Vanja <vanjab <at> gmail.com> writes:
> 
> And finally, if this is the case, is it possible to make an array with
> 3 drives, and then add the mirror later?

I assume you are asking if it is possible to create a temporary 3-way raidz, 
then transfer your data to it, then convert it to a 4-way raidz ? No it is not 
possible. However here is one solution (let's call your 4 drives A, B, C and 
D -- your current data is on D).

1. Slice up A, B, C in 2 halves each: A1 and A2, B1 and B2, C1 and C2.
2. Create a 3-way raidz on A2, B2 and C2.
3. Copy your data from D to the 3-way raidz.
4. Slice up D in 2 halves: D1 and D2.
5. Create a 4-way raidz on A1, B1, C1, D1.
6. Copy your data from the 3-way raidz to the 4-way raidz.
7. Destroy the A2, B2, C2, D2 slices.
8. Grow A1, B1, C1, D1 to extend over the remaining disk space.
9. Export and reimport the 4-way raidz to make it register the extra space.

Yes that's a lot of steps. But that's also the safest solution as your data 
would never transit on a (temporarily) degraded raidz (as it is possible for 
example on Linux by creating a raid5 MD with one "missing" component).

-marc


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