An SSD crash course: What you need to know

Chances are increasing that you'll use flash memory for primary storage, but nasty surprises are possible with naive adoption

1 2 3 Page 2
Page 2 of 3

In some cases, this battery backup is a supercapacitor that can power the device for a few days; this is common in very high-performance DRAM SSDs that ship in a PCIe card factor. However, in the event that power isn't ever restored, your data probably won't be, either. In other cases, the DRAM is paired with an equal-capacity array of hard disks or slower NAND flash memory in a rack-mount chassis that is used to stage and de-stage the DRAM memory during power up and power down (with an internal bank of batteries or capacitors providing enough power to perform the de-stage operation in the event of an unexpected power outage).

NAND-based (flash) devices use the same general breed of memory found in cell phones and USB sticks. These memory devices do not require power to hold their state. Thus, they don't require a battery backup of any kind to ensure data integrity. On the other hand, they're several times slower than DRAM-based devices, though their speed is improving as the devices and their controllers mature.

MLC versus SLC SSDs

NAND devices come in two major flavors: MLC (multilevel cell) and SLC (single-level cell). MLC devices are so named because they can store a few bits of data in the same cell, whereas SLC devices can store only a single bit of data per cell. SLC devices are much more expensive to make because they require more transistors to store the same quantity of data, but they're significantly faster and have a longer lifespan than MLC devices.

Most consumer-grade SSDs, such as the one in your fancy new laptop, are likely MLC devices. In those applications, low cost, lower power usage, and higher reliability when dragged off your coffee table by your dog are key concerns, while raw performance is not. Any enterprise-grade NAND-based storage device is likely to be SLC-based -- and much more expensive as a result.

It's all about the controller

As with traditional primary storage devices, NAND flash-based SSDs live or die by the functionality delivered by their controllers. The strength of the on-device controller represents a significant cost in delivering an enterprise-grade SSD, but the controller is also responsible for providing exceptional (or less than exceptional) reliability and performance. In addition, it's where significant technology growth and innovation is still taking place.

Unlike DRAM-based SSDs, flash-based SSDs suffer from long-term reliability issues. Individual single-bit SLC SSD cells usually wear out after about 5 million write cycles. Multibit MLC cells become unreliable after just 500,000 to 1 million write cycles.

1 2 3 Page 2
Page 2 of 3