01-24-2015 02:47 PM
I have 2 Extreme II 480 GB in 2 PCs running Linux.
I'd like to know why the write amplification is so high. Looking at these stats with smartmontools
233 Total_NAND_Writes_GiB 0x0032 100 100 --- Old_age Always - 3849 241 Total_Writes_GiB 0x0030 253 253 --- Old_age Offline - 1573
233 Total_NAND_Writes_GiB 0x0032 100 100 --- Old_age Always - 3584 241 Total_Writes_GiB 0x0030 253 253 --- Old_age Offline - 1284
Total NAND writes are about 2-3 times higher than host writes. I checked daily for several days and I saw 2-4 total NAND writes per host write.
At the moment, I use fstrim daily, not the discard parameter of ext4. But I also tried without fstrim for several days and still the new NAND writes per day are always 2-4 times more than the host writes.
Question #1: Why are there 2-4 GB total NAND writes per GB host write?
Question #2: What is the guaranteed life time for this SSD in TB? And does it apply to host writes or total NAND writes?
02-16-2015 09:14 AM - edited 02-16-2015 09:18 AM
It's not high. A Write Amplification of 2.43x is about normal for MLC SSDs normally used, perhaps even on the low side.
Mine (a SanDisk Extreme II 480GB like yours) has a write amplification of about 2.15x, but it's mainly because I've made lots of sequential writes, which lower the write amplification over time. With smaller loads the write amplification can be as high as 4-4.5x in some circumstances, from what I've seen. During normal usage it's about 3.0x. I'm using Windows 8.
SanDisk does not specify a guaranteed life time, but seeing that the media wearout indicator is based on a total amount of 3000 P/E cycles, with an average lifetime write amplification of 2.5x it would take about 600 terabytes of NAND writes to deplete the rated endurance. That's a lot of writes and thus, one shouldn't normally worry about them under normal (consumer / home) workloads.
When manufacturers specify a guaranteed endurance, they usually mean Host Writes, not NAND Writes. For consumer SSDs, it's usually a very conservative value to deter business / enterprise users from using them in servers or heavy duty workstations.
02-18-2015 01:24 PM
I read somewhere about some Samsung SSDs (maybe Evo, maybe Pro) and a write amplification of 1.5 or so.
Also, I think I read that SanDisk guarantees 80 TB of writes. Anyone from SanDisk can comment?
02-18-2015 08:48 PM
I remembered reading 80 TBW somewhere too, but I couldn't that find that specification anywhere on the SanDisk website for the Extreme II, and since when I sent an email to the tech support some time back I've been told that what matters for warranty is simply the warranty time and not any endurance rating I might have found on the internet, I assumed it was indeed the case.
Besides, realistically speaking, if an 80 TB endurance is valid for the 120 GB model, then the 240GB model which has got twice the amount of NAND should have a 160TB endurance, while the 480GB model should in turn be good for at least 320TB of Host Writes.
The P/E count usage percentage through SMART data also agrees with me. Who/what should I trust?