What do TBW and DWPD mean for SSD endurance?

As solid-state drives (SSDs) become more prevalent in data centers, personal computers, laptops, and any device or system requiring storage, it is important to consider factors beyond speed and capacity, especially when assessing SSD lifespan.

Two key metrics for explaining durability are: Total Bytes Written (TBW) and Drive Writes Per Day (DWPD). In this article, we explore the differences between them and how they are calculated.

Total Bytes Written (TBW)

In short, TBW refers to the total amount of data that can be written to an SSD over its entire lifespan. It is an excellent metric to indicate how long a drive can last under normal operating conditions. We calculate TBW by multiplying the drive capacity by the number of Program/Erase (P/E) cycles each NAND block can endure (as specified by the manufacturer), then dividing by the Write Amplification Factor (WAF).

The P/E cycle is determined by how many times a memory cell can be programmed and erased before becoming unreliable or inconsistent, while the WAF indicates the extra workload the SSD must perform when storing data.

NAND flash has inherent limits on the number of P/E cycles it can tolerate due to gradual degradation of the oxide layer that traps electrons within the memory cells with repeated use. Providing durability ratings for SSDs enables consumers to make informed decisions.

To illustrate TBW in real life:

If a 1.92TB SSD has a TBW rating of 3504, this means the drive can endure writing 3504TB of data before potential failure.

Drive Writes Per Day (DWPD)

On the other hand, DWPD offers a slightly different perspective by calculating how many times the drive can be fully written each day over the warranty period. This metric is especially important for high-intensity workloads such as servers or data centers.

We derive this ratio by comparing the TBW value to the product of warranty days multiplied by the total capacity. The formula used is:

DWPD = SSD TBW × 1000 / (365 days × warranty years × SSD capacity (GB))

For example, if a 7.68TB SSD has a TBW of 14016 and a warranty period of 5 years, the DWPD calculation is:

DWPD = 14016 × 1000 / (365 days × 5 years × 7680 GB)

This yields a result of 1 DWPD, indicating the SSD can handle a full drive write every day during the 5-year warranty period.

Conclusion

In summary, TBW is useful for understanding overall drive durability over its lifespan, while DWPD provides insight into the drive’s endurance under daily workloads—especially critical in enterprise and data center environments. Both metrics are key considerations when selecting an SSD, particularly for data-intensive tasks and continuous write operations.