[linux-lvm] (Why) Does lvmcreate --raid6 need 5 drives?
heinzm at redhat.com
Mon Sep 17 12:41:44 UTC 2018
On 09/17/2018 01:36 AM, tudor at tudorholton.com wrote:
> This question was originally put on ServerFault here:
> I'm investigating using LVMRAID instead of MD+LVM. In my case, I'm
> looking at a RAID6. As expected, the docs say you need N+1 devices for
> RAID5, and a minimum of 3 drives. The docs say for RAID 6 you need N+2.
> However, it then says that the minimum drives required is 5, which
> means that N is different.
> In an MD RAID6, only 4 drives are required; 2 for striping, 2 for
> parity. So if I built RAID6+LVM I'd only need 4 drives whereas,
> according to the docs, I'd need 5 drives for LVMRAID6. It was my
> understanding that LVMRAID is really just LVM on MD, so I don't
> understand the difference.
> Thomas on ServerFault pointed out that a minimum of 3 stripes is
> hardcoded into the source code.
> But the commit message doesn't seem to indicate a reasoning behind
> requiring a higher number of striped for RAID6 than RAID5.
> Why does LVMRAID need 3 stripes for RAID6 where MD only needs 2?
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
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> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
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