NAND endurance is something that always raises questions among those considering a move to solid state storage. Even though we have showed more than once that the endurance of today's MLC NAND based SSDs is more than enough for even enterprise workloads, the misconception of SSDs having a short lifespan still lives. Back in the day when we had 3Xnm MLC NAND with 5,000 P/E cycles, people were worried about wearing our their SSDs, although there was absolutely nothing to worry about. The move to ~20nm MLC NAND has reduced the available P/E cycles to 3,000, but that's still plenty.

We have tested MLC NAND endurance before but with the release of Samsung SSD 840, we had something new to test: TLC NAND. We have explained the architectural differences between SLC, MLC and TLC NAND several times by now, but I'll do a brief recap here (I strongly recommend reading the detailed explanation if you want to truly understand how TLC NAND works):

  SLC MLC TLC
Bits per Cell 1 2 3
P/E Cycles (2Xnm) 100,000 3,000 1,000
Read Time 25us 50us ~75us
Program Time 200-300us 600-900us ~900-1350us
Erase Time 1.5-2ms 3ms ~4.5ms

The main difference is that MLC stores two bits per cell, whereas TLC stores three. This results in eight voltage states instead of four (also means that one TLC cell has eight possible data values). Voltages used to program the cell are usually between 15V and 18V, so there isn't exactly a lot room to play with when you need to fit twice as many voltage states within the same space. The problem is that when the cell gets cycled (i.e. programmed and erased), the room taken by one voltage state increases due to electron trapping and current leakage. TLC can't tolerate as much change in the voltage states as MLC can because there is less voltage headroom and you can't end up in a situation where two voltage states become one (the cell wouldn't give valid values because it doesn't know if it's programmed as "110" or "111" for example). Hence the endurance of TLC NAND is lower; it simply cannot be programmed and erased as many times as MLC NAND and thus you can't write as much to a TLC NAND based SSD.

No manufacturer has openly wanted to discuss the endurance of TLC, so the numbers we have seen before have been educated guesses. 1,000 - 1,500 P/E cycles is what I've heard for TLC NAND. The reality can also be different from what manufacturers claim as we discovered in the Intel SSD 335 (though there is a high probability that it's just a firmware bug), so actually testing the endruance is vital. 

There was one obstacle, though. Samsung does not report NAND writes like Intel does and without NAND writes we can't know for sure how much data is written to the NAND because of write amplification. Fortunately, there is a a workaround: I wrote incompressible 128KB sequential data (QD=1) to the drive and took down the duration of each run and the Wear Leveling Count (similar to Media Wear Indicator). If I know the average write speed and the duration, I can figure out how much I wrote to the drive. Sequential large block-size data should also result in write amplification near 1x because the data is sequential and thus doesn't fragment the drive. I then compared the amount of data I wrote to the WLC values I had recorded:

Samsung SSD 840 (250GB) Endurance Testing
Total Amount of Data Written 92,623 GiB
Total Amount of WLC Exhausted 34
Estimated Total Amount of P/E Cycles 1,064
Estimated Total Write Endurance 272,420 GiB

It seems that 1,000 P/E cycles is indeed accurate. The raw Wear Leveling Count seems to indicate the amount of exhausted P/E cycles as it's inversely proportional to the normalized WLC value and once it hits 1,000, the WLC will hit zero. 

Note that if Samsung's WLC is anything like Intel's Media Wear Indicator, when the normalized counter value drops to 0 there's still a good amount of endurance actually left on the NAND (it could be as high as another 20 - 30%). At least with Intel drives, the MWI hitting 0 is a suggestion that you may want to think about replacing the drive and not a warning of imminent failure.

