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

[Heinz Mauelshagen]

On 09/18/2018 01:41 AM, tudor at tudorholton.com wrote:
> --------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 have to apologize, I meant gross size

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

The lvm2 code assumes data stripes to be more than parity/syndrome stripes.

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

Sure you could,  when expansion is planned one can create the raid6 set 
with the intended total stripes though.

That aside, this is a constraint for such use cases and we may 
eventually get rid of it.

Regards,
Heinz

>
>
> Cheers,
> Tudor.
>
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