Bridging The Gap Between Consumer And Enterprise StorageBuilding on the desktop-oriented M500, Micron is announcing its enterprise-focused M500DC. We got a chance to run the 800 GB model through an updated test suite to gauge whether this Marvell-powered SSD keeps up with the best-known enterprise solutions.
With the release of its M500DC, Micron continues a trend we've seen a lot of lately: dividing the SSD space into smaller pieces serviced by more purpose-built products. The relevant players take different approaches to this. For example, Intel cost-reduces its high-end enterprise drives, while Samsung adds enterprise-oriented features to its consumer architecture. Micron's M500DC takes the latter approach, incorporating higher-end functionality on a more desktop-class drive, rather than building a P400m (Micron P400m SSD Review: High Endurance MLC Is Here To Stay) with less expensive NAND.
The M500 and its nearly-identical, Crucial-branded cousin offered good performance, large capacities, and reasonable pricing. In fact, you'll still find M500s in Best SSDs For The Money. And while Crucial went after cost-conscious consumers, Micron pushed its M500 at entry-level, read-focused enterprise customers.
The M500DC fills a gap for the company between its M500 and P400m. Micron achieves this in much the same way as SanDisk (formerly SMART), taking 20 nm MLC NAND and applying a healthy dose of semiconductor and firmware know-how to stretch the attributes that matter for more demanding enterprise-focused customers.
Although Micron calls the segment it's targeting "Cloud/Web 2.0 Storage", a more general description would be a mixed-workload environment. The M500DC's endurance rating places it in front of read-focused SSDs with origins in the consumer space, while still clearly behind eMLC- and SLC-based drives. Not many SSDs fit that exact description, so comparisons are difficult. And this takes us back to advice we give over and over: know your workload. The more granular you get in predicting what your application does with storage, the better you can optimize total cost of ownership.
Looking over its specifications, you can see that the M500DC is more than just a read-focused enterprise SSD. Write endurance falls between one to two Drive Writes per Day (DWPD), which is four to seven times higher than what you get from repurposed desktop technology, but below the 10 DWPD you'd see from Intel's SSD DC S3700 or Micron's P400m. It comes close to SanDisk and its Optimus Eco family, which is rated at 3 DWPD.
Topping out at 35,000 random write IOPS, the M500DC is also aligned with high-end enterprise SSDs. Unfortunately, read performance is a little underwhelming at 65,000 IOPS and 425 MB/s. Those are respectable numbers, but they're nowhere near the Optimus Eco and its 95,000 IOPS and 500 MB/s performance. They're also lower than most entry-level enterprise-oriented SSDs. We're not overly concerned yet, though. If you have a true mixed workload, the added write performance more than makes up for any shortcoming in read speed.
So, from a high level, the M500DC is set up to be more capable than most entry-level enterprise SSDs, but not as much so as the drives built to tolerate write-intensive tasks. For many of Micron's customers, price is going to determine which way they go. Unfortunately, as we often see, the company holds that information back. At higher capacities, however, we're expecting somewhere in the neighborhood of $1.15/GB, putting Micron's latest on par with Intel's SSD DC S3500. But let's see if we can form an opinion about the M500DC without the luxury of knowing what it costs.
A Look Inside Micron's M500DC
Though the M500DC shares its architecture with the M500, opening up the new SSD's chassis shows that they're actually quite different. The first thing you'll notice is how empty the PCB appears. There's so much room, in fact, that Micron left what we assume are debug headers across one side of the board.
Micron has so much available space because the M500DC employs high-density NAND. The 800 GB model has 1024 GB of 20 nm MLC flash spread across eight packages. It's not of the eMLC variety. But still, Micron bins it specifically for more enterprise-oriented products.
The M500DC and M500 have the same type of NAND in common. However, when you open up an M500, you'll see all eight packages on one side of the PCB. The M500DC sports four per side. Each package contains eight dies, totally 1 Tb (128 GB) of capacity. Freeing up extra room allows Micron to also ship the M500DC in a 1.8" form-factor, in addition to the 2.5" drive we're reviewing. We didn't have a chance to benchmark the 1.8" model, however, you can see how the components would fit in a 1.8" footprint.
If you looked closely at the specifications on the previous page, two data points probably jumped out at you. First, the 800 GB M500DC has identical write endurance and lower random write performance compared to the 480 GB model. How is that possible? The more NAND your drive includes, the more flash there is to spread write amplification across and the higher endurance should be. It's just that simple.
When we opened the 800 GB model, we encountered the expected 1024 GB of NAND. So, what's going on with the other capacities to explain the difference? It turns out the the smaller drives include nearly double the amount of over-provisioning compared to Micron's 800 GB version. The 480 GB M500DC comes equipped with six packages, for instance, instead of the four you'd expect, yielding a raw total of 768 GB. As we know, the more NAND the controller has to work with, the more efficiently it can apply its software algorithms.
