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Surviving SSD sudden power loss

by Zsolt Kerekes, editor -
..... SSD history
the SSD design heresies
asymmetries in SSD design
big vs small controller architecture
Capacitor hold up times in 2.5" military SSDs

The risk of data corruption from power cycling isn't a random, unforseeable event. It's a direct result of choices made (or not made) when that SSD was designed.

Surviving SSD sudden power loss

SSD is going down! - We're going down!

If you've ever watched the movie Black Hawk Down - there's a memorable scene in which Super 64 has its tail hit by an RPG and becomes the 2nd chopper to go down.

From that moment it's clear to viewers that whatever the pilot does at the controls - '64 will hit the ground real soon.

Inside the brain of the SSD - a nerve ending tugs to say - forget your other priorities pal - the power rail is going down.

Is this the end of this promising young SSD's career? Will data will get corrupted?

That depends on what happens next and the skill of the SSD's designer. Did the designer understand the range of slew rates this product could see? Did they test for burst brownouts in which the power comes back up and then drops again as standby generators or batteries kick in and get hammered by delayed power surges.

This article looks at what happens inside various types of SSDs when the power goes down. This is an area in which products differ a lot. I'll explain some of the architectural parameters which constrain the freedom of SSD designers. You may think that solving marketing driven constraints like speed, price, data recoverabality / security and endurance are challenging enough. But the hardest part of an SSD design to get right is deciding exactly what should happen in the short time remaining when the power goes down - while the connected circuits can still respond to controls.

The article will also help you understand another reason why SSDs with apparently similar performance and datasheet specs behave differently inside. And why it's risky redeploying an SSD you may have used in one applications environment to another. For example why some SSDs designed for notebooks are more likely to fail in rackmount arrays - even when their write IOPS have been managed well within flash memory limits. The power management system is actually the one of the most important parts of the SSD which governs reliability (2nd only to the memory management system). But many digital systems designers don't give it the scrutiny it deserves. That's because most SSD designers have a background in digital systems design - and they don't have the conceptual background to imagine, model and control the range of less deterministic interactions between components and data in the wild world of analog power spikes.
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Precisely how many milli-seconds the SSD has got to perform shut down operations and the nature of the tasks to be done depends on 3 main factors.
1 The RAM cache flash architecture.

There are a wide range of commercially feasible architectures which can be summarized as skinny, regular, fat and hulk (a true RAM SSD).

More RAM makes it easier for the SSD perfomance architect to boost true random IOPS performance. These are headline datasheet characteristics which are leveraged to sell the SSD to its intended markets. But more RAM in the SSD also means that more data is in a vulnerable volatile state when the SSD power goes down. The designer has to calculate worst case conditions to guarantee saving the state of critical data using the local in-board technologies - which nowadays are nearly always flash.
2 Assumptions about the power supply design and availability of power hold up circuits.

In most SSDs the nature of the power hold up circuit (and the decision of whether there is one at all) is the direct consequence of the RAM cache flash architecture. In most markets (in early 2011) such as the enterprise server acceleration market, and notebooks - the power hold up architecture is not regarded as a headline datasheet characteristic which sells the SSD. Most end users - if they give this characteristic any thought at all - will regard it as neutral or not a significant decision factor in their vendor qualification process. In contrast - in many parts of the embedded SSD market - particularly in military, industrial and telecomms markets - the power hold up design will receive much closer scrutiny - because customers in these markets know from long experience that appropriate management of power cycling events is critical to operational reliability and ROI.
3 Assumptions about whether the SSD and the host processors it serves live in the same power rail zone are also critical factors. This isn't simply predetermined by whether the storage device has a DAS or NAS type of interface. For correct behavior in a power down situation - the designer has to make assumptions about the operational environment in which the SSD is used. This is another characteristic which can be a subtle difference in the datasheet of the SSD product - but which can make a big difference to operational reliability. For example the operating assumptions for an orderly power shutdown in a 2.5" SATA disk are very different whether that disk is living in a notebook PC - or whether it is part of an array of disks in a rackmount SAN.
As you can see from the notes above there are many permutations which derive from these factors. For simplicity I'm going to look at a handful of different situations below. These will give you an idea about the interplay between these power down management factors.
this SSD is going down - the RAM SSD and fat flash SSD scenario

RAM SSDs exist in a wide range of form factors including:- rackmount, 3.5" SSD, PCIe cards and DIMMs.

