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by Zsolt Kerekes,
editor - StorageSearch.com
SSD history
storage reliability
sizing SSD
controller architecture
how fast can your SSD
run backwards?
selective memories
from 40 years of thinking about endurance
how naughty flash
was sweetened for the enterprise - timeline from SLC to QLC
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SSD endurance myths and legends
The forever
war of flash SSD endurance has changed in character from its original simple
aims of making endurance as good "as possible" - which was to
avoid burnout and catastrophic failure from excessive writes associated with
high performance and write amplification (see more about this in
SSD jargon) or high
endurance could be needed to provide long service life - typically 7 years or
more - when the SSDs were used in equipment in embedded markets which due
to electrical power and physical size constraints traditionally (although
not
necessarily in future) depended on the reliability of solo SSDs (instead
of fault tolerant arrays - such as exemplified by
RAID systems) towards
a new value adjusted mission statement for endurance which is judged by
analyzing the
price points and
cost-benefits for various flavors of
DWPD (drive writes per
day).
In short - the new business related ambition is endurance
which is "good enough" but not over specified having regard
for the intended application roles of each new SSD.
Sounds simple
enough? - except when you see that exactly the same endurance figures can be
obtained at the global SSD level using different permutations of memory
geometries (nanometer line widths) and different coding densities (SLC, MLC,
TLC, QLC - and pSLC). So that's why SSD endurance (and its role on price and
reliability)
remains a complex topic which is much studied by SSD vendors and their
customers.
And you'll see different optimization techniques for
endurance which leverage nuances of knowledge about the operating environment in
similar systems depending on whether the SSDs are standard or
custom designed for
that application.
It's over
10 years
since StorageSearch.com
began to focus on the special challenges that flash wear-out posed for
designers of
SSDs - as
the requirements for high
performance SSDs began to push up against the endurance limits of
flash memory and move
into enterprise
acceleration roles which had previously been the exclusive domain of
RAM SSDs.
Even in
its infancy - endurance management was a complicated technical subject - but
if we look back from the perspective from the ultra-complexity of today - it was
much easier to manage and understand.
- Commodity SLC was rated at 100,000 write cycles (which seems
astonishingly high by today's standards).
- Also the fastest flash SSD throughput and IOPS rates were 10x, and 100x
slower respectively than typical products today.
Nevertheless the
risks of wear-out - even in the simpler days of SSD yore - were very real. And
I've heard of many enterprise users who experienced these failures.
Since
then - as flash memory cell sizes have shrunk to deliver ever cheaper flash
capacity (more gigabytes per square inch on the chip) the raw endurance figures
in each new generation of memory have got worse. |
| .. |
where did we get to with endurance? -
in 2016
Today's commodity 2D MLC flash has raw wear-out
in the 2,000 to 3,000 write cycle range. (Later - a news story in
March 2016
suggested that 2D QLC (x4 nand - which has double the virtual density of TLC)
will have endurance in the range of 500 write cycles.)
Pioneers of 3D
flash SSD design say that raw 3D nand flash endurance is better.
How
much better?
3x to 4x better than 2D at the same line
geometries - Dave
Merry founder of
industrial SSD
company FMJ Storage
told me in March 2014 - based on his early access characterization research.
Part of 3D nand's better endurance is due to more expensive
substrate (insulating) materials. But another factor -
explained
by Samsung in
2015 - is that the different design of charge trap (compared to floating point)
works with a lower write pulse voltage.
For a long time it had been
thought that the future direction of endurance (with successive cell geometry
shrinks) would be downwards (towards worse). This was set against the
growing
IOPS demands
from SSD
architects which were getting higher and higher.
The faster
the SSD, the quicker it can wear out the memory.
This is what
created the pressure cooker environment for ever more devious
flash controller
management schemes and clever SSD architecture.
The risk of flash
wear-out in SSDs is a kind of forever war - which is never really permanently
won.
That's why articles about flash SSD endurance remain so
popular .
But endurance was a bit more complicated than that
All
the figures you see above for "endurance" are based on classical wear
leveling techniques using a single memory manufacturer mandated master set of
specifications for the shape, height and width of write programming pulse.
In
2012 we started to see endurance stretching effects from over 10 competing
companies using different variations of
adaptive R/W
and DSP ECC techniques in their controllers.
