I wouldn't favour that HW RAID solution either.
That really depends. Infinite endurance. Hard to get any other way. That 128GB Samsung Pro has 150TBW write endurance. That's pretty pathetic; if you were making a VMware storage server in the several dozen TB range or larger, I'd be worried that you could blow through that within the first year.
The cheap SSDs still have their internal write cache. Writes to that cache are ack'd to the parent layers but in a case of power loss the data is *gone*, because said on-SSD write cache is not power loss protected.
Ok, good catch, I don't really keep track of that on the non-Intels and didn't pull the specs until this post. Getting lazy in my old age.
Also 128GB SSDs have less cells -> less write speed... and what do you want with a SLOG? write speed.
Unconvincing argument. Properly underprovisioned to a reasonable size, you are giving the drive controller a chance to maintain a massive pile of free pages. I actually asked for this to be supported years ago (see
https://bugs.freenas.org/issues/2365 ) and given that most SSD's can easily outpace gigE these days, all you really need is to make sure that you're not running that pool out of space.
I note that the Samsung Pro's write speed is 470MB/sec for 128GB and 520MB/sec for the others. This is effectively very near the SATA 6Gbps practical limit, so while you could go a LITTLE faster by bumping up to the 256MB model, it is a trite difference, and in a scenario where the speed difference actually mattered on a consistent basis, you'd be hitting the 150TBW write endurance limit quickly, I'd think...!
The
Intel 750 is a tad better at 219TBW ... but isn't limited by SATA and sports 230K IOPS at 900MB/sec write speeds, 20us (read's obviously irrelevant for SLOG).
For endurance, the Intel P3700 is a hell of a thing. The
400GB model sports 7.3PBW, with the higher capacity versions increasing at least linearly. 75K IOPS at 1080MB/sec write speeds, 25us.
I've gone with my typical selection of the random write speeds rather than sequential write speeds, because I'm a pessimist.
Now, what this really comes down to is how much data is getting written to the pool. Some of us have pools that do nothing but soak up backups. So if you've got a pool that's 150TB, and you were cramming data at it at 10GbE speeds, you could be writing 100TB/day to it. Even with the P3700 400GB unit, you will exceed the PBW rating within three months, and you can measure that cost in dollars.
You can also get some zippy enterprise HDD's in RAID somethingorother (0, 10...) and gain virtually infinite endurance. The question basically becomes one of cost and whether or not one needs the endurance.
Give it another year and the flash will likely be the winner here.