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=-7.1 required=3.0 tests=BAYES_00,DATE_IN_PAST_12_24, DKIM_INVALID,DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, NICE_REPLY_A,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, 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 4E9C8C43461 for ; Wed, 16 Sep 2020 12:33:47 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id CD9632224A for ; Wed, 16 Sep 2020 12:33:44 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="auLFRneY" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org CD9632224A Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:45468 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1kIWd5-0002cI-Sa for qemu-devel@archiver.kernel.org; Wed, 16 Sep 2020 08:33:43 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:34264) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1kIWOX-0002kC-5c for qemu-devel@nongnu.org; Wed, 16 Sep 2020 08:18:41 -0400 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:51120) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1kIWOU-00040Q-Aq for qemu-devel@nongnu.org; Wed, 16 Sep 2020 08:18:40 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1600258717; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=sGVWoFRRNIJjaEFjSCQuZV4/kPlzBlxwBNKlkQWqWq4=; b=auLFRneYCiLqdyqN2QaMa6qsnTB9RM0kLMRRD17ML5f8C466fas/k/axnaaTfzySHqx/Et zIe3ktXpsqkH+9H7SumWnzWr0SMM4TpF6gPQnc3VWTn1XFvjm0UiViLB2yQQHYt697koGN PSVCNBxJd8xSYgmi4L/BYsAMeptSkcE= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-302-6wdkQkF6OJWFdGKMYnbfHQ-1; Wed, 16 Sep 2020 08:18:30 -0400 X-MC-Unique: 6wdkQkF6OJWFdGKMYnbfHQ-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 2385E1084C81; Wed, 16 Sep 2020 12:18:29 +0000 (UTC) Received: from [10.3.113.91] (ovpn-113-91.phx2.redhat.com [10.3.113.91]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 733681002388; Wed, 16 Sep 2020 12:18:28 +0000 (UTC) Subject: Re: [PATCH v2] qemu-img: Support bitmap --merge into backing image To: Max Reitz , qemu-devel@nongnu.org References: <20200914191009.644842-1-eblake@redhat.com> From: Eric Blake Organization: Red Hat, Inc. Message-ID: <07832208-dcc8-8e33-c7e7-05f9c891fec3@redhat.com> Date: Tue, 15 Sep 2020 08:31:09 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.11.0 MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=eblake@redhat.com X-Mimecast-Spam-Score: 0.001 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Received-SPF: pass client-ip=63.128.21.124; envelope-from=eblake@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/09/16 02:35:56 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -40 X-Spam_score: -4.1 X-Spam_bar: ---- X-Spam_report: (-4.1 / 5.0 requ) BAYES_00=-1.9, DATE_IN_PAST_12_24=1.049, DKIMWL_WL_HIGH=-2.999, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.062, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: kwolf@redhat.com, eshenitz@redhat.com, vsementsov@virtuozzo.com, qemu-block@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 9/15/20 3:57 AM, Max Reitz wrote: > On 14.09.20 21:10, Eric Blake wrote: >> If you have the chain 'base.qcow2 <- top.qcow2' and want to merge a >> bitmap from top into base, qemu-img was failing with: >> >> qemu-img: Could not open 'top.qcow2': Could not open backing file: Failed to get shared "write" lock >> Is another process using the image [base.qcow2]? >> >> The easiest fix is to not open the entire backing chain of either >> image (source or destination); after all, the point of 'qemu-img >> bitmap' is solely to manipulate bitmaps directly within a single qcow2 >> image, and this is made more precise if we don't pay attention to >> other images in the chain that may happen to have a bitmap by the same >> name. >> >> However, note that during normal usage, it is a feature that qemu will >> allow a bitmap from a backing image to be exposed by an overlay BDS; >> doing so makes it easier to perform incremental backup, where we have: >> >> Base <- Active <- temporrary > > *temporary > > (Also it’s a bit strange that “Base” and “Active” are capitalized, but > “temporary” isn’t) > >> \--block job ->/ >> >> with temporary being fed by a block-copy 'sync' job; when exposing > > s/block-copy 'sync'/backup 'sync=none'/? Will fix both. > >> temporary over NBD, referring to a bitmap that lives only in Active is >> less effort than having to copy a bitmap into temporary [1]. So the >> testsuite additions in this patch check both where bitmaps get >> allocated (the qemu-img info output), and, when NOT using 'qemu-img >> bitmap', that bitmaps are indeed visible through a backing chain. > > Well. It is useful over NBD but I would doubt that it isn’t useful in > general. For example, the QMP commands that refer to bitmaps always do > so through a node-name + bitmap-name combination, and they require that > the given bitmap is exactly on the given node. > > So I think this is a very much a case-by-case question. (And in > practice, NBD seems to be the outlier, not qemu-img bitmap.) > I'm happy to reword slightly to give that caveat. >> [1] Full disclosure: prior to the recent commit 374eedd1c4 and >> friends, we were NOT able to see bitmaps through filters, which meant >> that we actually did not have nice clean semantics for uniformly being >> able to pick up bitmaps from anywhere in the backing chain (seen as a >> change in behavior between qemu 4.1 and 4.2 at commit 00e30f05de, when >> block-copy swapped from a one-off to a filter). Which means libvirt >> was already coded to copy bitmaps around for the sake of older qemu, >> even though modern qemu no longer needs it. Oh well. >> >> Fixes: http://bugzilla.redhat.com/1877209 >> Reported-by: Eyal Shenitzky >> Signed-off-by: Eric Blake >> --- >> >> In v2: >> - also use BDRV_O_NO_BACKING on source [Max] >> - improved commit message [Max] >> >> qemu-img.c | 11 ++++++-- >> tests/qemu-iotests/291 | 12 ++++++++ >> tests/qemu-iotests/291.out | 56 ++++++++++++++++++++++++++++++++++++++ >> 3 files changed, 76 insertions(+), 3 deletions(-) > > The code looks good to me, but I wonder whether in the commit message it > should be noted that we don’t want to let bitmaps from deeper nodes > shine through by default everywhere, but just in specific cases where > that’s useful (i.e. only NBD so far AFAIA). So is this a Reviewed-by? I'm happy to queue it through my bitmaps tree, if so. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org