strange things may still happen to wannabe MLC SSD enterprise pioneers on the way down.

PS - these warnings were valid for coonsumer MLC flash and the state of controllers and SSDs which were shipping in 2008. Newer developments since then - described in the articles at the top of this page have changed this guidance. However, there are still some vendors shipping enterprise SSDs today which can - in the wrong apps - die from premature wear-out in a few months.



More Articles About Flash SSD Data Integrity

Can you trust your flash SSD specs?
Increasing Flash Solid State Disk Reliability
SSD Myths and Legends - "write endurance"
Data Integrity Challenges in flash SSD Design
Is All CompactFlash Really Created Equal? (pdf)
Flash Disk Reliability Begins at the IC Level (pdf)
SLC vs. MLC: An Analysis of Flash Memory (pdf)
The Inconvenient Truths of NAND Flash Memory (pdf)
Flash Solid State Disk Write Endurance in Database Environments
Unveiling XLC Flash SSD Technology - spoof article on x4 MLC
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RAM SSDs versus Flash SSDs - which is Best?
Experts discuss the server acceleration market.
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What's the best way to design a flash SSD?

and other questions which divide SSD opinion
More than 10 key areas of fundamental disagreement within the SSD industry are discussed in an article here on StorageSearch.com called the the SSD Heresies.
click to read the article - the SSD Heresies ... Why can't SSD's true believers agree upon a single coherent vision for the future of solid state storage? ...read the article
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Data Integrity Challenges in flash SSD Design
Editor:- Data Integrity Challenges in flash SSD Design is an article - written by Kent Smith Senior Director, Product Marketing, SandForce.

Reliability is the next new thing for SSD designers and users to start worrying about.
read the article about SSD integrity A common theme you will hear from all fast SSD companies is that the faster you make an SSD go - the more effort you have to put into understanding and engineering data integrity to eliminate the risk of "silent errors." ...read the article
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Yes you can! - swiftly sort the Enterprise SSD buckets
If you're trying to create your first short list of vendors to talk to about how to speed up your enterprise apps using SSDs - you realize now - with a sinking feeling in your gut - that maybe delaying the decision for the past several years wasn't such a good idea after all.

Because the range of technologies and design approaches is now so bewildering that you envy your peers in other (richer) companies who started down the SSD track when the range of solutions was so much simpler.

Your problem today isn't just that vendors don't seem to agree about where the best place is to put the SSD or what memory should be inside it (something I've written about in the SSD heresies).

The problem is that even when you try to narrow down SSDs to a single interface - the competing SSD vendors tell a very different story about what their products will do for you and how much they will cost. And this confusing picture isn't simply down to SSD jargon - which is bad enough - but you're getting the hang of it. There's something tangibly different lurking behind those shadowy SSD vendor promises - but you can't quite put your finger on what it is.

Is there a simple methodology which - starting from the very first press release you see on the web - reliably helps you classify all enterprise SSD products - to create 2 distinct groups.
  • the SSDs you're not interested in
  • the SSDs that might be worth a closer look
without the risk that you may miss out the best choice for your situation - and without having to read hundreds of articles and reviews?
Legacy vs New Dynasty - the new way of looking at Enterprise SSDs ... Yes there is. ...read the article
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popular SSD articles on StorageSearch.com

SSD Myths - "write endurance" - In theory the problems are now well understood - but solving them presents a challenge for each new chip generation.

SSDs replacing HDDs? - That's a gross simplification.

the Top 20 SSD companies - updated quarterly.

the Fastest SSDs - in each form factor. Speed is still the #1 reason for buying SSDs.

the SSD Buyers Guide - summarizes key SSD market developments in the past quarter and has a top level directory of SSD content.

PCIe SSDs - news and market commentary. We've reported on PCIe SSDs since the first products shipped in 2007.

SSD market history (1976 to 2011) - If you're new to the market it provides a clue to how much things have changed - and how fast (or how slowly).

SSD controllers & IP - this is a directory of merchant market SSD controller chip technology providers.

Clarifying SSD Pricing - where does all the money go? - Also includes SSD price examples.

RAM SSDs - 20 or so companies still market RAM based SSDs. This directory page tells you who they are and explains why - as the market uses more flash SSDs - the need for RAM SSDs is growing (instead of shrinking).

