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TLC flash storage: What it is and what it's good for

We run the rule over triple-level cell (TLC) flash: What it is, how it performs, what it's good for, why it's a competitor to disk, and how it's all been made possible

Dell has become the first supplier to bring storage arrays to market with triple-level cell (TLC) flash on-board. In its SC series of storage arrays you can now populate the hardware with single-level cell (SLC), multi-level cell (MLC) and TLC flash, as well as good old spinning disk.

But what is TLC flash and what is it good for? 

TLC flash packs more data into the same footprint as existing forms of flash chip and in so doing brings down the cost to near disk levels. Dell claims it can offer TLC flash at a cost of $1.66 per GB, putting it in the same price ball park as 15,000rpm hard-disk drives (HDDs) in terms of cost.

But Dell says its TLC – supplied by Samsung – offers 24 times better performance than 15,000rpm spinning disk HDDs. The spec on the Samsung TLC drives has them providing random read input/output per second (IOPS) of 180,000 on a 4k block.

And that's for one drive; add a number of them together, with data striped across them, and you get the type of IOPS you'd expect from flash. But that's read performance.

Random writes are logged at 15,000 IOPS per 4k block, so quite considerably less, and a much greater differential than between reads and writes on MLC and SLC flash.

That means TLC flash is best suited to read-heavy workloads and is a good replacement – offering much better performance than 15,0000rpm HDDs.

Chips piled up to boost endurance 

Samsung guarantees its PM1633 TLC drives for five years and Dell has offered free replacement for failed TLC drives for arrays in support. This kind of confidence comes in part from the manufacturing process used in these chips. This has seen Samsung back up from the smallest possible – sub 10nm (nanonmetre) – fabrication process to 30nm to make their TLC drives more robust.

The reason behind this is that TLC adds complexity – more electrical switching per flash cell – compared with MLC, and so creates more onerous requirements in charge detection and greater scope for interference and failure of the microscopic cells, especially as things get smaller.

Read more about flash development

  • Flash storage 101: Why flash writes are troublesome, why endurance is limited and what suppliers are doing to overcome these issues
  • Flash storage has taken the enterprise by storm, but its days are numbered due to an unfavourable combination of technological obstacles and manufacturing economics

So, Samsung eschewed the smallest possible die size for 30nm and stacked dies on top of each other, producing so-called 3D NAND TLC. 

The fundamentals of flash are the use of electrical cells switched between 1s and 0s to hold data. In an SLC there is just one 1/0 switch per cell, so one bit. In MLC there are two sets of 1s and 0s providing four possible binary combinations. In TLC there are three 1/0 switches and eight possible bits.

As mentioned, it's that number of switching possibilities that leads to TLC running into physical barriers at small die sizes and issues with endurance as well as write performance compared with MLC and SLC. But by backing off on the race to ever-smaller chips and stacking bigger dies on top of each other Samsung has sidestepped some of those issues for now. Dell's the first but it won't be the last with TLC

Kaminario also recently said it will offer TLC-equipped flash arrays soon and other storage suppliers will certainly follow. There's no good reason why most couldn't adopt it fairly rapidly given all three silicon makers have TLC products available or in development.

Read more on Flash storage and solid-state drives (SSDs)

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