Conclusions

1,000 P/E cycles may not sound much but when it's put into perspective, it's still plenty. Client workloads rarely exceed 10GiB of writes per day on average and write amplification should stay within reasonable magnitudes as well:

SSD Lifetime Estimation
NAND MLC—3K P/E Cycles TLC—1K P/E Cycles
NAND Capacity 128GiB 256GiB 128GiB 256GiB
Writes per Day 10GiB 10GiB 10GiB 10GiB
Write Amplification 3x 3x 3x 3x
Total Estimated Lifespan 35.0 years 70.1 years 11.7 years 23.4 years

Of course, if you write 20GiB a day, the estimated lifespan will be halved, although we are still looking at several years. Even with 30GiB of writes a day the 256GiB TLC drive should be sufficient in terms of endurance. Write amplification can also go over 10x if your workload is heavily random write centric, but that is more common in the enterprise side - client workloads are usually much lighter. 

Furthermore, it should be kept in mind that all SMART values that predict lifespan are conservative; it's highly unlikely that your drive will drop dead once the WLC or MWI hits zero. There is a great example at XtremeSystems where a 256GB Samsung SSD 830 is currently at nearly 6,000TiB of writes. Its WLC hit zero at 828TiB of writes, which means its endurance is over seven times higher than what the SMART values predicted. That doesn't mean all drives are as durable but especially SSDs from NAND manufacturers (e.g. Intel, Crucial/Micron, Samsung etc.) seem to be more durable than what the SMART values and datasheets indicate, which isn't a surprise given that they can cherry-pick the highest quality NAND chips.

POST A COMMENT

46 Comments

View All Comments

  • jonjonjonj - Friday, November 16, 2012 - link

    im in the market for a 256GB ssd and wanted to get a 830. when the 840 was first release the price of 830's dropped. you could get a 256GB 830 for as low as $150. the problem is the 830's price went back up and now the 840 is now cheaper then the 830. not to mention the 830 is sold out at newegg. samsung shows a MSRP of $259.99 for the 840 on their website but you can get it $80-100 cheaper. do they know something we dont? the 840 pro is too expensive compared to the other drives. it just concerns me that the 840 is cheaper then the 830 and im not sure what to do. Reply
  • MrSpadge - Friday, November 16, 2012 - link

    It's using TLC NAND and a more advanced (smaller) process technology. This is done to bring the cost down, so when ever the yield in the new process is good enough, cost should naturally be lower than the elder brother. Reply
  • jonjonjonj - Friday, November 16, 2012 - link

    thats the whole problem. i would rather buy a MLC samsung/marvell drive for the same price as the TLC 840. i figured the retailers would be trying to get rid of their 830's but the prices are going up as the 840's price got slashed. you can get an 840 for $150 right now. Reply
  • Impulses - Friday, November 16, 2012 - link

    Then get an M4 or Plextor M5... I bought a second 830a few weeks ago because I knew the 840 was being positioned as it's direct price replacement and the 840 Pro would carry a premium for a while. Reply
  • MadMan007 - Friday, November 16, 2012 - link

    The lower price of the 840 is simply MSRP versus street price. Reply
  • Beenthere - Friday, November 16, 2012 - link

    With the compatibility and SSD reliability issues, NAND endurance is of little concern. Most folks will be replacing their SSD numerous times over the course of 3-5 years due to reliability issues. When consumer grade SSDs become as reliable as enterprise class SSDs, then NAND eundurance might be of concern. Consumer grade SSDs are disposable goods right now. Reply
  • fuzzymath10 - Friday, November 16, 2012 - link

    Many of us have been running first-gen X25-M drives for 3+ years, and will probably continue to do so for a few more, since 80GB is still sufficient for a system drive. I think the capacity will be the first restriction resulting in replacement; speed and reliability will be secondary issues unless you own an SSD for the sole purpose of benchmarking. Reply
  • Iketh - Saturday, November 17, 2012 - link

    you've been purchasing the wrong SSDs haven't you Reply
  • Death666Angel - Saturday, November 17, 2012 - link

    Still running my original OCZ Agility and OCZ Vertex 2 here without issues. Reply
  • iwod - Saturday, November 17, 2012 - link

    is there a software that records how many data i actually write per day or over a week?

    With a 16GB Memory system and No Virtual Memory. While Windows 7/8 are already doing its job trying to prefect as much data as possible i rarely sees my system memory being fully ulterised. So i am guessing i am doing very little write per day.
    Reply

Log in

Don't have an account? Sign up now