SSDs In the Data Center: SLC Out, MLC In

We're coming to the end of the SLC (Single Level Cell) era of SSDs in the data center. This has become clear to me as I speak to SSD vendors and the folks that assemble those SSDs into larger systems, from server caches to all-solid-state and hybrid arrays. The latest generation of flash controllers manage to boost the performance and endurance of the less-expensive MLC (Multi-Level Cell) form of flash, to the point where SSD vendors small and large are pitching MLC-based products to the data center market.

In addition to the evolution of flash controller technology, the market for expensive SLC products is shrinking due to the constant cost pressures inherent in moving flash from a niche solution for nuclear weapons designers and high-performance traders to a viable alternative to spinning disks for the rest of corporate America.

The latest evidence came during briefings I had from Intel and Samsung on the same day a few weeks ago. Both pitched me on their latest enterprise drives, and neither one mentioned SLC, or even Enterprise MLC (eMLC), which has MLC-like bit densities but extends MLC's limited write endurance by extending write times. Both companies have decided that even for enterprise applications, the latest controllers can extend plain old MLC's write endurance to the point where the three-year useful life of a typical corporate array or server will run out before its flash does.

Samsung brought out two new drives, the SM843 and SM1625. The SM843, which has a 6GBPS SATA interface, comes in 120, 240 and 480GB capacities; the SM1625 comes in 200, 400 and 800GB versions. Both use Samsung's 2xnm-class toggle mode MLC flash. The SM843 is interesting as it delivers only moderate performance and endurance when compared to other enterprise-class drives, including its own SM1625. The enterprise pitch has always been about speed, but this drive seems to acknowledge that in addition to a market for Lamborghinis, there's also a market for BMWs.

Samsung also chose to provide a connector for an external capacitor rather than include an internal tantalum or ultra-cap. The capacitors in SSDs are either ultra-capacitors (technically electric double-layer capacitors) or use tantalum oxide as their anode, and so are called tantalum caps. The capacitor is a critical difference between the kind of SSDs used in desktop computers and those used for enterprise drives that have to deliver a higher level of data integrity. Any SSD has to have some RAM to buffer data when it's written to the device but not yet written to flash, and for when the controller collects blocks and rewrites them to different pages when assembling empty pages during housekeeping. The capacitor provides enough power to write the data from RAM to flash in the event of a power failure.

Samsung's SM1625 is a higher-end device with a dual-port SAS interface and that all-important internal capacitor. Because today's SSDs can deliver data faster than the 6Gbps that a single SAS or SATA interface can handle, Samsung has tuned the S1625's controller to deliver data across both ports simultaneously. This is a neat trick, but since the vast majority of SAS drives go into external storage systems that attach one controller to each of the SAS drive's dual interfaces, and can therefore only suck data across one interface at a time, I don't know how much benefit it will give users in the short run. Over time, arrays will have to learn to share drives across multiple controllers simultaneously.

Intel's DC S3700 is the successor to its 710 and X-25E drives. The DC S3700, unlike most recent Intel drives, uses an Intel-designed controller specifically developed to address the problem of inconsistent latency in SSDs. Because SSDs have to write data to empty pages of flash, there can be a delay as long as 10 or 20ms if a write comes along and there isn't a free page. The DC S3700 is designed to minimize not just average latency but also the variations in latency as the drive does housekeeping. Intel claims the DC S3700's maximum latency will be just 500µs.

Intel has a development agreement with HGST, the Western Digital subsidiary previously known as Hitachi Global Storage Technology, for SAS drives. Because the array vendors do significant technical qualification on their drives and have OEM relationships with Seagate and WD, putting the Intel name on a SAS drive probably won't add significant value. While they weren't promising anything, Intel's spokespeople said we could probably expect an HGST drive incorporating DC S3700's controller technology at some point in the near future.

Note that while both the Intel DC S3700 and the Samsung drives use 2Xnm-class flash, the flash vendors have gotten cagey about their latest products. They use the term 2Xnm class to mean that their cell geometries are anywhere from 20-29nm, as opposed to just telling us it's 23 or 27nm.

While these SSD drives cost a few times more than the consumer-level drives you'd put in a laptop, the additional performance, endurance and data integrity are well worth it in the data center.

Disclaimer: Intel has provided several DC S3700 and a pair of 710 SSDs for testing in the DeepStorage lab.