Micron uses the same venerable Marvell 9187 that appeared on its M500, and heat is removed from it through thermal pads attached to the chassis. Of course, Micron applies its suite of enterprise-oriented firmware features (XPERT), which takes NAND management to a level appropriate for that market. I discussed the features at length in Micron P400m SSD Review: High Endurance MLC Is Here To Stay, but here are the highlights:
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Redundant array of independent NAND (RAIN): An architecture that essentially provides device-integrated algorithms that are RAID 5 across the flash channels.
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Advanced Read Mangement/Optimized Read (ARM/OR): This feature uses DSP algorithms to optimize NAND read and write locations.
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DataSAFE: A data path protection mechanism that ensures all information is transferred correctly through the SSD structures.
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Physical Power-Loss Protection: This feature uses on-board capacitors to continually store power for emergency use.
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Reduced Command Access Latency (ReCAL): Utilizes well-managed background operations for quicker response times.
Additionally, the M500DC sports two 512 MB DDR3 DRAM packages, one on each side, yielding a fairly standard 1 MB per gigabyte DRAM to NAND ratio.
Finally, the top of board is populated with 12 capacitors used for power-loss protection.
How we Test Micron's M500DC
The Storage Networking Industry Association (SNIA), a working group made up of SSD, flash, and controller vendors, has a testing procedure that attempts to control as many of the variables inherent to SSDs as possible. SNIA’s Solid State Storage Performance Test Specification (SSS PTS) is a great resource for enterprise SSD testing. The procedure does not define what tests should be run, but rather the way in which they are run. This workflow is broken down into four parts:
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Purge: Purging puts the drive at a known starting point. For SSDs, this normally means Secure Erase.
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Workload-Independent Preconditioning: A prescribed workload that is unrelated to the test workload.
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Workload-Based Preconditioning: The actual test workload (4 KB random, 128 KB sequential, and so on), which pushes the drive towards a steady state.
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Steady State: The point at which the drive’s performance is no longer changing for the variable being tracked.
These steps are critical when testing SSDs. It’s incredibly easy to not fully condition the drive and still observe out-of-box behavior, which may lead one to think that it’s steady-state. These steps are also important when going between random and sequential writes.
For all performance tests in this review, the SSS PTS was followed to ensure accurate and repeatable results.All tests employ random data, when available. Micron's M500DC does not perform any data compression prior to writing, so there is no difference in performance-based data patterns.
For comparison purposes, we evaluated the M500DC against similar products from Micron, Intel, SanDisk, and Seagate.
Results: 4 KB Random Performance And Latency
We knew going into our testing that the M500DC's random read performance wouldn't match most of our comparison drives. Iometer demonstrates exactly that, too. In fact, the only drive Micron beats at high queue depths is the company's P400m, its other enterprise-oriented SATA-attached SSD. The M500DC isn't positioned as a read-focused product though, so this result comes as no surprise.
The M500DC shines more brightly in random write performance. Its 800 GB model consistently exceeds the 24,000 IOPS claimed in Micron's datasheet. Compared to more read-oriented SSDs that hover in the 10,000 IOPS range, the M500DC delivers excellent performance.
We would have liked to test the 480 GB model as well. Micron says it should do 35,000 IOPS, and dropping that on the competition would have put it on equal footing as SanDisk's first-place Optimus Eco at 400 GB.
Average response time measurements put Micron's M500DC in the middle of the pack, which corroborates our IOPS testing.
Maximum response time lands slightly higher than the best enterprise-focused SSDs, though 26.54 ms is still a great result.
Results: Performance ConsisyencyIncreasingly, we pay close attention to the performance consistency of enterprise-class SSDs. This is what separates a good drive from a great one when all of the corner case testing appears equal. Viewing the data with more granularity gives us insight on particular drive behaviors.
For the following tests, we subject Micron's M500DC and three comparison SSDs to 25 hours of continuous random 4 KB writes. We record IOPS every second, giving us 90,000 data points. We then zoom in to the last 60 minutes to more coherently visualize the results.
Overall, the M500DC performs well, even against more expensive drives. In fact, the M500DC substantially reduces the halo that we observed on Micron's P400m, though it can't quite match the tighter grouping delivered by SanDisk and Intel.
Looking at the distribution of data points reveals another interesting behavior.
Almost 96% of all data points are within 0.06 ms of each other, which is astounding. In comparison, SanDisk's Optimus Eco did not have any 0.06 ms-band that contained more than 58% of its data points. Really, our only complaint is the outliers around the 1.7 ms mark. Still though Micron's M500DC maintains its rated specification on nearly 98% of our measured data points. Even the outliers are at a respectable 18,000 IOPS.
Normally, we only post consistency data on a drive once it reaches steady-state. With the M500DC, getting to this point was more difficult than usual. The chart below shows a fresh M500DC with our random write workload applied.
It took nearly nine hours before the drive finally hit steady-state. We also found that if you aren't careful, the drive will trigger the TRIM command, and you get back to fresh out-of-box performance in a hurry. For what it's worth, only enterprise SSD reviewers get upset when a drive recovers its speed like this. And in case you're wondering, that stretch of data at the beginning lasts for almost an hour, with IOPS ranging from 66,000 to 78,000!
