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flash SSD capacity - the iceberg syndrome

by Zsolt Kerekes, editor - May 2011
when comparing flash SSDs - capacity is not always equal!
  • the flash capacity shown on your invoice
  • the flash capacity accessible to your apps
  • the flash capacity inside the SSD
These differences tell you a lot about your SSD.

Although few of us will ever get close enough to a real iceberg to worry about being the next Titanic - it's nevertheless widely known that 9/10 of the mass of these floating mountains is under the surface of the water.

Have you ever wondered about the flash memory inside an SSD - and how the advertised storage capacity - which is shown on your invoice - may be considerably different to the capacity that's inside the SSD?

There once was a time when this difference was small - in the range 3 to 5 percent. But with the declining cost of memory the clever people who design flash SSDs have become accustomed to push the boundaries of performance and reliability by leveraging excess flash capacity.

The 3 most popular techniques are
  • overprovisioning - to ensure an abundant supply of pre-erased blocks which can be written to almost immediately
  • RAID - like redundancy (called by various trademark names like RAIC, RAISE etc) - to ensure that data in the SSD can survive the loss of individual blocks or even the loss of entire flash chips.
  • spare blocks - used to replace bad memory blocks which arise out of infant process defects and long term wear-out.
Overprovisioning for the purposes of getting high write IOPS performance has a visible and an invisible side to it too.

In some designs the excess capacity for overprovisioning is manufactured invisibly inside the SSD by compression techniques. Compression can also increase the write throughput. Examples of companies which do this include:- EasyCo and SandForce. SandForce says that overprovisioning by adding additional flash chips means there are more chips to go wrong and implies it's a bad thing to do.

In stark contrast - Fusion-io explicitly enables its oem partners and systems integrators to select precisely what percentage of its SSDs is deployed for overprovisioning. Typically done by a low level format the realistic range can be from 20% to over 50%. Fusion-io's view about reliability is that because their SSDs don't need an additional processor (the controller work is done by the host CPU) reliability is improved in that way. (Reliability is just one of the many SSD heresies you can read about elsewhere.)

Meanwhile Texas Memory Systems provides a guaranteed 30% overprovisioning inside its enterprise SLC flash SSDs. A blog by the company explains why they think 30% is a particularly optimal number. BTW this is memory which is additional to the capacity which the customer sees on their invoice. That's an additional 3 terabytes in the RamSan-630 - and upto 150GB on the company's RamSan-20 PCIe card.

All 2.5" SSDs and 3.5" SSDs which were in production upto the middle of 2012 used small SSD architecture controllers - which is inherently less efficient in its use of capacity than large controller designs.The industry's first large controller designs in 2.5" form factors were shown in prototype models by Fusion-io (2.5" PCIe) and beta models by Bitmicro (SAS SSDs). The combination of architecture and adaptive R/W DSP IP means that the differences between the best and worst 2.5" SSDs in usable to raw flash capacity ratios will be much bigger in 2013 than before.

Going back to classical SSD architecture for now- in the small SSD architecture 2.5" Nitro N2 launched by pureSilicon in January 2012 - the usable capacity was 1.6TB compared to 2TB of raw flash inside.

When it comes to RAID-like approaches the percentage of an SSD's internal flash storage which is lost due to RAID techniques varies from manufacturer to manufacturer. In some markets the impact is greater than others. That's why SandForce enables oems to turn-off this feature in the SF-2200 - for cost sensitive consumer markets because it frees up the capacity of an entire flash chip in an entry level 8 chip array to give 14% extra capacity.

As you can see from this briefing note the headline capacity of a flash SSD isn't the same as the capacity installed inside the SSD.

The difference - which can be double digit percentage points - is something else to think about when you're shopping around for the cheapest product - and is just one of the many reasons that simply comparing the price per gigabyte doesn't tell you the whole story.

See also:- SSD efficiency (in design and business), and adaptive R/W flash IP.
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Over-provisioning flash capacity in SSDs - article by LSI
Editor:- January 8, 2013 - Understanding SSD over-provisioning - is the title of a new article published in EDN and written by Kent Smith, Sr. Director of Product Marketing at the SSD controller part of LSI.

Kent's article describes the trade-offs between performance, the percentage of over-provisioned flash capacity and the useful impact of compressible data - which inside SandForce controllers is leveraged to create additional over-provisioning.

The interaction between write amplification counter-measures and the benefits of using TRIM commands on performance are also noted. the article

Editor's comments:- there wasn't anything new for me in this article - which covers similar ground to my 2011 article - flash SSD capacity - the iceberg syndrome - which shows how SSD makers leverage capacity to tweak reliability and performance.

But - having said that - I learned about over-provisioning by 10 years of talking about it - with many SSD companies. And some of the things I put in my own article had been gleaned from past conversations with Kent Smith himself when he was at SandForce - as well as various other people in Violin, Texas Memory Systems and Adtron.

I'm guessing that what Kent would have liked to say on OP may have been "trimmed" by a word count limit in his latest EDN article.

So here are some other suggestions for more substantial and ideas packed articles I recommend - which Kent Smith has written in the past for other publications, and which cover SSD controllers from other angles:-

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On what basis are users going to compare value for money when buying new enterprise flash storage arrays and hybrids? SSD marketers don't understand the technology in their products. User needs keep changing. Most people aren't confident that what they can measure and model today will predict the results they'll see in the upwardly scaled application experience.
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