SAS SSDs - includes a timeline of the SAS SSD market - and lists significant vendors.

the SSD Reliability Papers - links and abstracts of articles related to the subject of SSD reliability and data integrity.

the problem with Write IOPS in flash SSDs - this classic article helps you understand why all flash SSD benchmarks incorrectly suggest you're going to get much higher performance from some types of flash SSDs than you will actually see in your application..

auto tiering SSDs / SSD ASAPs - market guide to Auto-tuning SSD Accelerated Pools of storage.

this way to the Petabyte SSD - This article describes the future storage architecture of the datacenter, explains the economics of SSDs replacing HDDs for bulk storage and suggests a roadmap for getting there.

the 3 fastest PCIe flash SSDs list - or is it really lists?

storage search banner

the magic of enterprise MLC
..
Can you trust SSD market data?
Heck no! - whatever gave you that silly idea?
..
SSD ad - click for more info

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"You may decide that my ratios are too timid. - If so - scare us!"
scary Skyera? - I talked to Skyera's CEO about SSD utilization efficiencies and the problem of being a newbie vendor in the enterprise


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Fusion-io fast SSDs - click for more info
the standard for enterprise PCIe SSDs
by which all others are judged
ioDrives from Fusion-io
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nice vs naughty flash (management summary)

The arguments about flash in enterprise SSD accelerators have changed since this trend started in 2004.

First you learned about SLC (good flash).

Then you learned about MLC (naughty flash when it played in the enterprise - but good enough for the short attention span of consumers).

Then naughty MLC SSDs learned how to be good. (When strictly managed.)

But thanks to genetic alteration some naughty MLC has been bred to be much nicer than others. (Even when the strict controller isn't looking.) This (extra-good) MLC is always preceded by an "e" to show it's better. (Like emaill. OK email vs the pony express - Postman - kind of mail which is derogatively called snailmail.)

But other people say you don't need the expensive "e" in eMLC - because their controllers empathize better with native naughty flash. (They don't approve of flash eugenics and they really do care about street bred naughty flash cells being sent to bad block jail too soon.)

And a new type of naughty flash which wants to be in with the gang on the enterprise SSD block is TLC (alias x3).

Is your head ready to explode yet?

It's going to get even more complicated.

Best forget the technical explanations, click on the ads with the nicest pictures and think of it all as SSD magic.

Why do you need to allow space for that uncouth MLC flash in your nice clean reliable datacenter?

It's much cheaper - even when you take into account the effort of cleaning it up and re-training it than the other kinds of memory. Even though you still need SLC (good) and RAM (positively angelic) for ultimate performance.
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"Knowing the memory type in the SSD doesn't tell you anything useful about the SSD's likely characteristics and limitations any more... "
Strategic Transitions in SSD in 2012
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SSD ad - click for more info

Surviving SSD sudden power loss
what do enterprise SSD users want?
how fast can your SSD run backwards?
Data Integrity Challenges in flash SSD Design
MLC flash lives longer in my SSD care program
7 SSD types will satisfy all future enterprise needs
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"The result is a crash in sales for the SSD vendor - because the improvement in SSD utilization caused by the new software is bigger than the previous growth rate in customer hardware purchases."
the enterprise SSD software event horizon
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SSD ad - click for more info
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"By 2014 - 50% of all workloads will be processed in the cloud"
Editor:- August 24, 2012 - that quote is part of a profile of Tom - a mythical cloud datacenter manager - in SMART's white paper - is MLC ready for the enterprise? (pdf) - presented today at the Flash Memory Summit

Tom likes what flash can do to keep things flowing. His boss likes it too. Tom may get promoted.

But too late - it seems - he learns that the MLC SSDs he's been using have different lives in different workloads (4 months in an exchange server, maybe 1.6 years serving web pages). He starts to learn about endurance, write amplification etc - and instead of getting a kick up the management ladder - he risks getting a kick out the door.

You can guess how this story ends... with a happy ending.

SMART's SSDs - based on adaptive R/W DSP flash technology - saves Tom his job - and he no longer has to worry about flash reliability. The tale is engagingly presented in pictures. ...see Tom's SSD adventure in the cloud datacenter - (pdf)
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StorageSearch talks to SSD leaders... re flash in enterprise SSDs

Fusion-io's CEO - re MLC in banks.

Over 80% of the SSDs that Fusion-io has sold in the last couple of years have been MLC rather than SLC - and David Flynn thinks that they probably have a bigger base of enterprise MLC SSDs which has been operating longer in customer sites (upto 3 years) than any other company. ...read the article
the editor enjoys another conversation with SSD movers and shakers

Texas Memory Systems - re MLC and RAM SSDs.

