So the X25-M shouldn't be short on performance, but what about longevity? MLC-based flash memory cells are limited to 10,000 write-erase cycles, giving solid-state drives a finite lifespan. When estimating the operating life of their drives, other SSD makers generally rely on a basic formula to calculate the number of cycles used:
Cycles = (Host writes) / (Drive capacity)
Intel says this formula oversimplifies the issue, and that two other factors must be considered. The first of these variables is write amplification, which refers to the amount of data actually written to a drive for a given write request. Intel gives an example in which a host system generates a 4KB write request that, thanks to a drive's 128KB erase block size, actually incurs a 128KB NAND write. Dividing the NAND write size by the request size yields the amplification factor, which is 32 in this case. Intel says the X25-M's write-amplification factor is extremely low at 1.1, while "traditional" SSDs have much higher amplification factor of 20.
The efficiency of wear-leveling algorithms also has a hand in determining an SSD's lifespan. If a drive is going to shuffle bits around to avoid bad cells and more efficiently use those available, it must do so without wasting precious write-erase cycles. Intel estimates the X25-M's wear-leveling efficiency factor at less than 1.1, claiming that traditional SSDs have an efficiency factor of 3.
Taking write-amplification and wear-leveling efficiency into account, Intel says the correct formula for cycling is as follows:
Cycles = (Host writes) * (Write amplification factor) * (Wear leveling factor) / (Drive capacity)
Using a write-amplification factor of 1.1 and a wear-leveling efficiency factor of 1.1, 20GB of write-erase per day for five years should consume only about 550 cycles on an 80GB X25-M. Using "traditional" SSD technology with an amplification factor of 20 and an efficiency factor of 3, the same write-erase load would use over 27,000 cycles. That's a huge difference, and to be fair, it's one that relies on values provided by Intel that aren't entirely consistent. Another Intel presentation from IDF estimates that "mediocre" SSDs have a write-amplification factor of 10 and a wear-leveling efficiency factor of 5, resulting in just under 23,000 cycles for our 20GB of write-erase per day example. That presentation also pegs the X25-M's efficiency factor at 1.04 rather than 1.1. We can't easily test a drive's lifespan ourselves, but we did ask Samsung for the write-amplification and wear-leveling efficiency factor values for its SSDs. Samsung hasn't responded yet, though.
If you don't want to crunch through the math, Intel estimates that the 80GB X25-M will last for five years with "much greater than" 100GB of write-erase per day. That's a relatively long time for much more data than most folks are likely to write or erase on a daily basis.
Actual drive lifespans aside, Intel rates the X25-M's Mean Time Between Failures (MTBF) at 1.2 million hours. That's competitive with the MTBF rating of other MLC-based flash drives and equivalent to common MTBF ratings for enterprise-class mechanical hard drives.
