From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4BB81C433EF for ; Sat, 5 Mar 2022 09:05:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229615AbiCEJGS (ORCPT ); Sat, 5 Mar 2022 04:06:18 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47342 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229542AbiCEJGR (ORCPT ); Sat, 5 Mar 2022 04:06:17 -0500 X-Greylist: delayed 527 seconds by postgrey-1.37 at lindbergh.monkeyblade.net; Sat, 05 Mar 2022 01:05:27 PST Received: from mr4.vodafonemail.de (mr4.vodafonemail.de [145.253.228.164]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 530D1506F1 for ; Sat, 5 Mar 2022 01:05:27 -0800 (PST) Received: from smtp.vodafone.de (unknown [10.0.0.2]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits)) (No client certificate requested) by mr4.vodafonemail.de (Postfix) with ESMTPS id 4K9dr70BQ4z1xyg; Sat, 5 Mar 2022 08:56:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=nexgo.de; s=vfde-smtpout-mb-15sep; t=1646470599; bh=iMelwn/UztAG4srYQBADI6jWSiHQ1Y69WSiF83MIPRw=; h=Date:From:To:Subject:Message-ID:References:Content-Type: In-Reply-To:From; b=YR0NIQROFrxRjL6NvPv6xfqSxCWhhWJtFrl2kO4Ie/pKdBeC/G1SKvI+8OlJ7C0Qg NdegXsJIqJ1F/iHmgG86y8V6zfKYpBHy+PGt30/K+bLtseg5NdlqTT57gTZ21LzLgh 5raX1RsmLfRyTCvF11wwXutVOaHJvUvlPlIIrlRg= Received: from lazy.lzy (p5dd47cf0.dip0.t-ipconnect.de [93.212.124.240]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by smtp.vodafone.de (Postfix) with ESMTPSA id 4K9dr35SvmzMkrs; Sat, 5 Mar 2022 08:56:32 +0000 (UTC) Received: from lazy.lzy (localhost [127.0.0.1]) by lazy.lzy (8.17.1/8.14.5) with ESMTPS id 2258uV7O003428 (version=TLSv1.3 cipher=TLS_AES_256_GCM_SHA384 bits=256 verify=NO); Sat, 5 Mar 2022 09:56:31 +0100 Received: (from red@localhost) by lazy.lzy (8.17.1/8.16.1/Submit) id 2258uVFv003427; Sat, 5 Mar 2022 09:56:31 +0100 Date: Sat, 5 Mar 2022 09:56:31 +0100 From: Piergiorgio Sartor To: "Kengo.M" Cc: linux-raid@vger.kernel.org Subject: Re: Number of parity disks Message-ID: References: <20220304175556.407719-1-hch@lst.de> <20220304175556.407719-2-hch@lst.de> <20220304221255.GL3927073@dread.disaster.area> <20220305051929.GA24696@lst.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-purgate-type: clean X-purgate: clean X-purgate-size: 1094 X-purgate-ID: 155817::1646470598-00000483-3D828DEF/0/0 Precedence: bulk List-ID: X-Mailing-List: linux-raid@vger.kernel.org On Sat, Mar 05, 2022 at 03:09:55PM +0900, Kengo.M wrote: > Hi Folks > > I use mdadm conveniently. > > Let me ask a very very basic question. > > One parity disk can be used in RAID 5 and 2 parity disks can be used in RAID 6. > ZFS RAIDZ-3 (raidz3) can use 3 parity disks. > > Is it difficult to increase the number of parity disks to 4, 5 or more. > If so, is the reason for this because of the time it takes to generate the > parity bits? There was a project, some times ago, with the aim to create a library, initially for btrfs, capable of adding "n" parities. This should have been usable by md as well. The problem is that the Vandermonde matrix might not invertible with more than 2 parities. So, they've to use a Cauchy matrix. It might be that performances are degrading quite a lot. I'm not sure about the status of such library, maybe you can ask in the btrfs mailing list. On the other hand, 3 parities (or more) case might be a bit an overkill. For md could be better to have things like RAID-61 or similar. bye, -- piergiorgio