But in some key
embedded markets (such as the
industrial and
military markets)
those techniques were rarely if ever used in SSDs due to the added complexity,
latency variability and increased power consumption required by faster
SSD processors.
So
- until about the middle of 2015 - it was still generally agreed that if you
used classical (non adaptive, non DSP) controllers then the endurance estimates
you needed to take into account were still the figures supplied by the
semiconductor memory makers.
Then a new company came along - whose
founders had been researching a different way of characterising flash
endurance for over a decade.
That company
NVMdurance had
productized an entirely new way of working with flash - which could stretch the
endurance of flash in an SSD by a significant factor irrespective of whether
the controller used classical ECC or the new DSP type of ECC.
NVMdurance
uses a multi-stage life cycle model for flash coupled with "brute force"
raw computational research which discovers the best magic numbers for any type
of flash using a small sample of about 100 real chips but then simulates
behavior inside the SSD for millions of different predicted devices.
The
consequence is that for every type of MLC or TLC flash memory - whether it's 2D
or 3D - there now exist 2 different endurance ratings:-
- the raw native endurance - which comes from classical approaches and the
text books
- the new metascale advised, life-cycle fitted, virtually hardened flash
endurance - which you can get by using NVMdurance's magic numbers alongside
their own lightweight firmware which operates agnostically with either classical
or DSP controller approaches.
The result is that flash endurance (for
all types of flash) can now be upto 10x better!
The NVMdurance approach
in one sense is evolutionary (because some companies have got similar effects
before in their own SSDs - when they employed their rare flash engineering
talent to optimize endurance parameters in conjunction with DSP in particular
memory generations and product lines).
But the NVMdurance approach is
revolutionary in that the machine based characterization approach is
automatically scalable to produce optimum results for more flash geometries
and architectures than hand tuned methods, and the business model makes it
accessible to a much wider range of end markets for SSDs.
Here are some
articles which discuss how SSD companies are dealing with these challenges.
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and here's what I said about SSD
endurance before....
SSD endurance - should you worry? - and why?
Flash wear out still presents a challenge to designers
of high
IOPS flash
SSDs as the intrinsic effects at the cell level get worse with each new chip
generation.
That's in contrast to
RAM SSDs - where as
long as enterprise users remember to replace their batteries periodically - the
memory life is more dependent on elapsed time (classic bathtub
reliability curve)
and heat stresses rather than directly related to the number of R/W cycles.
Higher
SSD capacity, and
faster speeds come
from progressively smaller cell geometries - which we used to call shrinks.
In flash memory
small
size means less trapped charge holding the stored data values and greater
sensitivity to charge leakage, charge dumping and
disturbance effects
from the normal processes which happen around the cell vicinity during R/W,
powering up, powering
down etc.
If you're a
consumer you don't
have to worry about the internals of endurance management - because most
new SSDs are good enough (if they're used in the right applications
environment).
Exceptions still do occur, however for users in
the enterprise SSD market - where I still hear stories of users
thinking it's perfectly normal and economic to replace burned out
Intel SSDs every 6 to 12
months - instead of buying more
reliable (but more
expensive) SSDs -
from companies like STEC.
But
if you're a systems designer it's useful to know that the longevity
difference between "good enough" and the best endurance architecture
schemes can still be 2x, 3x or 100x - even when using
the same memory.
In
2011 -
new evidence started
coming in from longtitudinal flash SSD research done by
STEC that old, heavily
written MLC cells - managed by traditional endurance schemes - tend to get
slower as they get older - due to higher retry rates on reads - even though
the blocks are still reported by SMART logs as "good" - and the
writes do eventually succeed on retry.
In the same year - a paper by
InnoDisk
confirmed that whereas
SLC and MLC
memories have often had endurance populations within each chip which
were mostly much better than guaranteed (something which SSD makers had been
telling me since 2004) - the headroom / margin of goodness - in newer
types of MLC is lower than in the previous MLC generations. That's why
controllers which
used to work well with vintage MLC need something much stronger than a tweak to
deliver well behaved SSDs when co-starring with the new brat generation of
naughty flash.
That's what started the industry trends towards
designing a different type of flash management scheme -
adaptive R/W
- in which the goodness of cell blocks within the SSD are measured and
calibrated - and then different schemes of write pulse length and different
strengths of ECC
codes (including DSP - digital signal processing to remove "noise")
are applied within the same SSD.
