Results: Write Endurance
We typically spend a lot of time evaluating write endurance when we review enterprise-class SSDs. Write endurance is one of the major differentiators separating enterprise and client-oriented drives, after all. As MLC-based storage continues pushing its way into spaces previously filled by SLC NAND, we have to keep a close eye on this difficult-to-benchmark, but still very important variable involved in evaluating solid-state storage.
Micron takes a two-fold approach to ensuring ample write endurance from its P400m. First, it uses 25 nm MLC NAND specifically binned for enterprise purposes. Instead of the 3,000 or 5,000 program/erase cycles you typically get from client-oriented MLC at 25 nm, Micron rates the P400m's memory for 20,000 cycles. Second, Micron uses 70% over-provisioning across all of the P400m's available capacities. Taken together, the 300 GB model should endure 3.5 PB of data written to it over the course of its life. Put differently, Micron says you can write the drive's capacity 10 times per day for five years.
As always, we perform our tests using sequential workloads with large block sizes. This reduces the impact of write amplification and wear leveling, giving a better indication of actual P/E cycles for the NAND.
|Endurance RatingSequential Workload, QD=1, 8 MB||Micron P400m||Intel SSD DC S3700|
|NAND Type||Micron 25 nm MLC||Intel 25 nm HET-MLC|
|RAW NAND Capacity||340 GB||680 GB||264 GB|
|IDEMA Capacity (User Accessible)||200 GB||400 GB||200 GB|
|P/E Cycles Observed (IDEMA)||34,195||34,195*||36,343|
|P/E Cycles Observed (Raw)||20,113||20,113*||27,532|
|Host Writes per 1% of MWI||68.39 TB||136.78 TB*||72.69 TB|
When we compare the specifications of Micron's P400m to Intel's SSD DC S3700, they appear nearly identical. Both products are said to sustain 10 full writes per day for five years. The 200 GB models from both families specify between 3.5 and 3.65 petabytes of total data written to them. At comparable user-accessible capacities (200 GB), we see similar observed P/E cycles (34,000 versus 36,000).
It appears that Intel holds the leads when it comes to write endurance, but Micron compensates by arming its drive with 340 GB of raw NAND (rather than Intel's 264 GB). This is a good academic exercise. However, in the real-world, customers only care about actual performance and actual endurance. In that context, Micron has no trouble keeping pace with its higher level of over-provisioning.
Of course, using over-provisioning to improve write endurance has its drawbacks. In this case, that drawback is cost. Each gigabyte of user-accessible space on the SSD DC S3700 costs $2.35/GB. The P400m will push that closer to $3/GB. When you look at the raw NAND costs, however, the gap disappears; Micron's P400m is $1.76/GB, while Intel's SSD DC S3700 is $1.78/GB. Consequently, when you look at cost per petabyte of data written, the Intel drive maintains a lead.
Although we only tested Micron's 200 GB P400m, we also estimated the 400 GB model's endurance because it's priced lower than the 400 GB competitor from Intel. When you compare 400 GB drives, cost per petabyte written even out, making the 400 GB P400m a worthy alternative to the SSD DC S3700.