[linux-lvm] (Why) Does lvmcreate --raid6 need 5 drives?
heinzm at redhat.com
Tue Sep 18 09:14:02 UTC 2018
On 09/18/2018 01:41 AM, tudor at tudorholton.com wrote:
>> 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.
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