Comparing Wear Figures on SSDs
I have been receiving questions lately from people who are puzzled when companies use different parameters than their competitors use to specify the endurance of their SSDs. How do you compare one against the other? Some companies even switch from one parameter to another to define the endurance of different SSDs within their product line.
I have found that Intel uses three different endurance measures for its products: DWPD (drive writes per day), TBW (terabytes written), and GB/day.
There’s not any real difference between any of these measures – each one is one way of stating how many times each of the SSD’s locations can be overwritten before the drive has gone past its warrantied life.
The relationships between these three measures are illustrated in this post’s graphic. You can click on it to see an expanded version. It’s all pretty simple. We’ll spell out the relationships in detail below, but in brief, if you want to compare two SSDs that are specified using two different measures, all you need is one of those measures (TBW, DWPD, or GB/day), the drive’s capacity, and the warranty period or lifetime of the SSD. Since the lifetime is usually expressed in years, and since DWPD and GB/day are measured in days, then you will also have to multiply the warranty period by 365 to find what it is in days.
You may also need to convert from GB to TB or PB or back. For some reason Intel converts these using decimal 1,000s instead of the computer scientist’s way of using the binary number 2^10, or 1,024.
Regular readers of The SSD Guy will have already seen a post that explains how to convert terabytes written (TBW) to drive writes per day (DWPD). Intel sometimes uses a third measure for the same thing, and that’s GB written per day (GB/Day). These can all be calculated from each other as long as you know the SSD’s capacity (I’ll use GB here instead of TB) and its warranty period (measured in years).
- DWPD = (GB/Day)/Capacity –OR– DWPD = (10^3 * TBW)/(Capacity * Warranty * 365)
- TBW = (Capacity * DWPD * Warranty * 365)/10^3 –OR– TBW = ((GB/Day) * Warranty * 365)/10^3
- GB/Day = (10^3 * TBW)/(Warranty * 365) –OR– GB/Day = DWPD * Capacity
To give an example of how that can be used, let’s pick some Intel SSDs at random and calculate the missing parameters. In the table below (copied from an Excel spreadsheet) the white cells represent specifications provided by Intel and the yellow cells are calculated from these numbers.
To make the comparisons relatively similar I have chosen SSDs that all offer close to 1TB of capacity.
Most of these SSDs provide only a single one of the three endurance measures, but the SSD 750 shows two. The SSD 750’s TBW and GB/day match the values we can calculate based on one another.
I happen to be most comfortable with DWPD, so let’s talk about that column. A few years ago SSD users, particularly those using SSDs for enterprise applications, focused a lot of attention on this figure and always wanted the largest number the SSD maker could provide. Higher numbers increased the cost of the SSD because one of the tricks used to increase endurance was to increase the overprovisioning of the SSD.
As the market matured users began to realize that certain of their workloads didn’t need a lot of endurance, and many started to ask for a lower price for SSDs with low endurance ratings. One particular such application is the PC, which has very low write requirements, especially when compared with the needs of real-time database applications like on-line transaction processing (OLTP). Note that the lowest endurance figure is the SSD 750’s 0.06 DWPD, a number that meshes well with the needs of most PC applications.
At the other extreme is the DC P3700, Intel’s top-of-the-line NAND SSD, which supports 17 DWPD. This product is designed for the highest write-load enterprise applications.
Other SSDs fall at various points between these extremes, reflecting Intel’s efforts to provide the right mix of specifications (price, performance, and endurance) to match the needs of several different user types.
For this example I chose SSDs all of similar capacities. Let’s change our perspective to explore a single SSD with a range of capacities.
Intel’s DC P3600 is available in five different capacities. The table below illustrates how its static 3 DWPD translates to varying TBW and GB/Day values as a function of the capacity.
The relationship is simple: As the SSD’s capacity triples, from 400GB to 1,200GB, its TBW and GB/Day specifications triple as well, even though the DWPD doesn’t change.
Those who have been paying a lot of attention to Intel’s recent Optane SSD announcements may be interested to see how those SSDs stack up. Two versions have recently been introduced: The DC P4800X, which is aimed at enterprise applications, and the m.2 Optane Memory SSD, which is for PC applications. The enterprise SSD’s endurance was disclosed using DWPD while the endurance of the m.2 SSD is expressed in GB/Day.
Intel’s Optane SSD DC P4800X enterprise SSD is specified to last three years at 30 Drive Writes per Day (similar to the specifications used for Intel’s NAND flash based SSD DC P3700) while its consumer counterpart, the 16-32GB Optane Memory SSD is specified to last for 5 years at 100GB of writes per day.
Although I do not know why Intel chose to specify drive lifetime differently for the two Optane SSDs, it’s simple enough to calculate one from the other, allowing us to compare the two, not only against each other, but also against their NAND-based counterparts. Here’s how the first table appears with the addition of the Optane SSDs:
Since there are two capacities of the Optane Memory SSD, and since Intel chose to use GB/Day as its endurance specification, the DWPD numbers differ between the two, which is very different from what we saw with the DC P3600 in the second table. This product’s wear is higher than half of the NAND-based SSDs in the table, giving credence to Intel’s early announcement that 3D XPoint Memory has higher endurance than NAND flash.
The Optane enterprise SSD beats that figure by a good margin, and is nearly double that of the next-higher candidate – the DC P3700 – offering 30DWPD compared to the DC P3700’s 17. Articles in the trade press indicate that the DC P4800X has about 19% overprovisioning, and this probably helps it to achieve this level of endurance. Since this Optane SSD’s capacity is low, though, the TBW number is proportionally smaller.
It is unclear that the Optane Memory PC SSD uses any overprovisioning at all. This could explain why its DWPD is only 1/5th that of its enterprise counterpart.
Note, too, that the DC P4800X has a shorter warranty period than most Intel SSDs at 3 years instead of 5. This reduces its TBW figure to 3/5ths (60%) that of a similar drive with a 5-year warranty, but has no impact on the DWPD or GB/Day figures.
The SSD Guy hopes that this post will help clear some of the confusion surrounding these three measures of endurance, and will help purchasers understand how one SSD’s endurance compares to that of another.