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 X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B411FC433E0 for ; Fri, 22 May 2020 13:47:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 92A1520B80 for ; Fri, 22 May 2020 13:47:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729888AbgEVNry (ORCPT ); Fri, 22 May 2020 09:47:54 -0400 Received: from mx2.suse.de ([195.135.220.15]:38280 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729406AbgEVNry (ORCPT ); Fri, 22 May 2020 09:47:54 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 4C4B6AFB0; Fri, 22 May 2020 13:47:55 +0000 (UTC) Received: by ds.suse.cz (Postfix, from userid 10065) id B17F0DA9B7; Fri, 22 May 2020 15:46:56 +0200 (CEST) Date: Fri, 22 May 2020 15:46:56 +0200 From: David Sterba To: Anand Jain Cc: dsterba@suse.cz, dsterba@suse.com, linux-btrfs@vger.kernel.org Subject: Re: [PATCH v7 rebased 0/5] readmirror feature (sysfs and in-memory only approach; with new read_policy device) Message-ID: <20200522134656.GL18421@twin.jikos.cz> Reply-To: dsterba@suse.cz Mail-Followup-To: dsterba@suse.cz, Anand Jain , dsterba@suse.com, linux-btrfs@vger.kernel.org References: <1586173871-5559-1-git-send-email-anand.jain@oracle.com> <20200515195858.GS18421@twin.jikos.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23.1-rc1 (2014-03-12) Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On Tue, May 19, 2020 at 06:02:32PM +0800, Anand Jain wrote: > On 16/5/20 3:58 am, David Sterba wrote: > > On Thu, Apr 30, 2020 at 05:02:27PM +0800, Anand Jain wrote: > >> I am not sure if this will be integrated in 5.8 and worth the time to > >> rebase. Kindly suggest. > > > > The preparatory work is ok, but the actual mirror selection policy > > addresses a usecase that I think is not the one most users are > > interested in. Devices of vastly different performance capabilities like > > rotational disks vs nvme vs ssd vs network block devices in one > > filesystem are not something commonly found. > > > > What we really need is a saner balancing mechanism than pid-based, that > > is also going to be used any time there are more devices from the same > > speed class for the fast devices too. > > There are two things here, the read_policy framework in the preparatory > patches and a new balancing or read_policy, device. > > > So, no the patchset is not on track for a merge without the improved > > default balancing. > > It can be worked on top of the preparatory read_policy framework? Yes. > This patchset does not change any default read_policy (or balancing) > which is pid as of now. Working on a default read_policy/balancing > was out of the scope of this patchset. > > > The preferred device for reads can be one of the > > policies, I understand the usecase and have not problem with that > > although wouldn't probably have use for it. > > For us, read_policy:device helps to reproduce raid1 data corruption > https://patchwork.kernel.org/patch/11475417/ > And xfstests btrfs/14[0-3] can be improved so that the reads directly > go the device of the choice, instead of waiting for the odd/even pid. > > Common configuration won't need this, advance configurations assembled > with heterogeneous devices where read performance is more critical than > write will find read_policy:device useful. Yes that's the usecase and the possibility to make more targeted tests is also good, but that still means the feature is half-baked and missing the main part. If it was out of scope, ok fair, but I don't want to merge it at that state. It would be embarassing to announce mirror selection followed by "ah no it's useless for anything than this special usecase".