From mboxrd@z Thu Jan 1 00:00:00 1970 References: From: Heinz Mauelshagen Message-ID: Date: Mon, 17 Sep 2018 14:41:44 +0200 MIME-Version: 1.0 In-Reply-To: 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 09/17/2018 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/