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RAM Cache Ratios in flash SSDs

The new skinny on flash SSDs... - by Zsolt Kerekes, editor StorageSearch.com - July 2009

This article proposes a simple new terminology for grouping flash SSDs with respect to the relative size of the internal RAM cache. These are:-
  • Fat
  • Regular, and
  • Skinny
Understanding which type your SSD belongs to can be helpful, because each set has common modalities. This means you can re-use knowledge and models learned from one SSD device and apply that view to other SSDs which have a similar architecture.

Thinking about flash SSDs in this way makes it easier to group and sort SSDs you may be working with relative to the macro market situation. For any particular product - the new terminology provides a shorthand clue about
  • what behavior to expect,
  • other similar devices you should be comparing it with,
  • characteristics and limitations - and
  • the risk / benefit profile suitability of the SSD for your chosen application.
If you don't consciously go through this tick list / thought process when comparing flash SSDs (even when they have the same interface, form factor and flash memory type) - the results will be like comparing apples and oranges. And you will not get the optimum results from your invested efforts.

Categorizing the RAM cache architecture is essential because it affects a lot of parameters including:-
  • the way you should design the benchmark test to assess long term performance to avoid halo effects
  • power consumption - bigger internal RAM means more watts consumed and higher running temperature
  • reliability - increasing the RAM size impacts reliability in many ways
    • more components to go wrong - which impacts MTBF
    • can attenuate flash write cycles and improve endurance (using a simpler implementation of SSD controller)
    • increases the holdup time required for the internal UPS or capacitor to flush the cache - which increases criticality of the UPS
    • more software threads define the state of the SSD - so more intensive power cycle testing is required to confirm that the firmware design has been validated correctly
  • price - there are complex alternatives to be balanced here
    • bigger RAM cache increases the SSD cost (due to direct cost of RAM and UPS) but
    • smaller RAM cache may mean a faster (more expensive) SSD controller is required to achieve the same performance (compared to using more RAM and a slower internal micro-controller)
  • performance - generally you would expect that increasing the size of the RAM cache would always be the best way to increase the performance of the flash SSD - but above a certain point in any design - another option is to increase the speed of the SSD controller instead (or perform more of its functions in hardware rather than micro-code)
It's important to realize that the RAM cache is not the only cache technology available to the SSD designer. Some manufacturers stream write data to pre-erased flash blocks before doing much processing on them in RAM. SanDisk refers to their way of doing this as "nCache".

Generally the use of flash cache provides a simple and cheap way of implementing high R/W throughput (MB/s) for sequential streaming data - but suffers from the disadvantage of not being intrinsically as good as RAM when it comes to small random IOPS. Some designers get around the random IOPS performance problem in a flash cache environment by using hardware based mapping and translation schemes - but these are not as yet common in the market.

For simplicity it's useful to classify RAM cache types within flash SSDs into the following 3 types.
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New flash SSD terminology proposed by StorageSearch.com - July 28, 2009
Fat flash SSD

> 10,000 ppm
A flash SSD in which the ratio of RAM cache to flash array is at least 10x higher than that in (more common) regular flash SSDs.

The first Fat flash SSD in rackmount form factor was the RamSan-500 launched in September 2007 by Texas Memory Systems. RAM cache size was 64GB compared to SLC flash SSD capacity of 2TB - giving a RAM/flash ratio of 32,000 ppm ).

In the 2.5" form factor - an early example of fat flash SSD was the Platinum M-Cell SSD from DTS which (at launch time in June 2009) included 1GB internal RAM cache in an SLC flash SSD with 64GB capacity - giving a RAM/flash ratio of 16,000 ppm.
Regular flash SSD

around 1,000 ppm

(typically 500 to 2,000 ppm)
This is the design choice used in most fast flash SSDs currently in the market which include a RAM cache.

It's a self referential definition. It's a market efficient compromize which results in competitively attractive products. It makes the SSD controller much easier to design than in the "skinny RAM" category below. And it doesn't push up the design costs like the "fat RAM" designs above. The regular RAM cache size suits a wide range of flash SSD applications - those in which neither performance nor reliability are being pushed to the design limits.

If we look at 2.5" flash SSDs - the most popular products from most manufacturers typically have an internal RAM/flash ratio in the range 500ppm to 2,000ppm.

