HET MLC: What Does Endurance Really Look Like?
Intel won't tell us exactly how many P/E cycles its 25 nm HET MLC can withstand. However, we don't have to rely solely on the company's word with regards to the 710's high endurance spec, because we can backwards-calculate the number using S.M.A.R.T. values found on Intel's latest SSDs.
| Intel S.M.A.R.T.Workload Counters | Purpose |
|---|---|
| E2 | Percentage of Media Wear-out Indicator (MWI) used |
| E3 | Percentage of workload that is read operations |
| E4 | Time counter in minutes |
The media wear-out indicator is a S.M.A.R.T. value (E9) on all SSDs that tells you how many P/E cycles are used, on a scale from 100 to 1. This is like the odometer on a car. However, using this value would require months of testing, because its on a scale from 100 to 1.
In comparison, Intel's workload counters are kind of like trip counters on a car, because they measure endurance over a fixed time period. Better yet, they provide more granular information on wear-out out, which makes it easy to measure endurance in a under a day. However, none of these workload counters are generated until the drive has been used 60 minutes or more. In practice, one hour isn't long enough for us to take a precise measurement, which is why we our endurance values are based on a 6 hour workload.
The counter starts the minute you plug in the drive, so you'll need to reset it if you want to attempt this test on your own. This is possible by sending a 0x40 instruction via smartctl.
If you're using a disk information program like CrystalDiskInfo, all S.M.A.R.T. values are in hexadecimal, which means you'll need to convert to decimal before proceeding. The E2 field is particularly unique because it's only valid out to three decimal places, and it's stored in an IEC binary format. So, after converting the E2's raw value to decimal, you have to divide by 1024 to get percentage.
Before we get to the results of our tests, we need to cover a little bit of math. If we toss out the JEDEC formula for a second, what do we know about write endurance? Rules that apply to all SSDs:
- Host Writes ÷ NAND Writes = P/E Cycles Consumed ÷ Total P/E Cycles
- P/E Cycles Used ÷ P/E Cycles Total = Media Wear Indicator (scale of 100 to 1)
- 100% sequential write means Host Writes = NAND Writes (write amplification = 1)
If we take these three formulas, it's possible to calculate the write endurance of the SSD 710 using the SSD 320 as a reference point.
| 128 KB 100% Sequential Write6 Hours | Intel SSD 710200 GB | Intel SSD 320 300 GB |
|---|---|---|
| Total Data Written | 3.88 TB | 3.9 TB |
| Percent MWI used (E2) | 0.053 | 0.238 |
| Endurance In Years | 1.292 | 0.287 |
| Percent MWI per TB | 1.35 x 10-2 | 6.10 x 10-2 |
| P/E Cycles Per TB | 3.07 | 13.7 |
| P/E Cycles | 22 337 | 5000 |
| Recalculated Endurance Rating(P/E Cycles ÷ P/E Cycles Per TB) | 7268 Terabytes Written | 364 Terabytes Written |
Starting with a 100% sequential write (write amplification equals one), we see the SSD 710's write endurance is roughly 4x to 5x higher than the SSD 320. We'll keep things simple and average out to 4.5x.
Previously, we've heard Intel mention that the NAND in its SSD 320 is rated for 5000 cycles. So, that puts the SSD 710 somewhere between 20 000 to 25 000 P/E cycles, which is in-line with what the company's competitors say eMLC should be able to do.
Now that we know what MWI looks like with a 100% sequential write, we can check write amplification in a random write workload with a high queue depth.
| 4 KB 100% Random WriteQD= 32, 6 Hours | Intel SSD 710200 GB | Intel SSD 320 300 GB |
|---|---|---|
| Total Data Written | 0.23 TB | 0.11 TB |
| Percent MWI used (E2) | 0.016 | 0.084 |
| Endurance In Years | 4.28 | 0.83 |
| Percent MWI per TB | 1.35 x 10^-2 | 6.10 x 10^-2 |
| P/E Cycles Per TB | 15.65 | 37.73 |
| Recalculated Endurance Rating(P/E Cycles ÷ P/E Cycles Per TB) | 1437 Terabytes Written | 132 Terabytes Written |
| Write Amplification | 5.09 | 2.75 |
Interestingly, write amplification is higher on the SSD 710. However, in the same period, the 710 can write twice as much data as Intel's 320. That'd purportedly be counter to the reason for more over-provisioning, but it'll all fall into place shortly.
Perhaps more important, both drives have endurance values better than what Intel cites, which just goes to show that the JEDEC spec tends to underestimate real-world endurance. With the same random workload, all of the SSD 320's P/E cycles would be consumed in less than a year, whereas the SSD 710 could continue working for another three years or more.
