Measuring Write Endurance: SLC Wins Again
Most customers 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 consume 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.
This is a metric we're expecting to set Micron apart from its competition. The company chooses to use 34 nm SLC flash in its P320h, and that decision is impactful in two ways.
The first is write endurance. Typically, SLC is capable of 100,000 program/erase cycles, while eMLC is closer to 30,000. The MLC memory most commonly used today falls between 3,000 and 5,000 P/E cycles. In our testing, SLC (even from different manufacturers) consistently performs better than its rating. Micron's NAND is no exception. It doesn't come cheap, though. The RealSSD P320h runs roughly $10/GB.
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 Intel SSD 910 Review: PCI Express-Based Enterprise Storage.
In order to test write endurance, we write large-block, sequential data to the drive and continuously monitor the Percentage Lifetime Used SMART attribute. This tells us, on a scale from 0 to 100, the percentage of life exhausted from the drive's NAND. We started with a clean drive and wrote to it until the attribute reached 1%.
By writing sequential data, we demonstrate the maximum usable life of the flash, removing variables like wear-leveling and garbage collection. In this configuration, write amplification should be very close to 1.0x. We did run into an issue, though, that complicated testing. Mainly, Micron does not provide a SMART attribute that reports the total amount of data written to the drive. This definitely caused some concern because we had no way of knowing how much data had been written previously, which could have skewed our results. Normally, we rely on our testing software to keep track of this, but we always like to double-check our work. This could be a concern for system administrators that want a hard number. For most, though, the SMART attributes provided should be sufficient to successfully administer the P320h.
| Endurance RatingSequential Workload, QD=1, 8 MB, Random | ||||
|---|---|---|---|---|
| Row 0 - Cell 0 | Micron Real SSD P320h | Intel SSD 910 | Intel X25-E | Toshiba MK4001GRZB |
| NAND Type | Micron 32 nm SLC | Intel 25 nm eMLC (HET) | Intel 50 nm SLC | Toshiba 32 nm SLC |
| RAW NAND Capacity | 1,024 GB | 896 GB | 77GB | 512 GB |
| IDEMA Capacity (User Accessible) | 700 GB | 800 GB | 64 GB | 400 GB |
| Over-provisioning | 22% (12.5% RAIN) | 12% | 20% | 28% |
| P/E Cycles Observed (IDEMA) | 276,652 | 46,339 | 237,968 | 225,064 |
| P/E Cycles Observed (Raw) | 185,700 | 41,374 | 198,307 | 175,831 |
| Host Writes per 1% of MWI | 1857.0 TB | 370.71 TB | 152.3 TB | 900.2 TB |
| $/PB-Written | $38.57 | $106.60 | $60.51 | $79.63 |
If you only consider write endurance and cost in dollars per petabyte written ($38), ignoring all else, the RealSSD P320h is our new champion. Based on our results, the P320h is clearly the best choice for customers who need a write-caching solution with high endurance for a reasonable price.
