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SSD Jargon - Megabyte rows his boat to  a sign saying  flipper channel 3 miles

flash SSD Jargon Explained

by Zsolt Kerekes, editor - StorageSearch.com
Little words can have with big meanings in the world of SSDs.
They affect price, performance, reliability and user happiness.

This article collects together short descriptions of many commonly used SSD terms (but not all). You may also find it useful to put the words you're having trouble with into the site search box below - to get an idea of the context.
SSD Endurance - Is a term used in the context of flash memory or flash SSDs and is more verbosely called "Write Endurance."

Simple explanation... The number of write cycles which can be performed by any block of flash is limited due to physics and technology imperfections which eventually make the data storage process unreliable.

That number varies according to the type of flash (MLC is usually 10x worse than SLC) and according to the memory generation and also whether it has been designed (or selected) for high endurance.

Typical quoted values in the market today (2009) range from as low as 100,000 to over 2 million cycles.

Wear leveling and write attenuation are architectural techniques which can be used in SSD controllers to mitigate the effects of endurance - and extend usable life of a memory array by many orders of magnitude compared to the intrinsic life of a dumb flash array without such a controller.

see also:- SSD Myths and Legends - "write endurance"

flash wars in the enterprise SSD market- which so called "enterprise MLC" tastes the sweetest? How come there are so many different and contradictory reliability claims?
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SSD Garbage Collection is an important background process in flash SSD controllers.

Some editors and software vendors (who don't understand flash technology) mistakenly attribute a long term slow down in some SSDs to fragmentation - when really the issue is the ratio of resources allocated to Garbage Collection.

In well designed products which have reserved enough CPU power, internal R/W bandwidth and over-provisioning this "performance degradation" does not occur - or is minimal. For example systems from Violin Memory.

The term Garbage Collection was 1st used in 2002 - in an article we published about flash SSD reliability. Here's the definition below from that article.

The "Garbage Collection Process" eliminates the need to perform erasure of the whole block prior to every write. The "Garbage Collection process" accumulates data marked for erase as "Garbage" and perform whole block erase as space reclamation in order to reuse the block.
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MLC (Multi Level Cell) is a term used in the context of flash memory and flash SSDs to describe how the storage charge in a single floating gate transistor cell is interpreted by the logic system.

In traditional digital systems, the change in state (of a voltage, current or charge) is interpreted as being either 1 of 2 distinct levels - "0" or "1" - which is where we get the term binary logic from. Such flash cells are called SLC (Single Level Cell).

In an MLC memory chip the stored charge is interpreted as a range of values (0 to 3), (0 to 7) etc - which depends on the ability of the discrimator circuits surrounding the memory array to reliably tell the difference between levels.

The logical memory capacity of a such a cell is 2 bits, or 3 bits etc - where the bits are binary.

Discriminating multiple levels is difficult to achieve technically - because it involves an analog to digital conversion process - and due to manufacturing tolerances the same charge may not represent the identical value in another part of the chip.

There are also various factors which make the process unrepeatable - by dumping charge into the cell when adjacent parts of the chip are being written to, by leakage from the cell into the substrate over time, and by damage in the transistor material due to successive writes (endurance).

MLC designers overcome these problems (which become harder in each shrink generation) by wrapping blocks of memory in protective error correction and detection codes.

Because the charge in an MLC cell is interpreted as 2x, 4x etc more data than in the same geometry level SLC chip - MLC is much more sensitive to age and wear-out factors than SLC. That's why oems typically quote an endurance figure which is 10x lower.

Why go to all this trouble? - MLC memory provides capacity which is 2x, 4x etc lower cost than SLC. That competitive advantage is a compelling argument in many applications.

Throughput is similar for SLC and MLC SSDs. Although the extra R/W complexity in MLC is intrinsically slower than SLC - the data nibble going in or out of the cell is worth more informational bits than in SLC - which compensates.