Data in RAM systems is difficult to read back after power goes down. (But not impossible if you work for a security agency or forensic data recovery company with the right technologies and with a strong enough need to know the RAM contents to counter the expense of reading most of it back.)

Anyway in normal operation - RAM has to be treated as volatile. Data will be lost when the RAM is powered down. To counter this - early RAM SSDs included significant battery backup / UPS's. In the early 2000s many RAM SSD designs also started to include internal hard drives. That enabled designers to reduce the charge capacity and physical size of batteries - which now only had to last long enough to ensure that data could be written reliably to the on board HDD. In 2008 Texas Memory Systems became the first vendor to switch to using flash SSD as backup - and that has now become the norm in the industry because of faster save and restore times (compared to HDD) and better reliability too.

The custom and practise in RAM SSD design is that power rail hold up times which used to be hundreds of hours have now dieted and shrunk their way down to seconds. Designers in this market don't have to count the cost of each extra millisecond in the same bean counting way as flash SSD designers and can afford to be sure that data is safely put away. Designers still have to ensure that the flash they back up to shuts down itself in an orderly way - but that's a 2nd order timing problem. Batteries and supercaps are the tools of the RAM SSD shutdown trade and can't be avoided.
this SSD is going down - the regular flash SSD scenario

These flash SSDs have RAM caches the contents of which take multiple write cycles (written to flash) to store safely. This takes many milliseconds. These designs therefore include internal supercaps or similar technologies to hold up the power rail long enough for these processes to complete. Examples of these products include Memoright's 2.5" GTR family and Oracle's F20 PCIe SSD.

One of the disadvantages of this scheme is that the extra RAM memory and supercaps add to the physical space, cost and unreliability of the product (compared to skinny SSDs). But the advantage is that the supercaps enable less rigid design rules in the SSD controller actions to achieve high IOPS - without needing such meticulous micro managed internal detail.

It is theoretically possible for this type of architecture to skip the need for supercaps - if the RAM cache is implemented by non volatile RAM. This isn't currently economic. Advocates of skinny SSD architecture might also argue that the relaxed data handling rules enabled by regular and fat RAM architectures mean it would be difficult to transition their designs to nv rams regardless of nv ram costs - because designers still have to implement write completion tagging mechanisms even when the time slots are shorter.
this SSD is going down - the skinny SSD scenario

These flash SSDs either don't have external RAM or the capacity used is unbelievably small. (I have been quoted numbers of bytes - but can't disclose them.)

The key thing in these designs is that the SSD does not have to save the state of the SSD from RAM into flash when the power goes down. The operational state of the SSD is always in flash and it's always valid.

This requires a level of attention to detail in the data management processes done by the SSD controller which is dramatically different to that of all other types of SSD. It stems from a philophically different viewpoint in the design of SSDs which starts with the flash memory media - and asks the question - what happens when the power goes down? and extrapolates all the design rules back from that. Those rules of flash block engagement mean there is very tight integration between the SSD controller and the power management system. The PSU management system is not a "bolt-on" or afterthought - as it appears to be in many other common flash SSD designs.

Skinny flash SSDs consequently are the most reliable SSDs in the power down scenario. And they are more reliable than other types of SSD because they have less parts like capacitors and external RAM which can go wrong. Examples of companies which design these products are:- Microsemi (GUARDIAN), SandForce (all) and WD (SiliconDrives).

However, as you'll see in a later part of this article - when you combine multiple skinny SSDs into arrays the design thought which went inside the SSD doesn't absolve the array designer from analyzing and solving the power down problems introduced by the array logic.
this SSD is going down - some other special situations

PSU of the SSD same as that of host servers.

Typical examples of this are notebook SSDs and PCIe SSDs.

In the notebook environment it's tempting for the SSD designer to assume that as this is a low IOPS environment, and the directly attached host will mostly have initiated the shut down, and as there is a battery in the system - which enables a controlled rate of shutdown - there is little need for the SSD power management to be sophisticated - no matter what the internal SSD architecture. And due to price sensitivity in this market the power management in notebook SSDs is often minimal and crude but works 99% of the time. A risk factor for the SSD's data integrity is if the notebook hangs while doing disk I/O and the user decides to remove the battery pack to force a reset. However notebook SSDs are the simplest case for the SSD power management designer - because most of the time - even if it didn't exist - the SSD would survive in this environment.

