What you DIDN'T know about SSDs (Part 2) - How long do MLC drives last?

In part 2 of 2, we discuss the current (2016) state of SSD reliability - how long will your drive last? I'll also explain the difference between SLC and MLC NAND (and their respective lifespans), and how a floating gate MOSFET (as used in SSDs) works.

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My notes on this topic:
- How reliable are SSDs when compared to, say, traditional hard drives? Well, it all depends on the type of flash memory used, and the choice is between SLC - often used in professional, server-grade applications - or MLC - the preferred choice for consumer drives.

- What is a flash cell? Essentially, it's a floating gate MOSFET - which you can think of as a very fancy transistor.

- In a standard MOSFET, voltage would be applied to the control gate (the top section), which allows current to flow through the channel between the source and the drain terminals - effectively completing the circuit. But when power is removed from the control gate, the charge drains from the channel and stops flowing.

- In a "floating gate" transistor, a middle insulated section is added, and its purpose is to store a charge - even AFTER power is removed from the gate. It does this because, when very high voltages are applied to the control gate, electrons have a tendency to break through the insulation and get trapped in the floating gate.

- To read the stored charge, we simply apply voltage to the control gate - if the floating gate holds a charge, then no current flows and the result is a binary 0 - the cell is programmed. If there is no charge in the floating gate, then current flows and the result is a binary 1 - the cell hasn't been programmed.

- You can see how this process might be a little bit destructive to the cell over time. This is because the oxide layer of the cell - which, for lack of a technical term, stops the stored charge from "escaping" - will be subject to degradation. The rate of erosion is different for both SLC and MLC.

- In SLC (single level cell), a single bit is stored in each cell - so each distinct voltage stored in the gate is 50% of the maximum possible range. In other words, above a specific voltage there is data in the cell - below it, there isn't.

- But traditional MLC (multi level cell) can actually accept two bits or four different voltages, which makes for far tighter tolerances. After all, the "guard band" between the states has been massively reduced, thereby giving a greater risk of corruption. But the main problem is obviously that less cells will be used to store the same amount of data, so MLC SSDs will wear out quicker.

- How quick? Well, MLC flash usually only has around 5, 000 - 10, 000 program-erase cycles per block. While that doesn't sound like much, it has been argued that the adoption of such techniques as wear-levelling and overprovisioning -- which work to ensure that read and write cycles are evenly distributed amongst all cells -- offset such a limitation.

- Indeed, a life expectancy test (conducted by The Tech Report) shows that some consumer SSDs can withstand 2 petabytes of writing!

- And in practice, I'd argue that consumer SSDs are likely to last at least as long as mechanical hard drives in most applications, especially when you consider the vulnerability of HDDs to stray magnetic fields, head crashes, or simple mechanical wear and tear.

Видео What you DIDN'T know about SSDs (Part 2) - How long do MLC drives last? канала HandyAndy Tech Tips