Another refinement of MLC is x3 aka TLC (triple level cell) nand flash - which offers 8 distinct states in a single cell - equivalent to 3 binary bits of storage.

see also:- MLC - editor mentions, Are MLC SSDs Ever Safe in Enterprise Apps?
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"Silent Errors" - is a term used to describe uncorrected data errors in a flash SSD which arise from an incompletely understood, inappropriate or poorly designed data integrity architecture.

Many new products are vulnerable to these errors. For more details see the article:- Data Integrity Challenges in flash SSD Design
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SSD Over-Provisioning is a technique used in the design of some flash SSDs. By providing extra memory capacity (which the user can't access) the SSD controller can more easily create pre-erased blocks - ready to be used in the virtual pool.

2 beneficial effects of Over-Provisioning are:-
  • faster overall write IOPS, and
The latter case - is because another use of the extra capacity is to replace bad memory blocks - which occur at both ends of the bathtub curve.

There are wide variations in the percentage of flash over provisioning in different SSD designs. This is discussed in more detail in the article - flash SSD capacity - the iceberg syndrome.
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SSD Wear Leveling is a technique used inside flash SSDs to prolong the life of a flash memory array.

Countering the phenomenom called endurance - Wear Leveling processes in the SSD controller keep track of how many erase cycles have been performed on each flash block - and dynamically remap logical to physical blocks using algorithms which spread out the wear over the whole population in the array. Working hand in hand with over-provisioning - bad cells (which wear-out earlier than the median life) can be replaced - considerable extending SSD life.

There are 3 levels of wear leveling used in the best server grade SSDs - static, dynamic and active.

Rugged & Reliable Data Storage: Solid-State Flash Disks overview

Increasing Flash Solid State Disk Reliability

SSD Myths and Legends - "write endurance"

is eMLC the true successor to SLC in enterprise flash SSD?
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SSD Write Amplification - is a term popularized by SiliconSystems in various flash SSD related articles and press releases.

Gary Drossel, a VP at SiliconSystems defines it as follows. "Write Amplification - is a measure of the efficiency of the SSD controller. Write amplification defines the number of writes the controller makes to the NAND for every write from the host system. Long, continuous writes map over this mismatch, but most embedded/ enterprise applications do not stream data. Instead, they transfer data in a series of shorter, more random transactions."

It's the difference in ratio between the number of theoretical writes you think that your application does to a flash SSD - compared to what actually happens - due to OS or other software - which is often outside your control. Write Amplification can be a serious problem - because it can invalidate calculations related to endurance. SiliconSystems says - the best thing to do is measure it - rather than estimate it. Their SSDs can be used as tools to do this - because they perform real-time internal logs of write cycles.
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SSD Write Attenuation - is a term coined by the editor of STORAGEsearch.com.

It is the opposite effect of Write Amplification - and reduces the amount of writes done to the SSD compared to what you expect. This kind of out-of-sequence recognition, and reordering of packets before writes usually requires a non volatile RAM or similar memory inside the SSD controller.

Beneficial side effects of Write Attenuation are:-
  • lower wear-out of the flash SSD, and
  • (often) faster random IOPS - because the nv cache doesn't have to be flushed out as inefficiently - as in the case of unprotected RAM caches.
see also:- Z's Laws - Predicting Future Flash SSD Performance
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Skinny, Regular and Fat flash SSDs

These are new terms (July 2009) proposed in an article called - RAM Architectures in flash SSDs to describe RAM/flash ratios in flash SSDs.
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ASAPs (Auto-tuning SSD Accelerated Pools of storage )

This is a new term (November 2009) coined by StorageSearch.com to describe a product category which includes products like the following:- Although aimed at different markets, and having different interfaces, what they all have in common is their ability to self-tune.

