Part 1: eMLC – It’s Not Just Marketing
Until recently, the model for Flash implementation has been to use SLC for the enterprise and MLC for the consumer. MLC solutions traded endurance, performance, and reliability for a lower cost while SLC solutions didn’t. The tradeoff of 10x the endurance for 2x the price led most enterprise applications to adopt SLC.
But there is a shift taking place in the industry as SSD prices start to align with the prices enterprise customers have been paying for tier 1 HDD storage (which is much higher than the cost of consumer drives). If the per-GB pricing is similar, you can add so much more capacity that endurance becomes less of a concern. Rather than only having highly transactional OLTP systems on SSDs, you can move virtually every application using tier 1 HDD storage to SSD. The biggest concern for tier 1 storage has been that the most critical datasets that reside on them cannot risk a full 10x drop in endurance.
Closing the gap: eMLC
At first glance, enterprise MLC (eMLC) sounds like Marketing is trying to pull a fast one. If there was a simple way to make MLC have higher endurance, why bother restricting it from consumer applications? eMLC sports endurance levels of 30,000 write cycles, whereas some of the newest MLC only has 3,000 write cycles (SLC endurance is generally 100,000 write cycles). There is a big reason for this restriction: eMLC makes a tradeoff to enable this endurance – retention.
It’s not commonly understood that although Flash is considered persistent, the data is slowly lost over time. For most Flash chips the retention is around 10 years – longer than most use cases. With eMLC, longer program and erase times are used with different voltage levels than MLC to increase the endurance. These changes reduce the retention to as low as three months for eMLC. This is plenty of retention for an enterprise storage system that can manage the Flash behind the scenes, but it makes eMLC impractical for consumer applications. Imagine if you didn’t get around to copying photos from your camera within a three month window and lost all the pictures!
Today, Texas Memory Systems announced its first eMLC RamSan product: the RamSan-810. This is a major announcement as we have investigated eMLC for some time (I have briefed analysts on eMLC for almost a year; here is a discussion of eMLC from Silverton Consulting, and a recent one from Storage Switzerland). TMS was not the first company to introduce an eMLC product as the RamSan’s extremely high performance backplane and interfaces make endurance concerns more palpable. However, with the latest eMLC chips, we aggregate enough capacity to be comfortable introducing an eMLC product that bears the RamSan name.
10 TB of capacity multiplied by 30,000 write cycles equates to 300 PB of lifetime writes. This amount of total writes is difficult to achieve during the lifetime of a storage device, even at the high bandwidth the RamSan-810 can support. Applications that demand the highest performance and those with more limited capacity requirements will still be served best by an SLC solution, but very high capacity SSD deployments will shift to eMLC. With a density of 10 TB per rack unit, petabyte scale SSD deployments are now a realistic deployment option.
We’re just getting warmed up discussing eMLC. Stay tuned for another post soon on tier 1 disk pricing vs. eMLC.