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

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To combat this issue, the SSD controller performs a technique called write-leveling that tries to spread writes across the cells that make up the SSD, to ensure relatively equal write-load distribution. Additionally, some SSD controllers use slack (unprovisioned) space that can take over from cells that are near the end of their expected lifetime.

Some cheaper controllers perform this write-leveling without regard to how much load the device is under, while others wait until the device is under lower load before running. This is one of many reasons that benchmarking performance on SSDs is difficult: Performance often looks stellar for a few hours until the write-leveling algorithm starts running, but then it crashes and burns.

Write-leveling can also have some unintended security side effects. Let's say you have an unencrypted file and then decide to encrypt it. As you do this, your server reads the unencrypted file from disk, encrypts it, and writes the encrypted file over the unencrypted file -- usually deleting it in the process. Due to how some write-leveling techniques work, it's possible for your server to believe that the unencrypted files have been overwritten when in fact they have not. Some controllers honor these requests and erase the blocks, while others do not.

Putting it all together

As you start digging into SSDs and deciding whether they're right for your primary storage environment, remember that they are a completely different animal than traditional spinning disks and are still undergoing growing pains. To be sure, the enormous performance potential of SSDs will ensure that they will be an option for IT for many years. Just don't charge into SSD usage without understanding how they work, so you don't have nasty surprises in your production environment.

This article, "An SSD crash course: What you need to know," originally appeared at Read more of Matt Prigge's Information Overload blog and follow the latest developments in data storage and information management at

Copyright © 2010 IDG Communications, Inc.

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