In effect - "ASAPs eliminate waits for the SSD Hot-Shot / Hot-Spot Engineer ..."
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11 key symmetries in SSD design - defines 11 new SSD jargon terms and provides a unified overview of SSD architecture.
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Legacy SSD and New Dynasty SSD - are ways of classifying enterprise acceleration SSDs by the architecture of the storage environment they were designed to go into.

This nomenclature - was introduced in in September 2010 in this article - A new way of looking at the Enterprise SSD market.
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Flash Memory Basics - for enterprise SSD buyers
Editor:- February 3, 2010 - a new article - Flash Memory Basics - posted today by blogger Brad Diggs looks like it could be part of an educational series laying the groundwork for Sun Microsystem's PCIe SSD product family.
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storage search banner

pcie  SSDs - click to read article
PCIe & NVMe SSDs ..
high availabaility SSD arrays
fault tolerant SSDs ..
hybrid DIMMs
hybrid DIMMs ..
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Here's an example of the problem.
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Editor:- October 31, 2014 - Unified Storage...

What does that really mean?

And is SAN + NAS really enough to qualify a rackmount flash system as Unified Storage in the age of the SSD?

I don't think so.

And I got an interesting response when I said this recently to Skyera's CEO.

See SSD news for more.
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New Improved SSD Formula!
news flash:- XLC Disk has launched a new fast-enough fat cache eMLC SSD ASAP on a PCIe form factor with 200K (truly symmetric) R/W IOPS. High endurance and data integrity are achieved by using DSP adaptive flash management with 3 levels of wear-leveling despite light over-provisioning. The new SSD has industry leading write attenuation and fast garbage collection, has UBER better than 1 sector in 1019 eliminating the risk of silent errors. The no SPOF SSD has internal FT features such as RAID, plane failure management and chipkill - and can fail-over the entire flash array to an alternate PCIe host without loss of data. Designed for mission critical server apps - the SSD has strong protection against sudden power loss which doesn't rely on supercaps....
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do you understand all the ingredients in the solid state drive news headlines?

What are the factors holding back faster adoption in the SSD market?

That's been a constantly recurring topic in my discussions with SSD designers and oems in the past 10 years.

Now, if you're a manufacturer of hard disk drives you may think that the SSD market is racing along fast enough already - and doesn't need any more help from me.

But User Education has always come up as the most important SSD market accelerant.

Back in 1998 when we published the 1st real-time updated directory of SSD oems - the most important part of the education mix seemed to be - What were the benefits of application speedup - if you could afford SSDs?

Later, when I published my definition - What's a Solid State Disk? - in 2000 - I didn't think there was much more I could say on this subject.

But in the decade which followed - I told users
  • they could use some flash SSDs in server apps (if they were SLC and had high endurance and good wear-leveling), and then we said
  • they should be extremely cautious when choosing server apps for MLC SSDs. Then some types of MLC were better than others. Or exactly the same MLC - when used with different controllers was more or less reliable.
Confusing, isn't it?

Along the way users have had to learn the differences between RAM SSDs, MLC and SLC flash SSDs - and also hybrids.

In some ways this resembles consumer education about what foods to eat (or not to eat) to maintain a healthy lifestyle. But in the case of SSDs - the foods have been evolving fast and are now forming a bigger part of everyone's diet.

So you need to know more about the ingredients listed on the carton.

Even if you already know all you need to know about endurance, MLC, SLC and wear-leveling - here are 3 new concepts I think you'd benefit from following up in your future reading ...
  • SSD - Write Amplification and Attenuation
  • SSD - Garbage Collection
  • SSD - Over-Provisioning
I never thought users would need to know so much stuff about SSDs. But you do.

You can't rely on your SSD vendors to tell you. Just as you wouldn't base a healthy diet on things chosen randomly from a supermarket shelf - or buy a car just because it's got the right number of wheels. You have to choose your own risk / reward comfort zones in the SSD market too.

I've included an explanation of 7 important SSD terms in the table on the left.
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DWPD - in the SSD context is a way of rating endurance and matching application slots to the SSD.

