Understanding MTBF in SSD – What Does an SSD’s MTBF Mean for You?

I have noticed that a lot of people take the MTBF spec into consideration when comparing different SSDs without fully understanding what that figure means.

The assumption most people make is that a drive like the Intel 335 Series, with an MTBF of 1.2 million hours, isn’t expected to last as long as a drive like the Samsung 830, with its MTBF spec of 1.5 million hours. However, that is not how MTBF works.

Defining MTBF

MTBF is defined as the predicted elapsed time between inherent failures of a system during operation.

[pullquote align=”right”] If the MTBF is 1,200,000 hours … it would be expected to have one failure every 150 days[/pullquote]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 1.5m MTBF drive will last 1.5 million hours (that’s 136 to 171 years by the way)

In the case of the Intel 335, the 1.2 million hour MTBF means that 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, or about twice a year. The Samsung 830 is expected to have one failure every 187.5 days. However not all drives are tested to the same standards.

How SSD MTBF is 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.

So What Does SSD MTBF Mean for Me?

Unfortunately, most manufacturers don’t share this information freely. But when it comes down to it, most consumers should find any MTBF over 1 million hours to be acceptable. This generally indicates about 3 failures a year across 1000 drives that are used 8 hours a day. In other words, you’d have a 0.3% chance of having a write operation failure within the drive’s warranty.

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.

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