Manufacturers in this category include:- Corsair (Performance Series), Memoright (GT series), OCZ (Summit Series), STEC (Zeus IOPS) and Super Talent (UltraDrive ME).
Skinny flash SSD

< 100 ppm
These are designs which include miniscule amounts of RAM - orders of magnitude smaller than Regular flash SSDs.

In extreme cases there may not even be any external RAM chips at all. The RAM being implemented as part of the real-estate inside the SSD controller.

In the current market some extreme skinny flash SSDs have a RAM/flash ratio below 1ppm.

Advocates of skinny flash SSD architecture cite reliability as the main driving force. Tossing out the RAM reduces chip count and can help towards lower power consumption.

Companies who currently market all their SSDs in this category include SandForce (SF-1000) and WD Solid State Storage (SiliconDrive III), and Virident Systems (FlashMAX).

Foremay's SC199 Cheetah V-series (2.5" SSD) is skinny with no RAM cache - whereas their U-series (1.8" SSD) is regular with 64MB cache.

Getting good performance in skinny RAM architecture requires a very well designed SSD controller and a deep commitment at the outset that you are going to stick with this method. (And not bail out when you hit a hard design problem.) Instead of having the luxury of using RAM to cache large blocks of write data - you have to cache write data to flash. Which requires good garbage collection and pre-erased block availability. The nv RAM is restricted to storing the state of the machine and tables which may only be tens of kilobytes in size.

This is the RISC approach to flash SSD design. The battery / capacitor holdup time (when power fails) can be a low multiple of a single flash write-erase cycle. Another advantage of the skinny approach is faster power up ready time - when power is restored. This is a parameter which is not defined in most datasheets - but can be surprisingly long in some products.
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Notes:- the ratio of RAM to flash in a flash SSD is always a fraction. To simplify comparisons betweeen these fractions I'm using the form "integer ppm" - where

integer = RAM capacity divided by flash capacity x 1 million (parts per million)

As an example the RAM/flash ratio for a product with 256MB RAM and 256GB flash is 1,000 ppm.

A product with a RAM/flash ratio of 1 million is actually a RAM-SSD with internal flash backup.

RAM/flash Ratios will Vary Over Time

Don't take the exact numeric examples above as gospel. The exact ratio of RAM/flash cache for each classification of SSD will vary according to market conditions - and the relative prices of RAM versus flash memory. But that still provides a convenient frame of reference and mental bin for sorting those with massively more RAM, and those with massively less than the most common (regular) models.

What About MRAM? PRAM? etc?

I've used the term "RAM" above as a catch-all phrase which includes DRAM, SRAM and true nv RAMs such as MRAM. I don't think it's likely that flash SSDs using a 3 tiered cache (true nv RAM, battery / capacitor backed RAM, and flash) will become widely established. So the fat, regular, skinny appellations can be used regardless of the RAM cache type.

What's the Way Ahead?

It's hard to predict in advance how useful new jargon will be. The market is always the deciding factor in jargon adoption - just as with products.

3 years ago (in 2006) when I suggested to flash SSD oems the importance of a creating new standard way to refer to flash SSD life - related to endurance - that didn't work. But there was a real need in the market - and my educational article on SSD Myths and Legends - "write endurance" - originally aimed at a niche market - has been seen by nearly 0.5 million readers.

Since then users have had to learn a great deal more about SSD jargon to sort out the wheat from the chafe.

I plan to start using the new labels in editorial August 1, 2009. As always - feedback from vendors and readers will be used to update these terms and this article from time to time.


3rd party citations for these new terms:-
  • Fast Eddy Felson - from the blog - Dr. Codd Was Right - "musings on the primacy of the relational model, database and silliness of all things xml"

later articles on a similar theme:-

Why size matters in SSD architecture - the memory chip count ceiling at which the SSD controller is optimized - predetermines cost, performance and reliability

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

an introduction to enterprise SSD silos - 7 ways to classify where all SSDs will fit in the pure SSD datacenter

relative SSD capacity ratios in the server, SAN and archive - market data from HPCs indicates how much SSD you should put in the server compared to the SAN - but an entirely new set of assumptions may be more appropriate when it comes to figuring out the size of solid state storage deployed for backup.

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