Today Toshiba announced a new series of SSD drives based on their 32NM MLC NAND technology. The new technology is interesting but what caught my eye was another part of their website, i.e., their SSD FAQs. We have talked about MLC NAND technology before and have discussed its inherent reliability limitations, but this is the first time I have seen some company discuss their reliability estimates so publicly. This was documented more in an IDC white paperon their site but the summary on the FAQ web page speaks to most of it.
Toshiba’s answer to the MLC write endurance question all revolves around how much data a laptop user writes per day which their study makes clear . Essentially, Toshiba assumes MLC NAND write endurance is 1,400 write/erase cycles and for their 64GB drive a user would have to write, on average, 22GB/day for 5 years before they would exceed the manufacturers warranty based on write endurance cycles alone.
- 5 years is ~1825 days
- 22GB/day over 5 years would be over 40,000GB of data written
- If we divide this by the 1400 MLC W/E cycle limits given above, that gives us something like 28.7 NAND pages could fail and yet still support write reliability.
Not sure what Toshiba’s MLC SSD supports for page size but it’s not unusual for SSDs to ship an additional 20% of capacity to over provision for write endurance and ECC. Given that 20% of 64GB is ~12.8GB, and it has to at least sustain ~28.7 NAND page failures, this puts Toshiba’s MLC NAND page at something like 512MB or ~4Gb which makes sense.
The not so surprising thing about this analysis is that as drive capacity goes up, write endurance concerns diminish because the amount of data that needs to be written daily goes up linearly with the capacity of the SSD. Toshiba’s latest drive announcements offer 64/128/256GB MLC SSDs for the mobile market.
Toshiba studies mobile users write activity
To come at their SSD reliability estimate from another direction, Toshiba’s laptop usage modeling study of over 237 mobile users showed the “typical” laptop user wrote an average of 2.4GB/day (with auto-save&hibernate on) and a “heavy” labtop user wrote 9.2GB/day under similar specifications. Now averages are well and good but to really put this into perspective one needs to know the workload variability. Nonetheless, their published results do put a rational upper bound on how much data typical laptop users write during a year that can then be used to compute (MLC) SSD drive reliability.
I must applaud Toshiba for publishing some of their mobile user study information to help us all better understand SSD reliability for this environment. It would have been better to see the complete study including all the statistics, when it was done, how users were selected, and it would have been really nice to see this study done by a standard’s body (say SNIA) rather than a manufacturer, but these are all personal nits.
Now, I can’t wait to see a study on write activity for the “heavy” enterprise data center environment, …