In the server acceleration environment of the PCIe SSD - there's a wide range of RAM cache archietctures covering the full spectrum of design choices. Examples include RAM SSD (with flash backup) from DDRdrive, regular flash SSD (with supercaps) - from Oracle, skinny SSDs in an array - from OCZ, and skinny SSDs where the controller action and shutdown is controlled directly by the host - from Fusion-io. The successful operation of these products depends on the characteristics of the host server power supply, the operating system, and the host processor speed. The SSD designer has to be sure that the shutdown process (if loaded on the host processor) has a high enough priority to complete - given all the other things that it's got to do.

That's why qualification of PCIe SSDs of this type with one server box and OS doesn't automatically guarantee that the same SSD will work in a different server box design - or even in the same server box but running a different OS.

Another approach is to offload all the performance and shutdown functions from the host and have a more powerful on board controller. That's done in designs by Texas Memory Systems and Virident Systems. And there are many other SSD products which fill the spectrum between the extreme cases.

As always the question is - do I have enough milliseconds to complete currently committed write operations - and if I don't - then have I set up a semaphore which says I started a write process on this block - but I haven't said it finished. The power has come up therefore roll back to the previously known good version of this block.

PSU of the SSD is different to that of host servers

Typical examples of this are rackmount SSDs - both on the SAN / NAS and DAS connected (usually via SAS).

In this case an additional requirement for the SSD is to shut down in a way that prioritizes the completion of current data I/O requests - if possible - and doesn't leave the host hanging around for data that it's not going to get. There may also be multiple hosts accessing the same SSD rack. In many of these enterprise configuations the host(s) will switch to an alternative SSD and carry on serving apps - so a tidy switch-over is desrirable. A rackmount SSD can be implemented with any type of RAM cache architecture - but an additional demand on the power management system comes from the fact that the SSDs are located in an external system. Even if the SSDs are skinny flash and can shut down fast with no data loss - the rack itself will typically have some kind of additional logic which manages the array - ranging from the simplest case of network to DAS routers and RAID controllers to more complex systems. These devices and appropriate control of the memory and logic states within them also come into the scope of the SSD power management system. Rack logic which may have worked OK for hard drive arrays - may need to be redesigned (or power down cushioned) to work properly with SSDs. That's because the SSDs may still try to respond to write requests lower down the power rail voltage droop (and later in time) than hard drives.

The design of an SSD's power down management system is a fundamental characteristic of the SSD which can determine its suitability and compatibility with user operational environments. Systems integrators must take this into account when qualifying SSDs in new applications - because subtle differences in OS timings, rack power loading and rack logic affect some types of SSDs more than others. Users should be aware that power management inside the SSD (a factor which doesn't get much space in most product datasheets) is as important to reliable operation as management of endurance, IOPS, cost and other headline parameters.

related reading:-
  • SSD Power Failure Protection (pdf) is an application note - published January 2011 - by SMART. It describes the 3 most vulnerable SSD areas which can get corrupted due to sudden power loss - and describes typical architectures to prevent it.

    SMART's view is that supercaps aren't reliable enough for enterprise SSDs. "For every 10°C of ambient operating temperature rise, the life expectancy of a supercapacitor can be cut approximately in half." So instead they use NbO capacitors in an array. These have MTBFs 100x better than Al based supercaps, have little degradation of capacitance with temperature and fail to open circuit (which is acceptable) and the array guarantees there is sufficient capacitance remaining if this happens.
  • SSD Power Failure Recovery (pdf)- published in January 2010 - by Fortasa Memory Systems describes various techniques which the company uses in its SSDs.

    For example Fortasa's controller makes a redundant copy of the FAT structure when doing a block write which is retained until after the write has been verified - to "practically eliminate any chance of FAT table corruption." - Fortasa also specifies that system designers should provide approximately "5mS of reserve power to their SSDs to complete the NAND max program time, control signal propagation delay and queuing." That means the designer doesn't have to guess or over-design the power hold-up.
  • DataSentinel Whitepaper (pdf) - published in May 2010 - by BiTMICRO describes the the design approach which the company has taken to minimize data corruption in their E-Disk Altima SSD product range.

