[linux-lvm] (Why) Does lvmcreate --raid6 need 5 drives?

[tudor at tudorholton.com]

--------snip------
> We do have a constraint in lvm2 to require the raid6 minimum for N to 
> be 3.
> 
> Configuring a raid6 LV (an array by MD terms) with 2 data stripes is
> suboptimal for performance,
> because data striping is minimal in this case.  In addition, the
> metadata overhead is maximal
> for parity, P- and Q-syndromes being half of the brutto size of the 
> raid6 LV.
> 
My apologies.  I googled "brutto size" but came up with nothing.  I do 
have a basic understanding of P and Q syndromes so I think I have a 
vague understanding of your meaning.

I understand it's suboptimal, but not non-existent.  In particular why 
is this a restriction and not just a warning?

Performance aside, I could also argue that there's a use case for a 
minimal (and not degraded) set before expansion.

Cheers,
Tudor.