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From: Heinz Mauelshagen <>
To: LVM general discussion and development <>,
Subject: Re: [linux-lvm] (Why) Does lvmcreate --raid6 need 5 drives?
Date: Mon, 17 Sep 2018 14:41:44 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On 09/17/2018 01:36 AM, 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.

That's correct.

> 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?

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.


> Cheers,
> Tudor.
> [1] 
> [2] 
> _______________________________________________
> linux-lvm mailing list
> read the LVM HOW-TO at

  reply	other threads:[~2018-09-17 12:41 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-09-16 23:36 [linux-lvm] (Why) Does lvmcreate --raid6 need 5 drives? tudor
2018-09-17 12:41 ` Heinz Mauelshagen [this message]
2018-09-17 23:41   ` tudor
2018-09-18  9:14     ` Heinz Mauelshagen

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