Jamon Bowen said current consumer grade MLC nand flash has endurance on the order of 3,000 write cycles. ... And the company's burn-in process (done for QA as part of manufacturing) would use up 10% of the endurance life before the SSD even reached the customer!

In many bank applications RAM SSDs are actually cheaper than flash - because of the small size of the data. ...read the article
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what about enterprise MLC flash?

In July 2010 - a reader (Rob Mantia) asked - I was wondering what your opinion is on the decision of some SSD manufacturers to switch to enterprise MLC flash from SLC flash for their enterprise SSDs and if you think eMLC is as great as they make it sound (less cost, just as reliable) or if you think it's overhyped.

Here's what I said.

The view expressed in the original text of my 2008 article Are MLC SSDs Ever Safe in Enterprise Apps? hasn't changed.

Civilian enterprise users of flash SSDs have to segment their applications for flash into 2 types - SLC or MLC (and that "MLC" includes eMLC) depending on the mission criticality and costs associated with the risk of data corruption.

eMLC mitigates just 1 problem (endurance) of the 4 major risk factors associated with MLC which are significantly worse for MLC than SLC.

The other 3 intrinsic risk factors are
  • noise immunity - due to much smaller signal change associated with each logical bit
  • data integrity - due to physical variations across the chips (MLC poses more problems for R/W-ability even from the outset in a new chip)
  • temperature sensitivity - if you subject MLC to extreme temperature fluctuations you may irrecoverably lose data which the ECC cannot bring back. That's why MLC SSDs aren't used in military or industrial products.)
So as per my original article...

MLC is OK for server apps like video streaming (no big deal if a few pixels change color).

MLC is risky for storing financial data - like derivatives models and trades.
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doesn't write amplitude control make MLC safe?
In June 2010 - a reader asked if the comments in the article - Are MLC SSDs Ever Safe in Enterprise Apps? - were still valid - given that a few years had elapsed since it was written.

Don't SSD controllers from Anobit, SandForce and WD Solid State Storage - which reduce write amplitude - fix the problem of low MLC endurance?

Here's what I said.

Yes - but that's only one of the problems with MLC which was identified in this article. And this has to be reevaluated with each new flash memory generation - because the difference in intrinsic data integrity between SLC and MLC gets worse with smaller geometries.

What has got better is the strength of the error correction schemes which hide the magnitude of raw media defects in MLC.

A lot depends on your environment - because temperature cycling lead to charge leakage - and there isn't much tolerance in MLC cells. That's another reason that all industrial temperature SSDs are SLC. (No ECC scheme can fix a device which has redistributed too much charge.)

The issue of EMC compatibility (discussed in the original article) remains in my mind an intrinsic difference which no one else in the industry seems to be worrying about. If you don't have a noisy power rail or ground rail in your app then the EMC may not be determining factor.

If you have time - a good test would be to do continuous overwriting of your SSD with randomly changing data - and each time you fill the disk read back the whole disk and compute a data checksum. Run this for several weeks or months to qualify a new SSD (or HDD) for a mission critical app.

More about EMC compatibility etc in the original article text below...
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Are MLC SSDs More Susceptible to Power Rail Disturbance?
As someone who in a past career designed analog data acquisition products and systems which got right down below the thermal noise and who cared about the shape and material of PCB tracks I want to air another concern about the (in)/advisability of using MLC Nand flash in datacenter applications where there's a lot of power rail disturbance.

Although MLC devices have been used in commercial products since 2003 - the products they have been in (phones and portable music players) have been battery operated environments where (inside the casing) the environment's overall power rail and electromagnetic compatibility has been controlled and managed by the system designers who know enough about these things. And as I say elsewhere in this article - the consequences of misread data in these applications are trivial.

You could say almost the same about the environment for a MLC flash SSD inside a notebook PC. It's a known, testable environment. Although the user can plug modules in - they're rarely a high energy disturbance product. The designers would have tested it with a range of plug-ins, and they've sold millions of similar notebooks before. There will be few surprises.

An array of SSDs in a datacenter cabinet is not such a quiet place.

There are plenty of fast processors all around. Above you - below you. The SSD designer does not control that space. Every installation is unique.

