Finally! A Simple Metric for Solid State Drive Endurance

We've all seen the impressive MTBF (mean time between failures) specifications of SSDs versus HDDs of one to two million hours versus 600 thousand hours in a notebook pc class application. However, unlike HDDs where a mechanical failure can make the entire HDD unusable, the main failure mechanism in a SSD relates to the memory cells becoming unusable. MTBF is a statistical calculation that unfortunately, does not capture the effect of write endurance.

The write endurance of a SSD is a function of the type of flash memory used (SLC, MLC), storage capacity, frequency of writes, data sizes of the writes and the amount of static data which relates to the wear leveling algorithm. System level endurance of SSDs in the industrial, enterprise and miliary space have ranged anywhere from one million to five million program/erase cycles. There are three main problems with this.

1. The endurance will depend on how the SSD is used in the application. As a result, SSD vendors can tweak the parameters for their own marketing purposes.

2. Not all SSD vendors use the same formula for calculating the endurance and the formulas can be overly complex.

3. A straight endurance metric is nebulous and difficult to grasp its implications, especially for OEMs who've never had to grapple with endurance issues in HDDs. For example: What's the effect on the lifetime of the drive?

SanDisk aims to solve this with the Longterm Data Endurance (LDE) metric. LDE is simply defined as the total amount of data writes allowed in the lifespan of the SSD. The metric is based on the Bapco write usage pattern for a typical business user and assumes the data is written equally over the lifetime of the drive and that data is retained for one year once the LDE specification is reached.

LDE allows OEMs a simple way to compare SSDs and determine, based on the applications usage patterns which drives are suitable for a particular application. For example, a drive with an 80TBW (teraByte write) LDE can support 20GB writes per day for 10 years (equivalent to 73TBW). For an application requiring support for only half the number of writes per day (10GB), a 40TBW rated drive would be sufficient.

The beauty of LDE is that it captures endurance in one single, understandable figure. A common metric is necessary to facilitate SSD adoption moving forward. Now comes the hard part: garnering support from other SSD vendors and OEMs.

I've uploaded a copy of Don Barnetson's presentation on LDE, "Solid State Drives: The MLC Challenge" on my website at http://www.forward-insights.com/.

Seagate Fires First Volley Against SSD maker

Hard drive maker Seagate a lawsuit against solid state drive manufacturer STEC in the Northern District of California claiming STEC infringed four of Seagate’s patents related to how a SSD interfaces with computers. This is the first time a HDD manufacturer has sued a SSD maker.

It’s hard to see the financial motivation behind such a move. STEC’s enterprise SSD revenues were only $11 million in 2007, although it could reach 4-10x more this year depending on the ramp of the ZeusIOPS and Mach8 MLC product lines. With a market share of over 50% in enterprise HDDs, Seagate clearly sees a longer-term threat from the leading maker of enterprise SSDs.

However, a bigger motivation would be to send a signal to flash memory makers about the value of its intellectual property. It’s no secret that Seagate has been courting NAND flash vendors to secure NAND flash for a SSD it’s planning for the end of the year. A rumored JV with Micron fell apart last year and it is apparently in discussions with two of the three largest vendors. One of the main stumbling blocks is what Seagate could bring to any cooperation as all NAND flash vendors have ambitions to develop and market SSDs. This litigation could be a validation of the IP which Seagate can offer. Western Digital, which along with Seagate, was an early investor in SanDisk (then SunDisk) apparently holds some critical controller and wear leveling IP and could be next to enforce its IP.

SanDisk is taking no chances. In Q2/07, it set up Solid State Storage Solutions LLC with unknown partners that will license IP, presumably SSD-related IP. In July 2007, Solid State Storage Solutions LLC invested $42.5 million for the acquisition of intellectual property. It has apparently purchased relevant SSD and controller IP from Renesas Technology. This IP could prove effective in extracting royalties from flash memory card manufacturers and controller makers entering the SSD space, but it is questionable whether it is enough to counter any future claims by Seagate and WD.