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/.