DWPD stands for Diskful Writes Per Day and the associated assumption is that this daily usage figure is good for an operating period of 5 years.

See also:- what's the state of DWPD? - which gives examples of DWPD for various different types of enterprise SSDs from leading vendors.
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ull SSDs - ultra low latency SSDs.

In the circa 2013 period the context usually is related to Memory Channel Storage.

The idea is to implement ultra low latency flash SSDs - similar to but faster than PCIe SSDs - in modules which plug into DRAM compatible sockets.
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SSD CPU Equivalency

This term was invented by the editor in 2008 to succintly describe a concept he discovered and used in systems designs in the 1980s and which underpinned his earliest published SSD market adoption model in 2003.

It's related to SSD speedup / acceleration in servers.

For a wide range of applications (in a well designed system) if you take a black box approach and analyze the overall application performance of a computer system - you would not know from external tests whether that system had more CPUs with hard disks or less CPUs with more SSDs. That's SSD CPU Equivalency.
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FTL

Flash Translation Layer - is the standard term for lumping together the software and hardware IP which interecepts R/W requests for logical flash memory addresses made to the SSD from the apps processor and converts these into actions which do real R/W to the physical memory.

In the simpler days of SLC when flash was intrinsically more reliable - the FTL was a much simpler IP package than it is now.

The FTL - which can operate at many different levels - hides things like bad block management, wear leveling, data integrity etc so that the external software can think it's talking to clean reliable memory.
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µSSD, SATAe and SATA Express

These are next step variants of the Serial ATA (SATA) interface standard roadmap.

The new terms came into being in 2011.
  • µSSD - or more correctly "SATA µSSD" - defines the standard pins in an SSD chip (BGA form factor) which support the SATA interface - and which (in theory) makes it easier for systems designers to interchange SSDs from different suppliers.

    See also:- SSDs on a chip
  • SATAe and SATA Express - define the pins for the successor standard for SATA 3 - for a 2.5" form factor. The speedup in the SATAe generation - comes from 2 lanes of PCIe instead of doubling of the 6Gbps speed of SATA 3.

    See also:- 2.5" PCIe SSDs


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industrial mSATA SSD
industrial grade mSATA SSDs
from Cactus Technologies


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SSDserver rank
Editor:- March 4, 2014 - SSDserver rank is a latency based configuration metric - proposed as a new standard 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. ...read the article


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SSD ad - click for more info


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suggested SSD articles on StorageSearch.com
what do enterprise SSD users want?

how fast can your SSD run backwards? - 11 Key A/Symmetries in SSD design - what they are and why you need to know

Enterprise SSDs - the Survive and Thrive Guide - some simple rules to help you stay on the safer side of the tracks in this maddenly unruly market.

Where are we now with SSD software? - (And how did we get into this mess?)

adaptive R/W flash care management IP (including DSP) for SSDs - what is it? and who does it? This will be a disruptive transition.

The big market impact of SSD dark matter - As Fusion-io found out years ago, and OCZ and STEC are reportedly seeing now - some of the very biggest direct customer opportunities for SSDs aren't the big name computer and storage oems.

2013 SSD market milestones - if you're getting up to speed with the SSD market - lists the most significant products, suppliers and changes which happened last year.

enterprise SSDs - exploring the limits of the market in your head - is about enterprise SSD futurology.

Can you tell me the best way to SSD Street? - I'm like the Old Woman of the SSD Village who talks to everyone that passes through. No wonder I have a unique perspective. It would be strange if I didn't.

comparing the SSD market today to earlier tech disruptions - applying a sense of perspective to what's happening now with SSDs
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SSD news

endurance

PCIe SSDs

cost factors

FITs & SSDs

flash memory

industrial SSDs

rackmount SSDs

top SSD companies

history of SSD market

software - SSD aware

SSD lessons from 2013

10 years of enterprise flash