What is DWPD, PBW and TBW?

Welcome to this comprehensive article on SSD performance, speed, and capacity. The following is the 3^rd  post in a series of "Understanding SSD Basics". One of the most important characteristics and the biggest secret of the SSD (Solid State Drive) industry is how to measure the quality of the SSD?
Most the users/customers will point the Read/Write (R/W) speed as the major indicator.
Others will describe the Program/Erase (P/E) cycles as the most important feature.
Very few people will indicate the NAND Flash type (SLC, MLC, TLC or 3D-TLC) as the most relevant element.

In fact, all of them are correct, but I have a special secret to share.
The most important feature which describes the SSD qualify is its endurance figures.
Let’s describe the meaning of each term:

• DWPD (Data Written Per Day) – How many times can I write the entire SSD capacity per day?
• TBW (Tera Byte Written) – the amount of data, in Tera (10¹² = 1000x GB) Byte which can be written to that disk.
• PBW (Peta Byte Written) – the amount of data, in Peta (10¹⁵ = 1000x TB) Byte which can be written to that disk. Basically PBW = 1000x TBW

Before starting the comparison, one needs to know:

• Warranty [Years]
• Capacity [GByte]
• At least one of the SSD endurance values: DWPD, PBW or TBW

To explain my meaning, I would like to compare the following 3 disks:

Reviewing the calculation results, I’ve received the following:

SSD #3 has the highest DWPD value while SSD #2 has the best TBW. What does it mean?

I can write SSD #3 1.8 times per day, but I can write only 946TB during the SSD lifetime.

I can write SSD #2 1.1 times per day, but I can write 970TB during the SSD lifetime

Does it sound strange to you? Well, it is not.

The answer is very simple SSD #2 vendor decide to bonus his consumers with extra 2 years of warranty. Since the SSD NAND IC is not correlated with the vendor’s warranty rather with Program and Erase cycles (P/E) and Firmware (FW), the SSD DWPD value is dropping. The same amount of data 970TB needs to sustain a lifetime of 5 years. In that case, from first site, I would select SSD #3.

Now, lets check what will happen if SSD #2 manufacture will decrease the warranty period to 3 years.

Reviewing the calculation results, I’ve received the following:

Now, SSD #2 DWPD and TBW values are the highest. Now the selection is clear.

Let’s try to analyze our findings:

1. DWPD value decreased since SSD #2 manufacturer provided longer warranty period 
   
2. TBW value does not have any correlations with the SSD warranty

Important fact: the modern SSD calculate the amount of data written. The manufacture can pull out that value while receiving faulty SSD. In case SSD fail to malfunction during warranty period, the SSD manufacture will not repair the SSD if the value of TBW extended the certified amount.

Having said that, one can understand that the most meaningful value for SSD populated in high data writing systems is TBW/PBW.  For SSD populated in low writing system, the TBW/PBW value together with the warranty period will refer on the best SSD selection.

Remark: many consumer grade SSD vendors will not disclose the endurance values of their disks in their data sheet or brochures. Most of the times, the following mean very low performance. Watch out from these vendors.  

I’m adding useful online calculator which helps to convert the endurance characteristics (DWPD, TBW, PBW, GB/day) https://wintelguy.com/dwpd-tbw-gbday-calc.pl

On my next post I’m planning to discuss Read and Write speeds. I think you will be surprises…

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