Reliability and Endurance
Seagate: Winning the Battle of the Boot Drives
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The NAND cells that comprise an SSD’s storage media ultimately wear out and will no longer retain data after exercising a given number of write cycles. SSD controllers apply wear leveling tactics to distribute writes across all NAND cells evenly to prevent any given cell or area in the media from failing prematurely. Nevertheless, modern multi-layer cell (MLC) NAND can only sustain between 3,000 and 10,000 write cycles, depending on the specific chips. Higher-quality, single-layer cell (SLC) NAND can sustain between 50,000 and 100,000 writes, again depending on the chips’ design details.
The higher an SSD’s capacity, the more cells are available around which to spread writes, and thus the higher the drive’s endurance. (Keep in mind that only writes impact NAND cell wearing. Reads have almost no endurance effect.) This is why some people have difficulty trusting that an 8GB SSD in a hybrid drive will be sufficient. It seems too small, so the endurance must also be too small for the kind of longevity buyers expect, especially in a business environment.
However, the Seagate SSHD design allows for minimal writes to the NAND while simultaneously reducing wear and tear on the HDD portion of the drive by reading frequently from the flash. When combined, HDD and SSD technologies actually complement each other. The HDD handles most writes, while reads come mainly from the NAND. To demonstrate how this provides ample design margin for avoiding flash wear out, the following table encapsulates Seagate research about how much NAND writing is likely to occur over a five-year drive lifespan.

Let’s walk through the chart, which Seagate defines as a worst-case workload, involving much more reading and writing than would be typical of an average office worker. Reading and writing data is self-explanatory. In this worst-case scenario, reading accounts for roughly 12.1GB of data from the SSHD’s flash per day while writes comprise 10.4GB. Of the total data read, only 32% of these files were stored, or pinned, in the NAND flash area. Boot files are operating system and other files used in the boot sequence after powering up. Here, Seagate assumes that 0.5GB of data is associated with booting and 100% of this data is pinned to the flash to ensure fast boot operation.
For a discussion about flash endurance and reliability, we only care about the data pinned to flash memory. Using the assumptions in this model and calculating the results of use over five years with 365 days of operation, one can predict that any given NAND cell would experience 1,413 write cycles.
Comparing this against specifications for the newest Seagate flash component used in its third-generation SSHD drive, we see the part is capable of at least 3,000 write cycles. As a result, the Seagate SSHD realizes a design margin of 212% percent. In layman’s terms, this worst case scenario says that over a five-year life, being used 365 days a year, a Seagate SSHD product will still have only worn down the flash to only about half of its useful life.
What makes this analysis even more interesting is that the newest Seagate SSHD designs moved from using SLC NAND flash in second-generation SSHD products to MLC flash in its third generation. Recall that SLC is a higher endurance technology capable of more write cycles than MLC. This design change primarily focused on reducing costs and making SSHD technology much more affordable. However, the update also helped to emphasize the cooperative benefits of unifying HDD and NAND flash technologies in a single drive design. Again, since most reading is accomplished via flash and most writing via the HDD, the reliability and endurance strengths of each technology are enhanced.