It is pointed out that the 'DWPD' standard, which indicates the durability of SSDs, differs depending on the manufacturer, making it difficult to compare.
Is DWPD Still a Useful SSD Spec? - Klara Systems
https://klarasystems.com/articles/is-dwpd-still-useful-ssd-spec/
At the time of writing, most commercially available SSDs are based on NAND flash memory , which stores data in a cell that can store one bit of information, with a charged state of '1' and a discharged state of '0'. DWPD is an indicator of the durability of such SSDs, measured by the number of times they can be written to or rewritten. However, to fully understand DWPD, it is necessary to understand the causes and mechanisms of drive failure.
There are not only SSDs but also HDDs in storage, and SSDs use semiconductors to store data, while HDDs use magnetic disks to store data. Therefore, the causes of failure are different between SSDs and HDDs.
First of all, the causes of HDD failure include 'magnetic head failure,' 'motor failure,' and 'controller failure,' which are mainly mechanical or electrical failures of the device that accesses the data. In most cases, even if the HDD fails, the data technically remains, so it is possible to recover it using various physical means.
On the other hand, causes of SSD failure include 'poor charge retention,' 'write endurance exhaustion,' and 'controller failure.' Poor charge retention occurs when the device is not powered for a long period of time. The longer the power is off, the slower the charge dissipation becomes, making it impossible to accurately determine the charge state of each cell. Write endurance exhaustion occurs when each cell deteriorates with each discharge and charge (write and erase), eventually becoming unable to accurately store the required charge level. DWPD is an indicator related to the exhaustion of an SSD's write endurance.
Each NAND flash cell in an SSD has a limited number of write/erase cycles (P/E cycles), and with each P/E cycle, the cell's performance degrades, leading to a decrease in read speed and accuracy. As the cells wear out, they take longer to charge and become less able to retain the correct value on the first try, gradually reducing the SSD's performance.
NAND flash cells represent a '1' when charged and a '0' when discharged, but by increasing the number of charges a cell can hold, more bits can be stored with the same number of cells. Single-level cells (SLC) monitor the charging/discharging status of one charge, multi-level cells (MLC) monitor the charging/discharging status of two separate charges, and triple-level cells (TLC) monitor the charging/discharging status of three separate charges.
At the time of writing, the vast majority of commercially available NAND flash memory is TLC, often with only a small cache area of the same physical media as SLC. SLC offers the highest performance, durability, and reliability, but is also expensive. MLC, on the other hand, literally doubles the storage capacity in the same cell as SLC, while TLC offers triple the storage capacity.
However, the more charges that must be individually set and identified for each cell, the longer it takes to operate the cell, the lower its reliability, and the greater the impact of write endurance depletion on the cell. In other words, the more data stored in a single cell, the lower its performance and the shorter the lifespan of the cell itself. Smartphones and tablets that have been used for many years may experience unstable storage even though they do not store much data, but such devices may be experiencing cell write endurance depletion.
One way to solve this problem is to use
Another option is to use an SSD with more capacity, even if you're storing the same amount of data. The more physical cells an SSD can store data in, the fewer P/E cycles each cell will have, even if you write the same amount of data over the same number of years. This means it will take longer for the write endurance to run out. Even if you use the same smartphone, if you buy a 64GB model and a 512GB model, the 64GB model will likely experience storage issues sooner.
To ensure that wear leveling works properly, one technique is to reserve a portion of the capacity for wear leveling. Consumer SSDs and enterprise SSDs may have different capacities, such as 512GB and 480GB, because enterprise SSDs reserve a portion of the total capacity for wear leveling.
For these reasons, DWPD, which indicates the number of writes/rewrites per day that an SSD can perform within the warranty period, is used as an indicator of SSD durability. Unfortunately, however, there is no clearly defined industry standard for DWPD, so it is difficult to know what results are used to calculate DWPD.
Enterprise-grade manufacturers claim to base their SSD endurance on the JEDEC SSD endurance workload, JESD219A.01 . However, the publicly available documentation only describes the workload and does not describe the failure conditions that indicate an SSD has reached its limits, leaving manufacturers free to determine the 'point at which it fails.'
In other words, even if you have two SSDs with the same DWPD, if they are from different manufacturers, one may consider the limit to be 'when half of the test drives are fully readable,' while the other may consider the limit to be 'when half of the test drives show a significant performance degradation.'
'Ultimately, like any other performance metric offered by storage vendors, DWPD has its strengths and weaknesses. While it can be useful as a means of comparing two different drives within the same manufacturer's lineup, extreme caution should be used when attempting to directly compare DWPD ratings from different vendors,' Klara Systems said.
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in Hardware, Posted by log1h_ik