These characteristics are
re-evaluated regularly according to error rates - but also according to the age
of the SSD and the write counts in the blocks. One of the ideas of the "age"
factor being - that using lower power write pulses at the start of SSD life
(along with stronger codes) reduces the damage done to the flash material -
which means that heavier pulses carrying more charge can be reserved for later
years of use - when the cell quality declines due to wear out effects. | |
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This, below, is the original text of my
SSD endurance article published in
March 2007
Does
the fatal gene of "write
endurance" built into
flash SSDs prevent
their deployment in intensive
server acceleration
applications? It was certainly true as little as a few years ago (2005).
What's the risk with today's devices?
Flash based
solid state
disks would seem to be the ideal virtual storage device...
In
every other respect you can treat them in exactly the same way as a
hard drive:- same
interface, same software model. They even fit mechanically into the same
standard hard drive slots. And in many ways they are better - significantly
faster, consuming less electric power and more tolerant of ambient temperature
and vibration extremes. You mostly don't need to know about what's inside them.
They are the perfect "fit and forget" storage product.
In
the smaller form factors like 1.8"
and 2.5" - the gap
in capacity between SSDs and hard drives has disappeared. If it wasn't for the
price you'd use
them - right? (The user value propositions - explaining why SSDs can be
significantly cheaper to buy and own in a wide variety of applications are
discussed in
another
article.)
What's wrong with this utopian vision?
And
why is it that even if you were offered a flash SSD accelerator for your server
absolutely FREE you might still hesitate about installing it?
The
answer explains why the flash SSD server acceleration market still isn't a
billion dollar plus market - even 4 years after I
first posed this exact same
question.
When you look in more detail at flash SSDs there is just
one skinny dark stormcrow hanging around the edge of this picture which makes
you feel uneasy about a technology which in other respects is acquiring an
untarnished reputation. That's the prickly issue of write endurance.
Write
Endurance: - The number of write cycles to any block of flash is limited -
and once you've used up your quota for that block - that's it! The disk can
become unreliable.
In the early days of flash SSDs managing this was a real
headache for oems and users. The maximum number of write cycles to an address
block - the endurance - was initially small (about 10,000 write cycles in 1994,
rising to 100,000 in 1997). And the capacity of flash storage was small too.
So the write endurance limit was more than just a theoretical consideration. In
the worst case - you could destroy a flash SSD in less than a week! But in
those days the SSD was being designed in by electronics engineers who knew
exactly how the SSD was going to be used. If it helped solve the problem they
could even rewrite the software a different way to lessen the risk.
But
when you buy an SSD for use in a
notebook or
server - you don't write the software. You don't control the data. So how do you
know in advance if you're going to hit that brick wall?
This fear is
an issue which has slowed down the adoption of flash SSDs in commercial server
acceleration applications. Write endurance doesn't affect
RAM based SSDs - which have
until now dominated that part of the market - mainly due to their superior
speed. But the speed of flash SSDs has improved to the point where they could
replace RAM based SSDs
in many server acceleration slots at a much lower price - if it wasn't for the
worry about endurance.
Write endurance has been a FUD issue for
potential enterprise server users. They know it's lurking there - but who can
they trust to quantify the problem in their own language?
Server
makers didn't want users to know about SSDs (any type - period) during 2000 to
2006 - because more SSDs meant selling less servers. In the
2005
edition of the SSD Buyers Guide I wrote about the problem...
"One
disadvantage, compared to RAM SSDs is that flash has an intrinsic limit on the
total number of write cycles to a particular destination. The limit varies,
according to manufacturer but is over millions of cycles in the most durable
products. Internal controllers within the flash SSD manage this phenomenon and
can reallocate physical media transparently to prolong media life. In most
applications, high endurance flash SSDs can have a reliable operating life which
is typically 3 times as high as that of a hard drive. But I would hesitate about
installing a flash SSD as a server speedup in a university maths research
department, for example, or in other applications where the ratio of data writes
to data reads is unusually high."
In May 2006 I came to
the conclusion that my earlier doubts may need to be revised.