    It includes good systems analysis and block diagrams and this quote - "Power is a fundamental need but it can also be the biggest threat to the reliability and operation of any system."
  • EnduraCharge™ Technology Power Failure Data Protection (pdf) - (data safety features ready for unexpected power-loss) - published in June 2011 - by Unigen describes architecture and circuits which mitigate this problem - and reveals a unique feature in the company's SSDs which enables the health of the hold-up capacitors to be tested and logged - a process which is inititated by a host command.
  • OCZ DataWrite Assurance (pdf) - is a white paper which outlines power loss data protection in OCZ's enterprise SSDs.

    Here's a quote - "In the case an OCZ SSD's primary power source drops below a predefined threshold, the SSD will automatically not accept any new commands from the OS. The power loss backup circuitry, a self-contained secondary power source, is then activated ensuring that any in-flight data is safely transferred and stored in the NAND flash." It also warns about un-named "enterprise" competitors who store vital metadata in vulnerable host server RAM.
  • Dynamic Transaction Point Settings - overview - by Datalight - describes how its flash file system uses the concept of 2 states - the working state and the committed state - to preserve the integrity of old data when new data is written - to ensure that data is always preserved and valid - even through a power disruption.
  • Finding the Perfect Memory (pdf) - this 2009 white paper from AgigA Tech - described the architecture of the company's pioneering flash backed RAM DIMMs. It also included a historic tour of how various companies had encountered and solved the contradictory demands of low latency random access memory with various data integrity solutions designed to cope with randon power loss.
about the author

Zsolt Kerekes is the editor of I first started giving serious thought to the issue of data corruption in user programmable memory modules when I was designing intelligent analog I/O in the one of the programmable controller design groups in Square D in 1980. Although most of my electronics design career was digitally focused I also spent more than a year involved in pure analog design - which involved research into new process control sensors and inventing new measurement techniques - where I returned to the theme of fully characterizing sensitive electronics products at any power slew rate and any operating temperature. I returned again to the power disturbance theme many times later - such as when designing wire speed disk capture systems for national power grid testing and modelling.

And in my current job I've been privileged in talking to many of the world's leading thinkers in the world of SSD design and architecture.

All electronics products benefit from good power management and EMI compatibility. Data is a very sensitive thing if you don't take care in the design to protect it.

If you are in the SSD industry and wish to add useful comments to this article - email me Anonymous emails will be disregarded.
suggested SSD articles

flash backed DRAM DIMMs
how fast can your SSD run backwards?
the naughty history of flash in the enterprise
Adaptive R/W and DSP ECC in flash SSD IP
Efficiency - making the same SSD - with less chips
MLC flash lives 10x longer in my SSD care program
how will Memory Channel SSDs impact PCIe SSDs?
latency loving reasons for fading out DRAM in the virtual memory mix
"I've learned more about SSD batteries and how they can fail than I ever wanted to know.."
...... Holly Frost, CEO, Texas Memory Systems - talking about SSD design (Dec 2011)
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image shows Megabyte's lighter than air storage balloon - image for SSD PSU is going down article
What did you say happens
when we run out of gas?
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what happens in the SSD when the power goes down?
Why should users care what happens inside an SSD when the power goes down?

The simple answer is - it can make the difference between how much data in your SSD app is corrupted and whether the SSD itself is usable when the power comes back up.

You may be surprised to learn that the ideal state for an SSD is when it's in the powered up state. That's when it's at its most reliable. A well designed SSD will look after itself and its data when its powered up. And apart from phsyical environmental stresses (like being cooled or fried or zapped by static) it should mostly last for a predictable number of years.

Although it'scommon to talk about SSDs as being "non volatile" memory / storage devices - because they don't lose their data contents when the power goes down (unlike most RAM) - getting from one state to the other is a risky experience for all the chips in an SSD. What makes the difference - is the skill of the SSD designer in understanding the operational environment and making sure that the process is always controlled and predictable.

Are SSDs instrinsically more vulnerable to data loss with a sudden loss of power than hard drives?


