Endurance and reliability: the two numbers to ask for
Continuous video is the hardest duty a storage device can be given. A module recording several cameras around the clock rewrites its entire capacity every few days, for the service life of the vehicle, and it does so at temperature, inside a sealed enclosure that cannot be cooled by a fan. Two figures describe whether it will last, and they measure different things.
Nominal TBW — how much writing the memory can take
Terabytes written is the endurance rating of the flash itself: the total volume that can be written before the memory is considered worn out. It is the figure that matters for rolling video, because a rolling recorder never stops writing. Divide the nominal TBW by the volume written per day and the answer is the expected service life. Two cameras writing about 3.4 GB an hour amount to roughly 30 TB a year, so a fifteen-year installation on that duty needs an endurance rating of several hundred terabytes — and proportionally more as cameras and bitrates rise. Ask any supplier for the number, and ask which capacity it applies to — ratings vary by orders of magnitude.
MTBF — how reliable the electronics are
Mean time between failures describes the unit as a whole rather than the memory, and it is a calculated figure, so the calculation method matters. The CHM86 is rated at 500,000 hours to SN 29500, a recognised standard for electronic reliability prediction.
Why a specification should ask for both
MTBF tells you how often the unit fails. TBW tells you when the memory reaches the end of its designed life even though nothing has failed. A specification that asks only for MTBF can be satisfied by a module that wears out after three years of continuous video.