It was
clear from reader emails and negative comments about SSDs which I saw in other
publications that fear and doubt about the impact of write endurance was slowing
down adoption of flash SSDs in the server acceleration market. It was also clear
that most users didn't know how to interpret the kind of data being offered by
SSD oems - which was designed for an elite audience of electronics designers -
and not for managers of storage systems. So I contacted all flash SSD oems with
the idea of setting up a standard way of presenting endurance life expectancy
data - with a proposal which I called the "SSD Half Life." That dialog
met with some enthusiasm but there wasn't enough vendor support to take it
further. The SSD oems I talked to took reliability very seriously - but didn't
want their own proprietary reliability schemes and models swamped by a general
industry wide scheme.
The way that SSD oems deal with the management
of write endurance internally within their products varies but they all have the
common theme of scoring how many times a block of memory has been written to,
and then reallocating physical blocks to logical blocks dynamically and
transparently to spread the load across the whole disk. In a well designed flash
SSD you would have to write to the whole disk the endurance number of cycles to
be in danger.
Some manufacturers go a step further.
SiliconSystems
has a patented algorithm which delivers a lifetime which it claims is better
than simplistic wear levelling. Another manufacturer
Adtron actually has a
percentage of spare flash blocks in the SSD - which are invisible to the host
interface and don't show up as spare storage. But internally - when blocks get
close to the limit - the data is transparently switched over to the spare parts
of the disk to give an additional breathing space.
The precise numbers
are a proprietary secret but are based on analyzing the software from real
customers' SSD applications over many years. OEMs, like these, which target
high reliability applications, are also more picky about which flash chips they
use, and qualify them according to the results they see from testing.
the
Flash SSD Application from Hell* - the Rogue Data Recorder
In
most real-life applications the computer does a lot more reads from disk than
writes - and the duty cycle (that's the percentage of time that the disk is
being accessed at all) is low. But to estimate whether you should be worried
about write endurance with today's SSD technology I've chosen a worst case
example - the Rogue Data Recorder.
Real
hard disk based data
recorders from companies like
Conduant can record
data continuously in an endless loop. They are useful for a bunch of
applications such as capturing pre-trigger data in seismic events, capturing
unpredictable data for modelling and bugging phone calls. I managed a company
in the mid 80s which pushed storage technology to its limits to get wire speed
continuous recording onto disk and massive memory systems with inbuilt real-time
trigger processors, embedded workstations and array processors for various
types of industries and agencies. That was a good education for my day job now
of cutting and pasting.
Most of you wouldn't set out to design a
real-time data recorder - and if you are doing that - this article isn't going
to tell you anything you don't already know. But by looking at the worst thing
which could happen and estimating a confidence boundary from that - it can tell
you how much you need to worry.
The nightmare scenario for your new
server acceleration flash SSD is that a piece of buggy software
written by the maths department in the university or the analytics people in
your marketing department is launched on a Friday afternoon just before a
holiday weekend - and behaves like a data recorder continuously writing at
maximum speed to your disk - and goes unnoticed.
How long have you
got before the disk is trashed?
For this illustrative
calculation I'm going to pick the following parameters:- |
| Configuration:- |
a single flash SSD. (Using more
disks in an array could increase the operating life.) |
| Write
endurance rating:- |
2 million cycles. (The typical
range today for flash SSDs is from 1 to 5 million. The technology trend has been
for this to get better.
When this article was published, in March
2007, many readers pointed out the apparent discrepancy between the endurance
ratings quoted by most flash chipmakers and those quoted by high-reliability
SSD makers - using the same chips.
In many emails I explained that
such endurance ratings could be sample tested and batches selected or rejected
from devices which were nominally guaranteed for only 100,000 cycles.
In such filtered batches typically 3% of blocks in a flash SSD might only last
100,000 cycles - but over 90% would last 1 million cycles. The difference was
managed internally by the controller using a combination of
over-provisioning and
bad block
management.
Even if you don't do
incoming inspection
and testing / rejection of flash chips over 90% of memory in large arrays can
have endurance which is 5x
better than the minimum quoted figure.
Since publishing this article,
many oems - including
Micron
- have found the market demand big enough to offer "high endurance"
flash as standard products.)
AMD marketed "million cycle flash"
as early as
1998. |
| Sustained write speed:- |
80M bytes / sec (That's the
fastest for a flash SSD available today and assumes that the data is being
written in big DMA blocks.) |
| capacity:- |
64G bytes - that's about an
entry level size. (The bigger the capacity - the longer the operating life -
in the write endurance context.)
