Write Endurance: Comparing MLC, eMLC, And SLC
Because it employs MLC NAND, Micron's RealSSD P400e is clearly not intended for the same situations where you'd find Intel's older X25-E, or even its more modern SSD 710, which uses a special type of MLC called High Endurance Technology. Rather, it's more definitively destined to make its way into read-intensive environments that are less likely to hammer the limited endurance of its on-board flash.
Of course, cheaper NAND does allow Micron to sell its P400e at a lower price (currently, the drives are available from one online vendor between roughly $2 and $3/GB, depending on capacity). But that does little to assuage IT managers who simply cannot make compromises with the availability of data hosted on the SSD.
Micron is fairly pragmatic about the P400e's utility, citing heavy read workloads, fast data recovery, and data integrity as strengths. Its typical applications include read caching, application loading, operating system loading, and embedded/industrial environments. But it also lists enterprise endurance as one of its fortes. Can that really be said for an SSD based on MLC NAND?
Demanding storage workloads often involve continuous reads and writes, contributing to the eventual failure of a hard disk's mechanical components. On an SSD, those same tasks chip away at a memory cell's ability to successfully be programmed to and erased. SLC NAND is already well-established for its almost-inexhaustible endurance, and the aforementioned HET MLC flash is quickly gaining popularity as well in more cost-effective enterprise-oriented drives.
However, conventional MLC NAND can still be considered appropriate for certain professional applications. Based on discussions with data center managers, plenty of Intel's first-gen X25-Ms and newer SSD 320s are being used in mission-critical deployments. They're simply not serving a role that puts the business at risk.
Although it's unlikely that anyone would pick an MLC-based SSD for an application expected to push heavy, sustained writes, endurance is still an important factor to consider in evaluating a drive being sold as enterprise-ready.
Evaluating SSD Endurance
Before we take a stab at quantifying and comparing the endurance of different flash technologies, we should discuss our methodology. Our estimates are derived from monitoring each drive's media wear indicator (MWI), which measures media wear with a percentage countdown from 100 down to 1. Because a NAND cell can withstand a finite number of program-erase cycles, the MWI facilitates a rough estimate of SSD endurance.
Enterprise customers place a lot of importance on the MWI. It measures the “safe zone” of SSD reliability, giving you an indication of when an SSD might be operating on borrowed time. Of course, once a drive's MWI counter counts all the way down, its rated P/E cycles are theoretically exhausted. That doesn't mean something bad happens right away, though.
|Endurance Rating (Sequential Workload, QD=1, 2 MB)||Intel SSD 320||Intel SSD 710||MTFDDAK200MAR-1J1AA|
|NAND Type||Intel 25 nm MLC||Intel 25 nm eMLC (HET)||Micron 25 nm MLC|
|RAW NAND Capacity||320 GB||320 GB||256 GB|
|IDEMA Capacity (User Accessible)||300 GB||200 GB||200 GB|
|P/E Cycles Observed (IDEMA)||5460||36 600||3782|
|P/E Cycles Observed (Raw)||5119||22 875||2955|
|Host Writes per 1% of MWI||16.38 TB||73.20 TB||7.56 TB|
According to Micron's spec sheet, the 50 GB P400e features a write endurance rating of 87.5 TB, while larger models endure at least 175 TB of writes. It's hard to compare Micron's claim to other vendors because they all use different methodologies for estimating longevity. The numbers we generate assume a purely sequential workload, though, which means we aren't taking into account applications that push more random accesses. Incomplete though that may be, it allows us to evaluate endurance in a more consistent and comparative manner.
The shortcomings of MLC NAND are glaring in the table above, particularly with the HET MLC-based SSD 710 included. Even under the umbrella of vanilla MLC flash, however, not all memory is alike. Intel and Micron operate a joint NAND venture known as IMFT, but Intel grants its MLC-based flash a substantially higher endurance spec than Micron. So, with the effects of over-provisioning removed, the SSD 320's 25 nm MLC NAND has a rating of ~5000 P/E cycles. Employing MLC NAND manufactured using the same fab process, Micron's P400e has a rating of only ~3000 P/E cycles. This difference has a profound effect on SSD endurance.
Based on our endurance testing, the 200 GB Micron P400e should be capable of writing about 756 TB sequentially. That's not at all shabby considering that this drive is predominantly intended for read-heavy applications. However, it falls well short of even Intel's 300 GB SSD 320, which should be good for more than 1.6 PB and is considered a desktop drive by the company.
Bear in mind, of course, that endurance ratings apply to each flash cell. Larger SSDs naturally host more flash, so it takes longer to write across them. Consequently, higher-capacity SSDs enjoy more comfortable endurance ratings. We have only the 200 GB P400e in our lab, but we theorize that the 400 GB model should sport twice the endurance.