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

From: tudor@tudorholton.com
To: linux-lvm@redhat.com
Subject: [linux-lvm] (Why) Does lvmcreate --raid6 need 5 drives?
Date: Mon, 17 Sep 2018 09:36:08 +1000	[thread overview]
Message-ID: <c0a0c50159e74ffc44bf813b82673f1b@tudorholton.com> (raw)

This question was originally put on ServerFault here: 
https://serverfault.com/questions/931150/why-does-lvmraid6-need-5-drives

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[1].

But the commit message[2] 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?

Cheers,
Tudor.

[1] 
https://sourceware.org/git/?p=lvm2.git;a=blob;f=tools/lvcreate.c;h=3c22794b6f1c3cbd155df65421de21dac0bfa9fc;hb=HEAD#l528
[2] 
https://sourceware.org/git/?p=lvm2.git;a=commit;f=tools/lvcreate.c;h=ea0f604e704dc8418d962e20529fd1bcddf06bb7