From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from aserp1040.oracle.com ([141.146.126.69]:27940 "EHLO aserp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932334AbcHOOa7 (ORCPT ); Mon, 15 Aug 2016 10:30:59 -0400 Subject: Re: About minimal device number for RAID5/6 To: "Austin S. Hemmelgarn" , Qu Wenruo , btrfs References: <9fb53c9f-b6c1-6bb9-8c3b-7147b64b601f@oracle.com> From: Anand Jain Message-ID: <089d5900-8915-bf74-ffb3-044be11b04ae@oracle.com> Date: Mon, 15 Aug 2016 22:32:25 +0800 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Sender: linux-btrfs-owner@vger.kernel.org List-ID: On 08/15/2016 10:10 PM, Austin S. Hemmelgarn wrote: > On 2016-08-15 10:08, Anand Jain wrote: >> >> >>>> IMHO it's better to warn user about 2 devices RAID5 or 3 devices RAID6. >>>> >>>> Any comment is welcomed. >>>> >>> Based on looking at the code, we do in fact support 2/3 devices for >>> raid5/6 respectively. >>> >>> Personally, I agree that we should warn when trying to do this, but I >>> absolutely don't think we should stop it from happening. >> >> >> How does 2 disks RAID5 work ? > One disk is your data, the other is your parity. >In essence, it works > like a really computationally expensive version of RAID1 with 2 disks, > which is why it's considered a degenerate configuration. How do you generate parity with only one data ? -Anand > Three disks in > RAID6 is similar, but has a slight advantage at the moment in BTRFS > because it's the only way to configure three disks so you can lose two > and not lose any data as we have no support for higher order replication > than 2 copies yet.