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Exploring the future for SSDs

Posted: 31 Aug 2015 ?? ?Print Version ?Bookmark and Share

Keywords:Low-Density Parity-Check? LDPC? Error Correction Codes? ECCs? SSDs?

In the diagram below we show how the choice of LDPC parameters impacts t_ldpc across a range of NAND flash error rates. We consider two LDPC codes with different parameters. t_ldpc2 has better error correction properties but requires more parity than t_ldpc1. When the number of NAND flash errors is very low t_ldpc is the same for t_ldpc1 and t_ldpc2. However as the number of errors increases t_ldpc2 starts to exhibit less latency (t_ldpc) than t_ldpc1. In an SSD, there are two ways we can take advantage of this:
???Static LDPC configuration: If latency minimization is very important then you might choose to use t_ldpc2 in your SSD, rather than t_ldpc1
???Dynamic LDPC configuration: If your SSD supports it, you might choose to use t_ldpc1 when NAND error rates are low (i.e. at the beginning of life of the SSD) and then transition to t_ldpc2 as the NAND flash in the SSD ages

The choice of LDPC parameters impacts t_ldpc across a range of NAND flash error rates.

Note that in a given SSD controller there may be many more than two LDPC codes to choose from, in which case there is more flexibility in trading between the parameters of the LDPC code and the latency associated with t_read.

What happens when the initial LDPC decode fails? In that case the next step is to return to the NAND flash and start working toward something called a soft-decision LDPC decode. We'll talk about that more in the next blog post.

The migration from BCH codes to LDPC codes is going to enable a lot of great things inside SSDs. Some of these things are pretty obvious, such as better endurance and some are a bit less obvious, such as latency. And there are others that are pretty ambitious, including SSDs with properties that change over the course of their life-cycles.

It is going to be interesting to see how this all plays out as this technology starts to become pervasive and how software-defined flash may come to play a role in how SSDs are deployed in enterprise and data center environments.

About the author
Stephen Bates is with PMC-Sierra.

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