Today single flash SSDs are available
with 160G capacity in 2.5" form factor from
Adtron and 155G in a 3.5"
form factor from BiTMICRO
Networks.
Looking ahead to Q108 - 2.5" SSDs will be available
upto 412GB from BiTMICRO.
And STEC will be
shipping 512GB 3.5" SSDs. | |
To get that very high speed the process will have
to write big blocks (which also simplifies the calculation).
We assume
perfect wear leveling which means we need to fill the disk 2 million times to
get to the write endurance limit.
2 million (write endurance) x 64G
(capacity) divided by 80M bytes / sec gives the endurance limited life in
seconds.
That's a meaningless number - which needs to be divided by
seconds in an hour, hours in a day etc etc to give...
The end result
is 51 years!
But you can see how just a handful of years ago -
when write endurance was 20x less than it is today - and disk capacities were
smaller.
For real-life applications refinements are needed to
the model which take into account the ratio and interaction of write block size,
cache operation and internal flash block size. I've assumed perfect cache
operation - and sequential writes - because otherwise you don't get the maximum
write speed. Conversely if you aren't writing at the maximum speed - then the
disk will last longer. Other factors which would tend to make the disk last
longer are that in most commercial server applications such as databases - the
ratio of reads to writes is higher than 5 to 1. And as there is no wear-out or
endurance limit on read operations - the implication is to increase the
operating life by the read to write ratio.
As a sanity check - I found
some data from
Mtron (one of the few
SSD oems who do quote endurance in a way that non specialists can understand).
In the data sheet for their
32G product - which incidentally has 5 million cycles write endurance - they
quote the write endurance for the disk as "greater than 85 years assuming
100G / day erase/write cycles" - which involves overwriting the disk 3
times a day.
How to interpret these numbers?
With
current technologies write endurance is not a factor you should be worrying
about when deploying flash SSDs for server acceleration applications - even
in a university or other analytics intensive environment.
How about
RAID systems
stuffed with
flash SSDs?
The calculation above gives the worst case (shortest)
operating life based on stuffing data into a single disk at the fastest
possible speed. Having a faster interface coming into the a
box stuffed with SSDs
doesn't make the life shorter - because the data can only be striped to any
individual disk at the limiting rate for that disk.
Au contraire:- not
only can an SSD RAID array offer a multiple of a single SSD's throughput,
and IOPs, just as with hard disks but depending on the array configuration the
operating life can be multiplied as well - because not all the disks
will operate at 100% duty cycle. That means that MTBF and not write endurance
will be the limiting factors. And although oem published
MTBF data for hard
disks has been discredited recently - the MTBF data for flash SSDs has been
verified for over a decade in more discriminating applications in high
reliability embedded systems.
I've been waiting years for storage oems
to start marketing flash SSD based storage arrays - as alternatives to RAM based
systems. What's held that market back has been the looming shadow of write
endurance. That myth - that flash SSDs wear out - now belongs to the past.
...Later:-
in May 2008 - in an exclusive
interview with STORAGEsearch.com -
AMCC 3ware confirmed it is
working with leading
SSD oems to develop
products which will support the unique needs of the
flash SSD RAID
market.
* clarifying why the Rogue Data Recorder is the Worst Case
Application
I didn't need to explain this choice to those
who design SSDs, but it's clear from some comments I've seen that some readers
who don't have an electronics / semiconductor education or don't know enough
about SSD internals have queried this choice.
Why, for example, does
the data recorder example stress a flash SSD more than say continuously writing
to the same sector?
The answer is that the data recorder - by writing
to successively sectors - makes the best use of the inbuilt block erase/write
circuits and the external (to the flash memory - but still internal to the SSD)
buffer / cache. In fact it's the only way you can get anywhere close to the
headline spec data write throughput and
write IOPS.
This is because you are statistically more likely to find that
writing to different address blocks finds blocks that are ready to write.
If
you write a program which keeps rewriting data to exactly the same address
sector - all successive sector writes are delayed until the current erase /
write cycle for that part of the flash is complete. So it actually runs at the
slowest possible write speed.
If you were patient enough to
try writing a million or so times to the same logical sector - then at some
point the internal wear leveling processor would have transparently assigned it
to a different physical address in flash by then. This is invisible to you. You
think you're still writing to the same memory - but you're not. It's only the
logical address that stays the same. In fact you are stuffing data throughout
the whole physical flash disk - while operating at the slowest possible write
speed.
