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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham 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 64B21C43441 for ; Thu, 15 Nov 2018 23:15:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2C0872133D for ; Thu, 15 Nov 2018 23:15:44 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 2C0872133D Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726996AbeKPJZe (ORCPT ); Fri, 16 Nov 2018 04:25:34 -0500 Received: from mx1.redhat.com ([209.132.183.28]:34922 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725860AbeKPJZe (ORCPT ); Fri, 16 Nov 2018 04:25:34 -0500 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 17F9630832CF; Thu, 15 Nov 2018 23:15:42 +0000 (UTC) Received: from file01.intranet.prod.int.rdu2.redhat.com (file01.intranet.prod.int.rdu2.redhat.com [10.11.5.7]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 3B052611BF; Thu, 15 Nov 2018 23:15:35 +0000 (UTC) Received: from file01.intranet.prod.int.rdu2.redhat.com (localhost [127.0.0.1]) by file01.intranet.prod.int.rdu2.redhat.com (8.14.4/8.14.4) with ESMTP id wAFNFYxa027298; Thu, 15 Nov 2018 18:15:34 -0500 Received: from localhost (mpatocka@localhost) by file01.intranet.prod.int.rdu2.redhat.com (8.14.4/8.14.4/Submit) with ESMTP id wAFNFY4e027294; Thu, 15 Nov 2018 18:15:34 -0500 X-Authentication-Warning: file01.intranet.prod.int.rdu2.redhat.com: mpatocka owned process doing -bs Date: Thu, 15 Nov 2018 18:15:34 -0500 (EST) From: Mikulas Patocka X-X-Sender: mpatocka@file01.intranet.prod.int.rdu2.redhat.com To: Paul Lawrence cc: Alasdair Kergon , Mike Snitzer , linux-doc@vger.kernel.org, kernel-team@android.com, Jonathan Corbet , linux-kernel@vger.kernel.org, dm-devel@redhat.com, Shaohua Li Subject: Re: [dm-devel] [RFC] dm-bow working prototype In-Reply-To: Message-ID: References: <20181023212358.60292-1-paullawrence@google.com> <20181023221819.GB17552@agk-dp.fab.redhat.com> <296148c2-f2d9-5818-ea76-d71a0d6f5cd4@google.com> <4a934593-1bd1-be5f-35c0-945c42762627@google.com> User-Agent: Alpine 2.02 (LRH 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.44]); Thu, 15 Nov 2018 23:15:42 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 29 Oct 2018, Paul Lawrence wrote: > > > The snapshot target could be hacked so that it remembers space trimmed > > with REQ_OP_DISCARD and won't reallocate these blocks. > > > > But I suspect that running discard over the whole device would degrade > > performance more than copying some unneeded data. > > > > How much data do you intend to backup with this solution? > > > > > We are space-constrained - we will have to free up space for the backup before > we apply the update, so we have to predict the size and keeping usage as low > as possible is thus very important. > > Also, we've discussed the resizing requirement of the dm-snap solution and > that part is not attractive at all - it seems it would be impossible to > guarantee that the resizing happens in a timely fashion during the (very busy) > update cycle. > > Thanks everyone for the insights, especially into how dm-snap works, which I > hadn't fully appreciated. At the moment, and for the above reasons, we intend > to continue with the dm-bow solution, but do want to keep this discussion > open. If anyone is going to be at Linux Plumbers, I'll be presenting this work > and would love to chat about it more. dm-snapshot took 9 years to fix the last data corruption bug (2004-2013 - the commit e9c6a182649f4259db704ae15a91ac820e63b0ca). And with the new target duplicating the snapshot functionality, it may be the same. Mikulas