From mboxrd@z Thu Jan 1 00:00:00 1970 References: <086f04e0e43d27d5637a7516c4a27877@tudorholton.com> From: Heinz Mauelshagen Message-ID: <3f93f978-235f-214c-09cf-2b839b6048a3@redhat.com> Date: Tue, 18 Sep 2018 11:14:02 +0200 MIME-Version: 1.0 In-Reply-To: <086f04e0e43d27d5637a7516c4a27877@tudorholton.com> Content-Transfer-Encoding: 8bit Content-Language: en-MW 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="utf-8"; format="flowed" To: LVM general discussion and development , tudor@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/