From: Heinz Mauelshagen <heinzm@redhat.com>
To: LVM general discussion and development <linux-lvm@redhat.com>,
tudor@tudorholton.com
Subject: Re: [linux-lvm] (Why) Does lvmcreate --raid6 need 5 drives?
Date: Mon, 17 Sep 2018 14:41:44 +0200 [thread overview]
Message-ID: <a032c5fd-5381-49f3-0b16-a9baac9b5e43@redhat.com> (raw)
In-Reply-To: <c0a0c50159e74ffc44bf813b82673f1b@tudorholton.com>
On 09/17/2018 01:36 AM, tudor@tudorholton.com wrote:
> 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.
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.
Regards,
Heinz
>
> 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
>
> _______________________________________________
> linux-lvm mailing list
> linux-lvm@redhat.com
> https://www.redhat.com/mailman/listinfo/linux-lvm
> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
next prev parent 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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