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Ask an ExpertAs SSDs grow in their popularity – whether used for data centers, PCs, laptops, or any appliance or system that requires storage – it's important to look beyond just speed and capacity, particularly when you want to understand how long they will last.
Two core metrics that explain endurance are: Terabytes Written (TBW) and Drive Writes Per Day (DWPD). In this article, we explore the distinctions between the two, and how they are calculated.
Terabytes Written (TBW)
So – put simply – TBW is the total amount of data that can be written to an SSD, over its usable life. It is also a good indicator of how long a drive will last under normal operating conditions. How we calculate TBW is by multiplying the capacity of the drive by how many Program/Erase (P/E) cycles each NAND block can perform – as specified by the semiconductor – then dividing this by the Write Amplification Factor (WAF).
The P/E cycle is determined by how many times a memory cell can be written and erased before it becomes unreliable or inconsistent, whilst the WAF indicates how much additional work the SSD must do to store data.
NAND flash has an inherent limitation on the number of P/E cycles it can endure. This is because the oxide layer, which traps electrons within the memory cells, gradually deteriorates with repeated use. Endurance ratings for SSDs are provided so that consumers can make an informed decision.
But let’s give you an example of TBW in the real world:
If a 1.92TB SSD has a TBW rating of 3,504, it means that the drive can endure the writing of 3,504TB of data before it is likely to encounter failures.
Drive Writes Per Day (DWPD)
DWPD, on the other hand, takes a slightly different perspective and calculates how many times a drive can be fully written to, per day, over its warrantied lifespan. This is particularly important for high intensity workloads – like in a server or data center.
However, we do use the TBW number and calculate this against the warranty period in days multiplied by the overall capacity, giving us a ratio. The formula we use is:
| Drive Writes Per Day = | TBW of an SSD x 1000 365 Days x Number of years warranty x SSD Capacity in GB |
For instance, if a 7.68TB SSD has a TBW of 14,016 and a warranty period of 5 years, the DWPD calculation would be:
| Drive Writes Per Day = | 14,016 x 1,000 365 (Days) x 5 (Years) x 7680 (GB) |
This would result in 1 DWPD, meaning that the SSD can handle writing its entire capacity, every single day over its 5-year warranty period.
Conclusion
So, in summary, TBW is useful for understanding the overall durability of a drive over its lifespan, whereas DWPD is key in understanding how well a drive can endure daily workloads, especially in enterprise and data center environments. Both metrics are key considerations when choosing an SSD, particularly in scenarios where you intend to use the drive in data-intensive tasks and constant write operations.
If you need support choosing the right SSD, our Ask an Expert team is on hand to answer your questions and offer objective advice, whatever your requirements.
