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Demystifying SSD Wear Leveling
Solid state storage brings high performance to the enterprise through memory-based, zero-latent storage. It also brings with it new terms that may be confusing. One of those is wear leveling. Most solid state storage vendors have controllers on the memory that handle wear leveling for you, but the reason to understand wear leveling is so that you understand how it impacts the performance of the storage systems you are purchasing.
As I discussed in Collecting The SSD Garbage, when data is written to a flash cell, if there is old data in that cell it must be cleared out first by writing zeros to it. After a few months of use there is almost always data in every flash cell, so this double-write occurs on every write. A flash cell can be written to only so many times before it fails or wears out. As long as that wear out occurs several years after you purchased it and you have some warning that it is getting ready to fail, that is not a major problem to manage.
What you don't want is to have just a few cells in the environment receive the majority of the writes and wear out before the other flash cells. This might cause the storage to fail when most of the memory on it is still usable. Wear leveling fixes this. It makes sure that the write load is spread out as evenly as possible across all the cells in the environment. Different controllers will have different success rates in that distribution and it is something to test.
Testing how effectively a drive utilizes wear leveling is fairly easy. As my colleagues and I discussed in our recent webinar that guides you through understanding solid state "specsmanship," there is a self-monitoring, analysis, and reporting technology (SMART) statistic that will report how much life a drive has left. Using this information when comparing solid state technology will go a long way toward understanding which drives will last the longest.
It is important to note that memory wear-out concerns are unique to flash-based memory, As I discussed in a recent article. The Advantages of DRAM SSD, DRAM-based systems do not have to worry about memory wear and--under the right circumstances--that may be a compelling reason to consider DRAM-based storage. For many applications, though, the price/performance reality of flash-based storage is simply too attractive, which is why we need to know what wear leveling is and how the flash-based vendors are handling it.
The environment in which you use solid state storage will also directly impact how long it will last. Certain environments may wear through the drives much sooner than the five years that is often touted, while others may see a significantly longer life. In an upcoming entry, I will discuss how the various environments and use-cases can impact solid state.
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