Something which you may not be aware of - is that inside an MLC flash chip are effectively:- a 2 bit anlog to digital converter (ADC) and a 2 bit DAC. Between each of the 4 logic levels there is also an indeterminate band where the signal should never be. Power line disturbances are 3x more likely to result in a false read for MLC than SLC, but the overall error comparison gets worse. There's also a bigger intrinsic risk (for MLC than in SLC) of an error creeping in with the initial write charge. SSD designers deal with this by surrounding blocks of MLC flash data with heavier error detection and correction codes than they would normally use for SLC.

I found a good detailed discussion of ECC potential problems in this Denali article:- Memory ECC: A curiosity for decades, now essential for MLC NAND flash from which the quote below comes.

"With the voltage levels closer together for MLC flash the devices are again more susceptible to disturbs and transient occurrences, causing the generation of errors which then have to be detected and corrected. If that is not enough for the chip maker, it poses an even larger problem for the system designer, in that there is more of a variety of technologies employed among competing flash chip designs than DRAM makers, for example, would ever dream of."

For a related discussion about what EMC (not the storage company) can mean for signal integrity going into a flash SSD see the white paper - Noise Damping Techniques for PATA SSDs in Military-Embedded Systems (pdf) by SiliconSystems.
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More Conclusions
Flash SSDs are complex systems with a lot of stuff going on inside.

Like cars (which use the internal combustion engine) all flash SSDs from all manufacturers are not the same.

Even if they have the same capacity and interfaces.

There are many different process and media management technologies inside a a flash SSD which oems deal with (or not) in their own proprietary ways. These are just some of the consequences:-
  • best to worst wear leveling algorithms can vary product life by a factor of 3 to 1. (That's not too bad. Some so called "SSDs" - which are actually dumb flash storage bolted to a disk interface - don't have wear leveling and should not be used in servers at all.)
  • best to worst SLC endurance can vary by 30 to 1.
  • SLC to MLC endurance can vary from 10 to 1, upto 300 to 1
  • intrinsic electrical noise susceptibility between SLC and MLC is hard to quantify - but probably on the order of 10 to 1. Although hidden by wrap around redundancy and error detection and correction - the possibility of uncorrectable errors is still greater in MLC - which is unproven in enterprise environments.
Buying flash SSDs for enterprise applications should be regarded as an important qualifying process. Just as you wouldn't buy a traditional RAID system without knowing what type of hard disks were inside it, or without knowing something about the experience of the vendor in enterprise apps - so too you shouldn't buy flash SSDs without asking about the factors discussed in this article.

The risk for users is that many oems who designed SSD architectures for the notebook market - will try to capture business in the enterprise market - with the same (or similar) products without dealing with the datacenter's need for better resilience and data reliability.

And, sadly, I know from my own inbox that some SSD marketers don't know how much they don't know about their own market and how much more advanced their competitors are in the field of reliability.
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SLC MLC Hybrid SSDs
In June 2008 - Silicon Motion announced a new family of flash SSD controllers which enable oems to mix and match MLC and SLC chips in the same drive.

The controller can analyze the incoming files from the host and intelligently move frequently accessed data to SLC NAND and non-frequently accessed data to MLC NAND. With this innovative hybrid architecture, the SSD system cost is significantly reduced to a level comparable to a pure MLC-based SSD, while endurance is significantly enhanced and comparable to a pure SLC-based SSD.

However, the intrinsically higher susceptibility of MLC flash to electrical disturbances remains a risk factor in such hybrid devices.
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Looking at MLC Writes in More Detail
The clearest description I've seen explaining the mechanics of MLC flash writes and the problems presented for an SSD controller are in a white paper by Hyperstone called Flash Controller & Firmware (which I originally linked to - but is no longer viewable on their site). From which this quote is taken.

"...charge levels might change due to external conditions such as extreme heat or magnetism. While the cell itself is not damaged permanently, the bit value might have changed and a read error might occur. Some more recent flashes have the capability of recognizing the systematic change in behavior or in the voltage level so that not the difference to a starting reference voltage, but the inability to differentiate the relative difference to other voltage levels produces the read errors."

Editor again... Hyperstone's description of Incremental Step Pulse Programming reminded me of the Intel 2816 (the first commercially available flash chip).

I had an early sample in about 1981. Programming was done by shaped pulses. The Intel suggested circuit for doing this didn't work, but it was easy to modify. Locations were programmed interactively using bursts until you read back the value you'd written - and then wrote some more bursts for safety. If you wrote too many pulses that could zap the device. Or it might mean the location was unusable. Later generations of flash memory hid these details from view. But once you've seen what happens - past the cloaking effect of a flash memory controller - you appreciate the delicate balances involved in making a working flash storage drive.
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