How Controllers Maximize SSD Life – Reduced Write Amplification

Tempus FugitWrite amplification plays a critical role in maximizing an SSD’s usable life.  The lower the write amplification, the longer the SSD will last.  SSD architects pay special attention to this aspect of controller design.

Unlike the other factors described in this series this is not a technique that extends flash life beyond the 10,000 erase/write cycles that one would normally expect to result in a failure, but it is very important to SSD longevity.

Write Amplification is sufficiently complex that I won’t try to define it in this post, but will leave it to another post that will be published shortly.  Suffice it to say that an SSD that is trying to manage its free blocks will need, at times, to move data from two sparsely-populated blocks to another block so that those sparsely-populated blocks can be erased.  Since the host system didn’t ask the SSD to perform this write, it results in a write that is over and above what the host system requested, so the SSD undergoes more writes than the host system intended.

If the number of NAND writes within the SSD matched the number of host system write requests then the Write Amplification (or Write Amplification Factor – WAF) would be 1.0.  If the average number of writes to flash were twice the number of host system write requests then the Write Amplification would be 2.0.

If you were to write a piece of system software that measured SSD writes, this measurement would fall short of the actual number of times the flash chips were being written to because of Write Amplification.

There are many ways to reduce Write Amplification.  Some SSDs write data to the SSD in a log structure – the data is written into sequential NAND addresses and housekeeping is performed during idle times.  This does reduce write amplification, but data management gets complicated as the SSD fills.

SandForce’s approach is to compress the data going into the SSD.  If the data is compressed to 1/2 its normal size, then half the system writes actually make it into the SSD.  This allows SandForce to boast that its write amplification is less than 1.0, a figure that is not intuitive unless you understand what approach the company uses.

In brief, an SSD that has been designed to reduce write amplification will reduce the number of times the NAND flash sees write cycles, thus taking the best advantage of the limited number of erase/writes that its NAND flash chips can withstand before experiencing bit errors.

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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