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=DKIM_INVALID,DKIM_SIGNED, 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 57CAEC433E0 for ; Mon, 22 Jun 2020 18:17:14 +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 25EB120738 for ; Mon, 22 Jun 2020 18:17:14 +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="dA5754eY" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 25EB120738 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]:36372 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jnR0L-0007Hk-E3 for qemu-devel@archiver.kernel.org; Mon, 22 Jun 2020 14:17:13 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:39910) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1jnQzK-0006UM-Sk for qemu-devel@nongnu.org; Mon, 22 Jun 2020 14:16:10 -0400 Received: from us-smtp-1.mimecast.com ([205.139.110.61]:32678) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1jnQzI-0005Vi-Fb for qemu-devel@nongnu.org; Mon, 22 Jun 2020 14:16:10 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1592849767; 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=xhc6lL4IS/i3EyXZIwaBTeAwdg/N6iGa7qhD3hnjDd8=; b=dA5754eYejFtbywsk5i9Pw043veRr4xVyTwUrN7OGbPwOTlg94DPNNHXORoPUyXjwCV6Q1 tOQXerCkPN5BphpGJ4tmwZpkfMZrivAaWnO3M9W5v+j3uanNv0E/EUIicNNygZv25SRLs+ rGVkuD8XZVsLCo15ZnBfKJXez64y3rU= 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-10-Y8JnATNWNauac3ZcmZkYog-1; Mon, 22 Jun 2020 14:15:59 -0400 X-MC-Unique: Y8JnATNWNauac3ZcmZkYog-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 7186F8005AD; Mon, 22 Jun 2020 18:15:58 +0000 (UTC) Received: from [10.3.114.4] (ovpn-114-4.phx2.redhat.com [10.3.114.4]) by smtp.corp.redhat.com (Postfix) with ESMTPS id C5C7171677; Mon, 22 Jun 2020 18:15:57 +0000 (UTC) Subject: Re: [PATCH] qemu-img convert: Don't pre-zero images To: Kevin Wolf , qemu-block@nongnu.org References: <20200622151203.35624-1-kwolf@redhat.com> From: Eric Blake Organization: Red Hat, Inc. Message-ID: Date: Mon, 22 Jun 2020 13:15:56 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.9.0 MIME-Version: 1.0 In-Reply-To: <20200622151203.35624-1-kwolf@redhat.com> Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=eblake@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=205.139.110.61; envelope-from=eblake@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/06/22 03:17:49 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=_AUTOLEARN 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: nsoffer@redhat.com, pl@kamp.de, qemu-devel@nongnu.org, mreitz@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 6/22/20 10:12 AM, Kevin Wolf wrote: > Since commit 5a37b60a61c, qemu-img create will pre-zero the target image > if it isn't already zero-initialised (most importantly, for host block > devices, but also iscsi etc.), so that writing explicit zeros wouldn't > be necessary later. > > This could speed up the operation significantly, in particular when the > source image file was only sparsely populated. However, it also means > that some block are written twice: Once when pre-zeroing them, and then > when they are overwritten with actual data. On a full image, the > pre-zeroing is wasted work because everything will be overwritten. > > In practice, write_zeroes typically turns out faster than writing > explicit zero buffers, but slow enough that first zeroing everything and > then overwriting parts can be a significant net loss. > > Meanwhile, qemu-img convert was rewritten in 690c7301600 and zero blocks > are now written to the target using bdrv_co_pwrite_zeroes() if the > target could be pre-zeroed. This way we already make use of the faster > write_zeroes operation, but avoid writing any blocks twice. > > Remove the pre-zeroing because these days this former optimisation has > actually turned into a pessimisation in the common case. Indeed, it is both data-dependent (how sparse is the source) and target-dependent (does the reduced I/O from a single pre-zeroing pass outweigh the time spent on non-zero portions). I'm fine with dropping this optimization, as we do not have proper heuristics to reliably detect when it is actually going to be a win. (It would be nice if we _could_ have reliable heuristics, but that's a story for another day) Reviewed-by: Eric Blake -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org