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.
Seriously though, I wonder why Tom's doesn't run one of their basic "real-world" tests used on consumer SSDs (such as Tom's 7zip test) on one of these professional SSDs just so that we can get an idea how they compare to the consumer-level stuff.
In particular, Tom's always says that comparing one SSD to another is nearly moot point when you consider the magnitude of improvement an SSD has over a traditional HDD; it would be nice to know if these pro-level SSDs are of a similar magnitude of improvement over consumer SSDs or whether the difference is actually less.
The difference is much, much less in terms of performance difference. Tom's has told us this time and time again.
In desktop loads, very very less difference.
In server loads, huge difference. Plus, these server SSD's wil maintain high speeds even after large amounts of data is continuously being written.
How would this compare to a true SLC SSD ?
It'd probably still be inferior overall and not even be cheaper at that point. Over-provisioning is only good for mitigating MLC's disadvantages over SLC AFAIK, not replacing SLC.
Even in server workloads, there are many desktop drives where the performance difference is still not great. For example, Vector is right up there at the tops of the charts with Samsung 840 Pro in performance and many cheaper alternatives are often not far behind in performance. Endurance is another matter, but that wasn't the question. If you want a seriously significant performance difference like with HDDs versus cheap consumer SSDs, you have to consider SSD RAID and/or extreme PCIe storage.
Also, many consumer drives have no trouble keeping performance over time with lots of data written. That's also not an enterprise-only feature.
Some back of the napkin calculations - If you have a consumer 512GB MLC SSD with no over-provisioning, and the MLC was high grade consumer, you could expect 5K P/E cycles. In order to mimic the P400m, which is 35K P/E cycles, the usable space would be ~75GB. By dong that, you would be paying roughly $7/GB for usable storage(assuming you paid $512 for your consumer drive). For SLC, you are looking at 17GB of usable space and $30/GB.
No matter how you look at it, consumer grade MLC will never get close to eMLC and SLC in terms of write endurance, unless the price goes down by orders of magnitude compared to eMLC and SLC.
You are correct. Many drives, such as the 840 PRO, would perform great on the enterprise performance tests. There are also a lot of consumer drives that would have problems over time if they aren't allowed to TRIM every so often. We limit the scope of our testing because the main use cases for these drives are enterprise. Will some companies use them in workstations, absolutely. The same can be said for high-end RAID cards too. Nearly every consumer drive would perform very poorly when you take into account write endurance and other enterprise features.
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