TBW and DWPD are two very important factors deciding your SSD’s lifespan and endurance. MTBF is another generalized specification deciding how many hours an SSD will work on your computer.
All these things decide the endurance of your SSD. So, it is important to discuss them and consider checking before you buy an SSD.
SSD endurance is never an easy thing to calculate in case of non-wear-out failures which are generally caused by electric faults, firmware issues, and other problems in the drive. These faults will come before the TBW or warranty periods have been reached.
SSD endurance ratings are more about wear-out issues. TBW, for example, indicates the total amount of data your SSD can possibly write before its endurance rating is totally surpassed. Let’s talk in detail about three of the most important endurance factors in SSDs i.e. TBW, DWPD, and MTBF.
What is TBW in SSDs?
TBW or Terabytes Written is a measure of total data that could be written on the SSD before its lifetime ends. This parameter is considered to determine the endurance and reliability of any SSD. Manufacturers give these numbers in TB and they can vary depending on many things.
The actual lifespan of an SSD depends on the type of workload. If you do high write operations or use write-intensive applications, your drive will reach TBW limits faster.
Even to determine the warranty period of any SSD, manufacturers use the TBW as a benchmark. Drives with higher TBW will generally come with a higher warranty period as well.
How is TBW determined?
The first and most important thing determining the TBW in SSDs is the type of NAND flash cells. SLCs have the highest TBW while QLCs have the lowest one. Larger drives generally offer higher TBW as compared to the lower capacities.
Write Amplification Factor is another major contributor. This number is used to highlight how much extra data the SSD will write for any amount of data. A WAF of 1.5 indicates that for writing 1TB of actual data, the drive will have to write 1.5TB of data in real. This extra data is due to the internal tasks of the SSDs including wear leveling, garbage collection, etc.
Many other things determine the SSD TBW including controller efficiency, over-provisioning, ECC mechanism, wear-leveling techniques, etc.
So, all in all, the manufacturers put these drives into testing and give specific numbers as their drive’s TBW. The Write-Amplification-Factor and Over-Provisioning are decided and designed by the manufacturers and the components used in the SSD. However, keep in mind that these are not definitive numbers. Your SSD can fail prematurely in a case called non-wear-out failure.
How is TBW calculated?
We do not have to do any calculations in the case of TBW because manufacturers provide the numbers on their own. You can check it on the product page or SSD datasheet. However, the formula used by the manufacturers is this:
TBW = (NAND Endurance) × (Total Capacity) × (WAF) × (Over-Provisioning Factor)
NAND Endurance is the number of total P/E cycles. This number comes from the type of NAND flash utilized for that SSD. SLC has the higher P/E number while QLC has the least.
The overprovision factor determines how much extra space is allocated for the internal working of SSDs which can’t be used for primary storage. For a 1TB (1000GB) SSD, a 10% overprovisioning factor means 100GB is over-provisioned space and 900GB is user-accessible capacity.
The total P/E cycles of the NAND flash are one of the key factors, as we saw in our formula. Unfortunately, manufacturers don’t disclose the WAF and OPF of their drives. So, we can’t calculate it on our own.
As per our calculation for a TLC SSD with 1TB drive, the TBW came out to be 1650TB which is the very high and rare number to see. So, either there are compromises with the P/E cycles in the NAND flash or with WAF or OPF.
What is DWPD in SSDs?
DWPD stands for Drive Writes Per Day. This metric is used to understand how many times the total capacity of an SSD can be written or overwritten every day during the warranty period without exceeding the drive’s endurance limits. DWPD is more of a user-centric metric which helps use understand how much data we can actually write on our drive daily for how long.
DWPD is determined by taking TBW and the warranty period into consideration for any SSD. So, you can say that DWPD and TBW are directly proportional if the warranty and capacity of SSDs are the same. If the TBW is high, the DWPD will also be higher.
The DWPD number comes with a single number of how many times you can write the entire SSD size to it. If the DWPD is one for a 1TB drive with 5 years of warranty, you can write 1TB data on your drive every day for 5 years. After 5 years of this much data, the drive will be out of its endurance limits.
How is DWPD calculated?
There is a simple formula to calculate the DWPD from your SSD’s storage capacity, TBW, and warranty period. If you have these three, you can easily get the DWPD out of them. Here is the formula and an example DWPD calculation for Crucial T700.
So, for an SSD with 1TB capacity, 600TBW, and 5 years of warranty, we get 0.329 DWPD. This means we can write around 32.9% or 329GB on Crucial T700 every day for five years. If you write this amount of data every day, after five years, the SSD endurance limit will be exceeded and it may not remain usable anymore.
What is MTBF in SSDs?
MTBF is a reliability metric that estimates the total time (in hours) before the failure of an SSD. This metric is used in various other electronic items and components as well. But, for SSDs, this just says how reliable is the SSD. So, a higher MTBF indicates better reliability.
The MTBF is again calculated by the manufacturer with the help of accelerated testing, user data, and historical failure rates.
Also, MTBF is the opposite of the failure rate in SSDs. If 1 from 10,000 SSDs has failed during the testing period, the MTBF will be 10,000 hours.
How is MTBF calculated?
Manufacturers do tests on multiple drives for a good amount of period. Then, the total number of failed drives is checked. When the total operational time of all drives and the total number of failed drives are calculated, they get to a number in hours. The formula and calculation is like this:
In our example, 1000 drives were tested for 10000 hours. 10 drives failed during the test. From the calculation, we reached 1 million hours. MTBF is a good estimation metric for the bulk of the drive but doesn’t say much about the individual drive depending on its specifications.
What happens when an SSD reaches its TBW limit?
Although it is very hard to surpass the TBW limits within the warranty period in the consumer environment. But, you will surpass it someday if the drive keeps working for years on. After the TBW limit is reached, the first thing your drive will do is enter the read-only mode. The chances of data loss at any time increase. The performance degrades and the drive is prone to failure at times.
Which one is the End-of-life indicator of an SSD?
TBW is partially an End-of-life indicator because reaching TBW directly means the SSD can fail at any time. Both DWPD and MTBF aren’t End-of-life indicators in any way. TBW also doesn’t directly say that the SSD is going to die because the drive may keep working without issues even after reaching its TBW limits.
DWPD is a simple calculation that will help users who do more intense write operations daily. People who work with large video files, moving files here and there can use the DWPD as a measure to ensure the good usability of their drives.
MTBF is an estimation given by the manufacturers by doing bulk tests on thousands of drives. Although these numbers are coming out of many drives, a good MTBF means your drive was manufactured with proper care for endurance.
How is the warranty period determined in SSDs?
Most of the modern SSDs including NVMe and SATA (enterprise and consumer) come with the standard 5-year warranty. These are mostly the TLC SSDs. Most QLC SSDs come with 3 years of warranty. This is because the warranty is also majorly dependent on the P/E cycles of the NAND flash just like the TBW.
The warranty period can have implications on the brand reputations so the manufacturers try to give their best for the best possible numbers. 5 years is an ideal warranty period in the SSD market. However, failures can come at any time because SSDs are electronic components working with the help of electricity.