A real world example of SSD endurance

The decision cascade kind of went like this:

1. Huh, pure-write is hard on SSDs. We'll hit endurance problems pretty fast that way.
2. If endurance is a problem, I should look at SLC based SSDs not MLC, what with them being able to endure hundreds of thousands to millions of writes.
3. A filesystem journal is weeny, I don't need a massive device for that. That one 50GB one would be more than enough, and I could even put the OS partition on it. Yay affordability!

Made total sense!

Now 10ish months later, why don't I take a look at the actual performance data as reported by SMART attributes. The attribute I'm most interested in is "Program/Erase Cycles", which records the number of full-disk writes that have happened to the device.

In the 10ish months since these devices went into service, the average P/E Cycle count is a whopping 726. That's more than 1 a day, but less than 2.

Er, hm. Clearly I'm not dealing with as much writes as I thought I would. In fact, I'm about two orders of magnitude under my engineering maximum since these drives are rated at 50PB total write endurance. Doing the math, and each cell would have to be written to 547 times a day for 5 years to reach that.

SLC was w-a-y overkill for this application, much cheaper MLC would have been fine.

That said, between one and two full-writes a day is still a good amount of activity. A dumb MLC device with only 3,000 P/E cycles per cell and no fancy controller magic to minimize P/E cycles would hit that line in... well, math-time:

• 3,000 P/E cycles until death.
• 1.3 full-writes per day
• 3000/1.3 = 2307 days = 6.3 years

Huh.

Of course, write-amplification is going to play a factor here but figuring that out is something I'll cover later.

That dumb MLC device would last 6.3 years at current utilization, but would die a lot sooner once we hit massive scaling and its corresponding increase in utilization. So dumb MLC isn't an option. What about the enterprisey MLC offerings out there? What do they offer for endurance these days?

The number in parens is the calculated write-endurance of the cells in question.

Clearly, the Seagate 600 Pro SSD isn't up to the task, but everything else would be. Of course, this can be gamed. If I got a 600 Pro SSD that was 4x larger than I need, I'd have enough spare cells around that the effective endurance matches that of the Samsung SM843 (thanks to load-spreading, more unallocated blocks means more spare-area to spread writes around).

When we get to the point of making more nodes to scale out this system, I won't be reaching for the SLC devices again. MLC is good enough!