It will take orders of magnitude longer wearing out the memory
in this way than in the rogue data recorder example. That's because writing to
flash is not the same as
writing to RAM, and also
because writing to a flash SSD sector is not the same as writing to a block of
dumb flash memory. There are many layers of virtualization between you and the
raw memory in an SSD. If you write to a dumb flash memory chip successively to
the same location - then you can see a bad result quite quickly. But
comparing dumb flash storage to intelligent flash SSDs is like comparing the
hiss on a 33 RPM vinyl music album to that on a CD. They are quite different
products - even though they can both play same music.
...Later:- Clarifying Flash Endurance Specifications
I've
added this footnote in response to some reader emails which asked about the
variation in flash endurance specs quoted by different flash SSD oems.
Like
any semiconductor related spec (such as memory speed, or analog offset voltage
in an op-amp, or failed memory blocks in a high density RAM chip) - there's a
spread of performance which depends on the process and may vary over time in
the same wafer fab, or at the same time when chips are made in different fabs
within the same company.
A spec such as 100k or 1 million or 10 million erase-write cycles -
is a business decision made according to market conditions - which gives
generic semiconductor buyers a confidence level that if they buy 1 million
chips - then the reject rate - of those that will fail due to process
tolerances - will be acceptably low. The shape of the distribution curve may
not actually be gaussian - but there is a distribution curve in there which is
implied by the published specs.
Due to process variations between oems
(some designs will be automatically shrunk from old designs, other layout
geometries may be recompiled or optimized for that particular process point)
there will be vast differences between the endurance from different
chipmakers.
As the generic semiconductor flash market doesn't place a premium on
this spec - the "datasheet" published standard number will gradually
improve at a slow pace (every 2-3 years) even if some oems are making chips
today which are 10x better.
If I was designing a
high reliability
flash SSD - I would want to get into the process details - qualify devices and
order them to my own spec. Currently SSD volumes are too low - to give much
buying power with flash chipmakers. Therefore few SSD oems are able to buy
flash chips qualified to their own specs. (This is done by batch testing
samples and by negotiation with the fab where the chips are made.) Some SSD oems
make their own flash chips - and while this gives them more control over the end
to end process - it does not necessarily mean that they start with the best
chips.
See also:-
is eMLC the true
successor to SLC in enterprise flash SSD?- which so called "enterprise
MLC" tastes the sweetest? How come there are
so many different and
contradictory reliability claims? |
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| the forever war in
flash SSDs | |
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| In the 1970s -
endurance in EAROMs (an early semiconductor random R/W non volatile memory)
was about 1,000 write / erase cycles per cell. |
| SSD market
history | | |
| .... |
| stressy
writes help reliability assessment of 3D nand |
A research paper in August 2018
showed how deliberately wearing out a small number of blocks in 3D nand flash
(using 10K P/E cycles) can be used as a tool to help calibrate the memory.
For more about the architectural aspects see
3d
nand and new dimensions in SSD controller architecture | | |
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| One of the interesting
surprises about 3D nand flash which emerged early in 2014 was that the endurance
- reported by SSD designers - was better than you would have expected if you had
taken as your starting point - assumptions about 2D nand with similar line
geometries. |
| the unsung hero
of 3D endurance (September 2015) | | |
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| endurance
issues in flash SSD operating life |
Editor:- July 1, 2011 - In my
2007 article (lower down on the left of this page) I showed readers why flash
SSD operating life could be much greater than predicted by extrapolating flash
chip wear out - due to a blend of architecture and process choices
implemented in well designed SSDs.
Before
2007 - the
industry wisdom had been that users should not use flash SSDs in
enterprise apps.
(And in those days - we were talking about SLC flash.)
But wear-out was
then (and remains today) a reliability risk in badly designed SSDs! And I
still get a lot of reader questions about it.
For those who just need a
short summary of the issues - here it is.
- Wear leveling is more effective in some SSD designs than in
others. The effectiveness of patented active wear leveling algorithms can
vary by
more than 3 to 1.
- SSD life also depends on how write amplification
is controlled.
