MTBF is defined as the predicted elapsed time between inherent
failures of a system during operation.
It literally stands for "Mean Time Between Failure". Additionally...
As you can see, MTBF refers to the failure rate of a drive over its
expected lifetime. This doesn't mean a 1.2 million hour MTBF drive
will last 1.2 million hours, and a 1.5 million hour MTBF drive will last 1.5
million hours (that’s 136 to 171 years by the way)
So What Does SSD MTBF Actually Mean for Me?
unfortunately, most manufacturers don’t share this information freely.
What does 2,000,000 hour MTBF Mean For Me?
In attempt to make the example used in the article specific to a drive with a 2,000,000 hour MTBF. The following math was performed to determine that one failure would happen every 250 days
2,000,000 / 8 hours a day = 250,000 / 1000 drives = 250 days.
The article originally stated that a drive with a 1.5 million hour MTBF would fail once every 150 days:
if the drive is used at an average of 8 hours a day, a population of
1000 SSDs would be expected to have one failure every 150 days ...
The article continues to indicate that MTBF isn't that great of a way to determine how reliable the drive will be.
A better way to get an idea of how long an SSD will actually last for
you would be to consider the Total Bytes Written spec, or TBW.
Although this is another ‘overall expectation’ figure and doesn’t
directly tell you the lifespan of a drive, it will give you an idea of
how one drive compares to another. Unfortunately, not all
manufacturers give out this spec either.
The also article continues to explain how MTBF is normally determined.
The JEDEC JESD218A standard defines the method for testing the
read/write endurance of an SSD (free registration required to view)
which is the leading cause of SSD failure, but manufacturers may
choose to supplement this with some additional failure tests.
Another thing to consider is what workload is used to specify the
MTBF. For instance, Intel qualifies their SSDs using a workload of 20
GB of writes per day for 5 years. With this workload, along with the
supplemental failure tests, the Intel 335 has an MTBF of 1.2 million
hours. However if the workload was reduced to 10 GB a day, the MTBF
would be 2.5 million hours. At 5 GB per day, it becomes 4 million
hours.
References
- Understanding MTBF in SSD – What Does an SSD’s MTBF Mean for You? - Hardcoreware.com, Carl Nelson, January 6, 2013