The 3 main reasons are:-
  • SSDs are mostly capable of higher R/W intensive activities than HDDs. SSDs also include many more internal housekeeping functions. Therefore the state of a typical SSD at any point in time is much more complex than than of a typical HDD. In many SSDs the complexity of the internal data management processes is more like that inside a traditional RAID system. (Some examples of this complexity include queue depth, erases in progress, and bad block management). With complexity comes risk. There's more to go wrong - if the designer hasn't throught through the issues and understood what needs to be done.
  • There are many more distributed storage elements in an SSD than in a hard drive . There can be thousands of flash chips in an SSD. All of these have to be protected from spurious data writes as the logic system changes from the powered up state - where operations are well defined - and the power rail drops through voltage regions where the operation of each chip is undefined. In contrast - in a typical hard drive only a handful of heads and write amplifiers have to be controlled at the critical periods.
  • The hard disk industry can draw on the experience of many years (and billions of units in the field) in which hard drive architecture hasn't changed much. So it's reasonable to assume that power management designs and lessons can safely span to new product generations with tweaks rather than jerks.

    In contrast - the SSD experience in current market conditions is that most product architectures are changing and evolving significantly from one product generation to the next. This means that lessons learned from one generation of SSD power management systems does't necessarily guarantee coverage for all the critical events in the next. And field experience for most SSD vendors is more limited too. That means designers still haven't got the visibility of all the bad things which can kill their SSDs.
When the power goes off the purpose of the power management system is to save the state of the SSD (or enough of the state) to ensure that data integrity is preserved.
"Many production enterprise SSD designs we see still get killed stone dead by hot swap simulation tests."
Quarch Technology talks about rail testers - December 2015
"By creating an automatic failure testing framework, we subjected 15 SSDs from 5 different vendors to more than 3,000 fault injection cycles in total. Surprisingly, we find that 13 out of the 15 devices, including the supposedly enterprise-class devices, exhibit failure behavior contrary to our expectations."
... from Understanding the Robustness of SSDs under Power Fault (pdf) - February 2013
"We had an SSD 320 600GB 2.5" SATA drive in for evaluation from our Intel rep. I was able to kill it in 2 or 3 hours by power cycling it."
... from Intel's SSD community site - June 2011
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customizing power consumption in SandForce driven SSDs

In February 2011 - SandForce started shipping its 2nd generation SF-2200 processors optimized for SSDs deployed in client computing applications. One of the oem customizable features in this family was the ability to set a maximum power budget.

SandForce's Product Marketing Director Kent Smith gave me this outline of how it works.

"To briefly describe our new power management feature, our SSD manufacturers have the option to set a max power envelope for the drive such that the drive will maximize the performance it can get within that power envelope. One of the key levers in that feature is controlling how many simultaneously active die are used at one time. This feature would be set at the factory, but not controllable in the field. Therefore the power spec of the drive can be set at the factory. If the SSD manufacturer chooses not to enable this feature, the drive will always maximize the performance with the maximum fully active simultaneously active die."
re different types of supercap architecture

Here's a comment from Woody Hutsell - "From a reliability point of view, you can start to see some architectures in systems that use distributed super-capacitors and others that are using centralized redundant batteries. I tend to prefer the centralized redundant backup power myself. This approach allows system designers to more carefully provide redundancy."
SSD caching software has to be power crash aware too

In June 2011 - I asked Ted Sanford, founder/CEO of - FlashSoft - a leading company in the SSD caching software market - what are the steps taken to protect the state of the cached data and update the external storage in the case of sudden power loss?

He said - "FlashSoft employs a method called multi-level metadata management, which stores some cache metadata in RAM, but most of it on the SSD itself (and employs a balanced tree design for optimal efficiency). There are two benefits to this design: first, it minimizes utilization of server memory. Only the hottest metadata runs in server memory. The rest is cached in SSD. Also, the application regularly creates snapshots of the metadata on the SSD, so that in the event of a server crash, the cache metadata can be re-created from the snapshots + most recent metadata almost immediately. Typical recovery is less than a second. (Keep in mind, our team's background is at Veritas, Oracle, Symantec, etc. so data recovery is a top priority for the product design.)"
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cold boot times for Kaminario's K2

In March 2011 I spoke to Kaminario's CEO Dani Golan about their new K2 (a rackmount RAM SSD which internally uses battery backed RAM and hard disk backup).