In some good controllers the gigabytes written to
the flash are actually
less than the gigabytes written to the SSD from the host. Conversely in
naiively designed controllers the flash media to host write ratio can be
20x higher - resulting in very short life in enterprise apps. This is
not just a theoretical issue. Real enterprise customers
have
encountered system failures when consumer SSDs were installed into heavy
duty arrays.
- All flash chips
are not the same - even from the same fab (chip factory).
All
flash cells do not have the same endurance even within the same chip.
High
reliability oems have been
sample
testing and rejecting flash chips since 2004 to my knowledge - to select
flash chips which are 5x or 10x better than the minimum quoted figures for
endurance by chipmakers. So called "enterprise grade flash" is simply
a marketing label to industrialize a process which had already been used
in the industry below the visibility radar of most storage users.
- Over-provisioning extends SSD life - because all cells in a chip do
not have the same endurance.
There's a distribution curve of
endurance within chip blocks which is a proprietary secret which can be
characterized by the SSD controller designer for the chips they support. Most
blocks are significantly better than the floor level in the same memory chip.
On the other hand - if an SSD company installs the wrong flash chips
(maybe due to market supply problems) - the SSDs may work in the short term -
but fail early. This is because the mortality rate of cells may be outside
the bands and coping strategies built in to the controller for the type of
chips it was designed to work with. But the same chips could still be usable in
an entirely different set up of SSD controller.
It's like putting
the wrong tires on your car. Everything's OK on the daily commute. You see
the difference when you hit snow or drive around a bend at 120 mph.
It's this combination of factors which can make a difference (up to 2 orders of
magnitude) between SSD life - even when using flash made by the very same chip
supplier
For related topics see
SSD controllers,
SSD reliability papers. | | |
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Leading
Data Recorder Company Comments
on the "Rogue Data Recorder"
|
Ken Owens,
CEO, of leading
data recorder company
Conduant commented.
"In
many applications that use a standard file system, the directory updates are a
major concern for using up the available flash life.
Even though
recording applications are inherently heavy on writing, the optimization of the
directory structure actually minimizes the number of times a specific location
is written thereby extending the life of the flash media.
This is on top of any wear leveling algorthms provide by the SSD
manufacturer. Conduant systems can even use Compact Flash in some recording
applications." | | |
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DRAM Memory Doesn't Wear
Out
(or so I thought for 10 years) |
In the
2004
edition of the SSD Buyer's Guide I warned users in no uncertain terms that
they couldn't use flash SSDs in enterprise server apps - because of the low
endurance of products available at that time.
The banner ad, below,
was run here in February 2004, by a company called
Computer Expertise Group
(no longer in business). CEG sold refurbished
RAM SSDs - and the
banner includes the tagline - "DRAM memory doesn't wear out".
When this ad was run, that statement wasn't an allusion to flash
wear-out (because no one in their right minds would have considered flash for
server apps at that time). Instead it suggested that whereas a refurbished
hard drive array won't last as long as a new one, a refurbished RAM SSD could
be just as good as new - and cheaper (assuming you also replaced the batteries.)
Later:-
we learned that DRAM can wear out after all.
In 2014 - The memory
industry became aware of an exploit called
row hammer. The original
research paper (below) was in the process of publication contemporaneously with
researchers in memory companies becoming aware of the problem. | | |
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| "Among 129 DRAM
modules we analyzed (comprising 972 DRAM chips), we discovered disturbance
errors in 110 modules (836 chips). In particular, all modules manufactured in
the past two years (2012 and 2013) were vulnerable, which implies that the
appearance of disturbance errors in the field is a relatively recent phenomenon
affecting more advanced generations of process technology. We show that it takes
as few as 139K reads to a DRAM address (more generally, to a DRAM row) to induce
a disturbance error." |
| Disturbance
Errors in DRAM - an Experimental Study (pdf) (2014) | | |
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SSD power down
management architectures
after SSD endurance - what next? |
Editor:- March 14, 2011 -
StorageSearch.com recently
published a new article -
SSD power is
going down! - which surveys power down management design architectures
and characteristics in SSDs.
Why should you care what happens in an
SSD when the power goes down?
This important design feature - which
barely rates a mention in most SSD datasheets and press releases - is really
important in determining
SSD data integrity
and operational
reliability.
This article (which has become unexpectedly
popular) will help you understand why some SSDs which work perfectly well in
one type of application might fail in others... even when the changes in the
operational environment appear to be negligible. |
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