FORTASA BLOG

SSD MTBF Rating - Meaningless in Flash Drive Reliability Estimate

Many Flash Drive manufacturers provide a Mean Time Between Failure (MTBF) rating as an estimate of an SSD life expectancy. Typically this rating is >2,000,000 hours for an SLC (Single Level Cell) NAND based SSD or >1,000,000 hours for an MLC (Multi-Level Cell) based SSD. While at a first glance 1 Million hours (114 years) of life expectancy seems like a huge overkill for a typical application, in reality an MTBF number is not representative of the realistic expected usage of the Flash SSD.

MTBF is a theoretical formula based on the sum of the Mean Time To Failure (MTTF) rates of the individual components used in a specific SSD design.

Formula to calculate MTBF for a Flash SSD, Card or Module

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Write Amplification - Cause for Diminished Reliability in Flash SSD

Write Amplification is a major contributor to the reduced life expectancy of an SSD. Unlike a conventional HDD, Flash based SSD can’t simply write new user data on top of the previously recorded data. Instead, flash memory requires an erase and subsequently a re-write cycle to be issued to program the new information. Typically when the system needs to only update a single page of data, the flash controller has to read the full erase block size of data into the read buffer, update the desired page in the read buffer, erase the original block and re-write the updated block of data back to the Flash media. The same update process needs to occur not only to the data information itself, but also to the file table structure that links the data pages into a full information file. Therefore, a single page of data update actually mandates multiple Flash media writes. This process is commonly called – Write Amplification phenomenon.

Considering that Flash Media has a finite number of program/erase cycles as specified by the component Flash supplier, the SSD-wide Write Amplification phenomenon of increasing the number of program/erase cycles, significantly reduces the useful life of the SSD. Additional system requirement of Wear Leveling to spread the Flash Media usage by re-writing between static and dynamic data adds to the Write Amplification factor.

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