What makes this different from regular md? I'm not familiar with unRAID.
replies(2):
- It lets you throw JBODs (of ANY size) and you can create a "RAID" over them.
- The biggest drive must be a parity drive(s).
- N parity = surviving N drive failures.
- You can expand your storage pool 1 drive at a time. You need to recalculate parity for the full array.
The actual data is spread across drives. If a drive fails, you rebuild it from the parity. This is another implementation (using MergerFS + SnapRAID) https://perfectmediaserver.com/02-tech-stack/snapraid/
It's a very simple model to think of compared to something like ZFS. You can add/remove capacity AND protection as you go.
Its perf is significantly less than ZFS of course.
That's just your drive failure tolerance. It's the same risk/capacity trade as RAIDZ1, but with less performance and more flexibility on expanding. Which is exactly what I said.
If 1 drive failure isn't acceptable for you, you wouldn't use RAIDZ1 and wouldn't use 1 parity drive.
You can use 2 parity drives for RAIDZ2-like protection.
You can use 3 drives for RAIDZ3-like protection.
You can use 4 drives, 10 drives. Add and remove as many parity/capacity as you want. Can't do that with RAID/RAIDZ easily.
You manage your own risk/reward ratio
As hammyhavoc below noted, you can work around this by having cache, and 'by deferring the inevitable parity calculation until a later time (3:40 am server time, by default)'.
Which seems like a hell of a bodge -- both risky, and expensive -- now the unevenly balanced drive is the cache one, it is also not parity protected. So you need mirroring for it in case you don't want to lose your data, and the cache drives are still expected to fail before a drive in an evenly load-balanced array, so you're going to have to buy new ones?
Oh and btw you are still at risk of bit flips and garbage data due to cache not being checksum-protected.
Good, it can be the canary.
> thus you are going to need to recalculate parity for the array more often, which is a risky and taxing operation for all drives in the array
This is not worth worrying about.
First off, if the risk is linear then your increased parity failure is offset by decreased other-drive failure and I don't think you'll have more rebuilds.
And even if you do get more rebuilds, it's significantly less than one per year, and one extra full-drive read per year is a negligible amount of load. If you're worried about it all hitting at once then A) you should be scrubbing more often and B) throttle the rebuild.