From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mx1.redhat.com (ext-mx09.extmail.prod.ext.phx2.redhat.com [10.5.110.38]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 3B98682218 for ; Mon, 17 Sep 2018 23:41:28 +0000 (UTC) Received: from server1.tudorejo.org (server1.tudorejo.org [172.104.8.48]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id CF10C3A26E for ; Mon, 17 Sep 2018 23:41:26 +0000 (UTC) Received: from [::1] (port=40122 helo=server1.tudorejo.org) by server1.tudorejo.org with esmtpa (Exim 4.91) (envelope-from ) id 1g238v-0001k0-LT for linux-lvm@redhat.com; Mon, 17 Sep 2018 23:41:25 +0000 MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Date: Tue, 18 Sep 2018 09:41:25 +1000 From: tudor@tudorholton.com In-Reply-To: References: Message-ID: <086f04e0e43d27d5637a7516c4a27877@tudorholton.com> Subject: Re: [linux-lvm] (Why) Does lvmcreate --raid6 need 5 drives? Reply-To: LVM general discussion and development List-Id: LVM general discussion and development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , List-Id: Content-Type: text/plain; charset="iso-8859-1"; format="flowed" To: LVM general discussion and development --------snip------ > We do have a constraint in lvm2 to require the raid6 minimum for N to=20 > be 3. >=20 > 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.=C2=A0 In addition, the > metadata overhead is maximal > for parity, P- and Q-syndromes being half of the brutto size of the=20 > raid6 LV. >=20 My apologies. I googled "brutto size" but came up with nothing. I do=20 have a basic understanding of P and Q syndromes so I think I have a=20 vague understanding of your meaning. I understand it's suboptimal, but not non-existent. In particular why=20 is this a restriction and not just a warning? Performance aside, I could also argue that there's a use case for a=20 minimal (and not degraded) set before expansion. Cheers, Tudor.