He said he had read my recent article about SSD power down management (which you're reading now). So while we were on that subject I asked...

How long does it take to rebuild data onto a new blade's hard drive?, and

How long does it take to boot up a new K2 from cold assuming a flat / failed battery hsystem?

Dani Golan said a couple of minutes for a single blade's HDD rebuild and about 20 minutes for a cold boot from a battery failed systems, resepectively. He said he thought the latter would be very rare event.
the mysterious case of the silent lightning strike

Identifying the Source of a Power Surge - is an interesting article by LWG Consulting which discusses the differences in the damage to data storage systems caused by natural and artificial causes such as lightning surges, power grid switching faults and equipment failures.

This type of forensic detection is where Sherlock Holmes partners with a data recovery version of Dr Watson in civil and criminal legal cases. Most of the experience in this market relates to hard drives - but SSDs will come under the scrutiny of the magnifying glass in greater numbers too. Maybe even weaknesses in SSD design....
3 scenarios of flash data vulnerability at power voltage collapse
A blog by Virtium - SSD Protection Against Data Corruption - (May 2015) outlines their thinking about protecting data integrity in industrial SSDs from power loss events.

The author - Tony Pond, Director of Marcomms - identifies 3 scenarios for data corruption:-

1 - Power fail during a write, but before the SSD has acknowledged receipt of data.

2 - Power fail after the SSD acknowledges that it has data but before data has been committed to NAND flash.

3 - Power fail after the SSD has data in NAND but before it has been committed to the correct logical block address (LBA).

How does the company design around these exigencies? the article
hold up capacitors in 2.5" military SSDs

to be or not to be?
zero to three seconds are 2 numbers which demonstrate some of the extreme diversity in SSD design. My examples here being the hold up times inside 2 current models of 2.5" SATA SSDs designed for the military market.
  • One from Microsemi (HQ in Aliso Viejo, CA, USA).
  • And the other is from Solidata (HQ in Shenzhen, China).
I've touched on this kind of architectural design difference many times before in earlier articles. But every time I revisit this vast topic with fresh examples - I learn something new. more
how InnoDisk survives abnormal power events
Editor:- September 18, 2013 - Adding to the growing body of articles about SSD data integrity in the event of sudden power loss - InnoDisk today launched a new SSD white paper (pdf) which outlines how its Power Secure Technology copes with abnormal power failure - including inadvertent disengagement of a live drive.
SSD power failure data protection system -Innodisk white paper
A key assumption in InnoDisk's design is that some data corruption is inevitable at the point when power is interrupted - despite the best efforts of the hold up capacitors etc - because other parts of the system - outside this power protected zone are also disturbed. So their algorithms - on power up - begin by looking for such errors and data inconsistencies and proceed to clean up and rebuild the mapping tables. the article (pdf)
now where was I - before I was so rudely interrupted?
Editor:- February 21, 2013 - WD has recently published a new white paper - the Art of SSD Power Fail Protection (pdf)

If you've read up on the subject of Surviving SSD sudden power loss you may already be aware that the WD team has been working on this theme for over 9 years - and even promoted educational whitepapers on this subject using banner ads in 2005.

In 2004 I was told that getting the SSD data integrity to work reliably even when the SSD is subject to unexpected rapid power rail disturbances was one of the starting points of the original SiliconDrive designers - due to one of the founders having had a bad experience with an earlier prototype flash drive failing such a test at an oem presentation while at another company.

So what can WD tell us about this subject that's new?

Well - without mentioning names - there have been many examples of other SSD companies who have got this factor wrong - and some of the reasons why simplistic power protection schemes fail are mentioned in this paper.

The key to validating a reliable SSD design is testing:- with variable types of applied power line disruptions which are applied at any time in the SSD software. WD aren't going to reveal all their hard won patented design secrets in this white paper - but you can learn a lot from it which may help you better evaluate other products too. the article (pdf)

Like good software a well designed custom SSD can greatly benefit from the analysis of expensive functions which can be reduced in scale or avoided.

The power fail detect and memory write completion protection circuits is an example of something which if implemented at a system collaborative level may help to reduce the costs loaded into each SSD.
some thoughts about SSD customization

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 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? 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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