Most folks will never even come close to exceeding the write endurance limits of today's desktop-oriented SSDs. Write exhaustion requires continuous writing to a drive for weeks and months on end before you completely exhaust the usable life of each NAND cell.
In the enterprise world, however, this is a much more likely scenario. Knowing the write endurance of an SSD can help IT professionals select drives that are best suited to their tasks.
When Intel released its first enterprise drive, X25-E, the company did not publicly state write endurance specifications. With its two subsequent offerings, though, Intel was very specific about what results were achievable and how to achieve them. The 400/800 GB versions of the Intel SSD 910 have a stated write endurance of 7 and 14 PB, respectively. According to Intel, write endurance is measured while running 100% random 4 KB and 8 KB writes spanning 100% of the SSD using Iometer. This is, by far, the worst-case scenario. In a mixed workload, you'd see more favorable results, as we will see.
Before we dig into the results, if you are unfamiliar with the different types of NAND or the concept of write exhaustion in general, take a look at our reviews of the Toshiba MK4001GRZB and Intel SSD 710.
To test write endurance, we wrote large block, sequential data to the drive, while continuously monitoring the MWI (Media Wearout Indicator). The MWI reports, from 0-100, the percentage of life that has been used on the drive. We started with a clean drive and wrote to it until the MWI reached 1%. It should be noted that each of the four NAND modules has its own MWI. The data below is based on when the first module reported a change to the MWI. The other three modules all changed within ~150 GB of the first. This difference only accounted for ~0.15% of the total number of writes.
By writing sequential data, we are showing the maximum usable life of the NAND itself, removing outside factors like wear-leveling and garbage collection. In this configuration, the write amplification should be very close to 1.0.
|Endurance RatingSequential Workload, QD=1, 8 MB, Random||Intel SSD 910||Intel SSD 710||Intel X25-E||Toshiba MK4001GRZB|
|NAND Type||Intel 25 nm eMLC (HET)||Intel 25 nm eMLC (HET)||Intel 50 nm SLC||Toshiba 32 nm SLC|
|RAW NAND Capacity||896 GB||320 GB||77GB||512 GB|
|IDEMA Capacity (User Accessible)||800 GB||200 GB||64 GB||400 GB|
|P/E Cycles Observed (IDEMA)||46 339||36 600||237 968||225 064|
|P/E Cycles Observed (Raw)||41 374||22 875||198 307||175 831|
|Host Writes per 1% of MWI||370.71 TB||73.20 TB||152.3 TB||900.2 TB|
In terms of P/E cycles observed, the Intel SSD 910 outperforms Intel's SSD 710 by 80%, even though they use the same NAND. But, as with all MLC-based flash, it can’t really hold a candle to good old-fashioned SLC.
So why, in an enterprise application, where write endurance is so important, would you consider anything other than SLC? Simply, cost. HET MLC (or eMLC) offers a solid middle-ground to those that need enterprise-level write endurance, but can’t justify the price of SLC-based drives. Intel's SSD 910 makes that value proposition even more intriguing compared to its SSD 710.
When you look at just write endurance and cost ($/PB-written), ignoring all other factors, the SLC-based X25-E is still the clear winner. But the comparison to the SSD 910 is much better-looking than the SSD 710. This is important for customers who want to use these drives purely as write-caching devices, where speed and size can be secondary features.