How Controllers Maximize SSD Life – Better Wear Leveling

Tempus FugitIn this post we will explore how the right wear leveling algorithm  can help a controller maximize the life of an SSD.

Wear leveling is a fact of life with NAND flash – blocks start to suffer bit failures after a certain number of erase/write cycles (usually specified from the thousands to the hundreds of thousands) and it is only natural that software will attempt to over-write some blocks more than others.  In order to prevent this from causing failures, all of today’s SSD, USB flash drive, and flash card controllers incorporate some sort of wear leveling.

This is a simple re-mapping of the contents of the flash chips.  A more graphical explanation is given in any of Objective Analysis’ SSD reports.

When the software attempts to write to a certain block, it is automatically forced to write somewhere else.  The next write is likely to be mapped somewhere yet again.  This is done in a way that is completely invisible to the rest of the world, and results in all of the blocks in the flash chip receiving roughly the same amount of wear.

Wear leveling algorithms are not all the same, though, and some work better than do others.

There are two fundamental categories of wear leveling: Static and Dynamic.  With dynamic wear leveling only two kinds of blocks participate in the wear leveling algorithm: those that undergo change, and those that are unused.  Blocks of data that don’t change (“Static” blocks) are left alone by a dynamic write leveling algorithm.

Static wear leveling manages write leveling across all of the flash in the system – the changed blocks, the unused blocks, and the static blocks.   This means that a static block that contains valid data but that is never being written to will still be moved around within the flash in order to allow all blocks to receive the same amount of wear.  It’s easy to understand why this would help, since those blocks would otherwise be unavailable to the wear leveling algorithm.  On the other hand, if you’re trying to minimize write/erase cycles, it’s unintuitive that this approach is the better of the two.

Underneath these two fundamental wear leveling algorithms lie several other strategies that The SSD Guy won’t go into here.  These determine how much data will be moved, where it will be moved, and how data fragments can be consolidated to make one full block out of a number of partial blocks.

Controller designers have to walk a tightrope when it comes to wear algorithms since a very aggressive wear leveling algorithm may create an excessive number of unnecessary additional writes.  Meanwhile, the wear algorithm has to interact well with other operations within the drive (like garbage collection) or it might also create undue block wear.

This is all very tricky!

The selection of these algorithms determines not only how much wear each block will receive, but also how quickly the SSD will perform in demanding applications.  Each SSD architect has a unique philosophy about how these algorithms should be managed.  The best algorithm depends on a number of variables including NAND type, workload, speed requirements, and so on.

Intel slide illustrating a basic level of write levelingIntel has shared some graphics that help illustrate how wear leveling algorithms differ.  The first is a chart of an SSD with a very basic wear leveling algorithm called “Regioned Leveling.”  The X-axis represents different flash regions.  The Y-axis indicates the number of erase/write cycles each has undergone.  A red dot represents the activity in and individual block.  Clearly some blocks, even blocks within the same region, undergo significantly more write activity than do others, and these blocks should be the first to fail.

Intel slide showing wear leveling for an Intel SSDThe second graphic contains similar data for an Intel SSD, but in this chart the data has been sorted from lowest to highest because otherwise the difference between blocks might not clearly show up in the chart.  The Intel SSD used in this example uses a much more balanced write leveling algorithm that will wear each block at essentially the same rate.  According to Intel, the first SSD exhibited some cases in which peak wear was three times the average.  The second had only a 4% difference between the blocks with the maximum and minimum wear.

What impact does this have on SSD longevity?  Well, if the wear leveling algorithm can both minimize erase/write cycles and truly even out wear across the entire SSD, then it will naturally coax the longest life out of the blocks before any of them starts to wear.

Wear leveling is one of many techniques used to extend SSD life.  This post is part of a series published by The SSD Guy in September-November 2012 to describe the leading methods SSD architects use to get the longest life out of an SSD despite the limited number of erase/write cycles that NAND flash specifications guarantee.  The following list provides the names of all of these articles, and hot links to them:

Click on any of the above links to learn about how each of these techniques works.

Alternatively, you can visit the Storage Networking Industry Association (SNIA) website to download the entire series as a 20-page booklet in pdf format.

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Jim Handy
Objective Analysis
SSD Market Research
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