We're planning to move from a 4-drive RAID6 to a SSD RAID 1, and I was analyzing smartctl data to see how many GB we've writen to the drives over the past year so I can estimate the lifetime of the SSDs based on their endurance rating.
I'm thinking that if 1GB is written to the RAID 6, then to put it simply each of the 4 drives incurs 500MB of writing (250MB for data and 250MB for parity), so I should take whatever number of GB was written to each drive for the year and multiply it by 2 to get the GB per year estimate for the SSDs in the RAID1.
Does this seem correct or am I missing something? I didn't know if RAID6 has some additional write amplification due to its algorithm or something like that.
Your RAID6 description and translation to usage on a RAID1 is correct. It does not do extra writing as part of the RAID6 operation (in everything I've heard of) - everything needed would be done in the controller or software.
In general I'd have to guess you'd be safe. SSD MTBF is pretty high.
Check this out for fun - "The SSD Endurance Experiment: They're all dead"
Here's a good reference for you:
https://serverfault.com/a/5165/161412
Here's more details:
http://www.storagereview.com/guide/comp_perf_raid_levels.html
In short, RAID1 has better write performance; RAID6 has better read performance.
If you're concerned with SSD lifespan, you'll need to be aware of differing types of SSDs. In most cases, when you're writing stuff to storage, you're writing them down for long term. It's not so much "how much" you've written to storage, but more of "how many times" you've written to storage in the same spot (overwriting, deleting, etc).
If you want something with high performance, go SLC. SLC SSDs are rated for 100K writes.
If you want something to hold data for long periods of time, go MLC. MLC SSDs are rated for 2K-3K writes.
These numbers are an estimate per bit, not for the whole drive itself. Depending on how you manage your array and whether or not there's a lot of deleting and overwriting, an SSD could theoretically last you 10-20 years. If you're constantly deleting and overwriting data, an SSD could last you 2-3 months.
Because of the nature of SSDs, you may want to build a custom solution with a mix of SLC (expensive), MLC (moderately cost effective) and possibly SATA (very cost effective) drives. Use SLC for high performance writing; Use MLC for high performance reading, as in caching your data here to get a speed boost. Use SATA for long term storage of data; because most data that you keep on your system is going to just sit here for a long time anyway, and it's cheaper to replace a SATA drive than it is to replace an SLC or MLC drive.
If you end up going a route of mixing and matching drives, you may as well invest in a Nimble storage array; they do all that natively, with IOPS rating from 30K on their lowest model to over 100K on their performance model.