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flash SSD Jargon Explained

New Improved SSD Formula!

news flash - Fast symmetric R/W IOPS high endurance, MLC SSD, with 3 levels of wear-leveling, massive over-provisioning, write attenuation and fast garbage collection provides competitive alternative to RAM SSDs...

do you understand the list of ingredients in the solid state drive 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 constantly 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 couldn't use flash SSDs in any server apps (RAM SSDs were uniquely reliable), and then we said

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

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 right.

You can find over 100 more definitions like them simply explained in Megabyte's Storage Dictionary

...Later:- March 6, 2009 - if you know that enough readers are confident about SSD jargon - it paves the way to discuss weighty concepts.

Pliant Technology neatly brought together the themes of SSD jargon and benchmark reliability in a blog which asks - is the performance deterministic? - among many other important questions.

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.

I noticed it because it cites one of my own favorite articles - Are MLC SSDs Ever Safe in Enterprise Apps?.

...

1.0" SSDs	1.8" SSDs	2.5" SSDs	3.5" SSDs	rackmount SSDs	PCIe SSDs	SATA SSDs
SSDs all	flash SSDs	hybrid drives	flash memory	RAM SSDs	SAS SSDs	FC SSDs

Little words can have with big meanings in the world of SSDs.

They affect price, performance, reliability and - if you understood them wrongly - user happiness.

Scroll down this column to see my explanations about commonly used SSD jargon.

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"

RamSan-20 very fast PCIe SSD from Texas Memory Systems

RamSan-20 very fast 450GB PCIe SLC flash SSD
from Texas Memory Systems

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.

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.

see also:- MLC - editor mentions, Are MLC SSDs Ever Safe in Enterprise Apps?

"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

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

better reliability

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.

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"

Are MLC SSDs Ever Safe in Enterprise Apps?

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.

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.

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.

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:-

FXT Series - from Avere Systems
MaxIQ SSD - from Adaptec
XcelaSAN - from Dataram
SP5000 Application-Aware cache - from Storspeed

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 ..."

S3B is an acronym for Solid State Storage Backup - coined by StorageSearch.com.

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