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Reads and Writes: A Power Perspective

Watt-ergate: The SSD Power Consumption Conspiracy Exposed
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Test after test will show a given SSD using a different amount of power performing read operations vs. writes. Sometimes that difference can be significant, which is definitely something to be aware of before you hand over the corporate credit card. A drive could be incredibly power-efficient with reads but falter when it comes to writes. Are you going to want a drive like that in your business if your business deals primarily with write-intensive applications, such as online transaction processing or financial trading?

Write operations, on the whole, require more power than reads. This is because reads require one operation, detecting a NAND cell’s charge state, while several operations are involved in performing a write. Jeff Nowitzke, product applications engineer at Seagate, explains:

“Writes take more power because there are charge pumps within the flash that have to be used in order to erase and program at a higher voltage level than for reads,” he says. “That increases the current draw on writes. In addition, to keep your write performance up as high as possible, you have to potentially do three writes in a random situation for every one write from the host.”

As you might know, an SSD can’t simply overwrite the data written to one of its NAND cells. First, it has to erase the cell before it can write new data into it. And as SSDs have become more advanced, incorporating technologies like garbage collection and wear leveling, power use goes up. Other factors can also boost power use, such as write amplification, wherein a logical write operation at the host controller level turns into multiple operations in the media.

Many SSD vendors aren’t taking write amplification lying down, though. Modern SSD controllers from LSI attempt to precompress data, which can reduce write amplification levels below 1x. (Traditionally, a write amplification of 1x was ideal. A single write operation at the interface was a single write operation in the media.) Whether an SSD can precompress data depends entirely on the SSD’s controller and the workload itself, but the method will reduce the number of writes and thus the SSD’s power consumption.