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: Tue, 18 Sep 2018 11:14:02 +0200 [thread overview]
Message-ID: <3f93f978-235f-214c-09cf-2b839b6048a3@redhat.com> (raw)
In-Reply-To: <086f04e0e43d27d5637a7516c4a27877@tudorholton.com>
On 09/18/2018 01:41 AM, tudor@tudorholton.com wrote:
> --------snip------
>> 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.
Regards,
Heinz
>
>
> Cheers,
> Tudor.
>
> _______________________________________________
> 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/
prev parent reply other threads:[~2018-09-18 9:14 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
2018-09-17 23:41 ` tudor
2018-09-18 9:14 ` Heinz Mauelshagen [this message]
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