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=-2.4 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, 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 8A5F3C433E0 for ; Tue, 9 Jun 2020 14:47:34 +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 4AED82072F for ; Tue, 9 Jun 2020 14:47:34 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="S8PkK5QZ" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 4AED82072F 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]:34786 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jifXJ-0006bg-Hr for qemu-devel@archiver.kernel.org; Tue, 09 Jun 2020 10:47:33 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:59008) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1jifWI-0006BU-Et for qemu-devel@nongnu.org; Tue, 09 Jun 2020 10:46:30 -0400 Received: from us-smtp-2.mimecast.com ([207.211.31.81]:26000 helo=us-smtp-delivery-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1jifWH-0005MS-Jr for qemu-devel@nongnu.org; Tue, 09 Jun 2020 10:46:30 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1591713989; 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=ddVBo13rJbPYLZScmOuRArd6DNdVPEKHdtA9qP5PgUU=; b=S8PkK5QZRSw9pALQ4hb8Yc8CUqSClGGZi6VYJIEkn+8pIn7OLpxrlwAV95AMOIyLc6Yi4m n3m3ShHcg1w20zH9ZtHMVMYOlZYLfJTYPDon28vAp5rstBerrqYDR16Wi8oBhvozAttHVO 3zZPtv7afX2lRmDlJy1NQMOf+Ny3DjM= 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-145-yX5iXfFvMVulsVjD0bQ8yw-1; Tue, 09 Jun 2020 10:46:27 -0400 X-MC-Unique: yX5iXfFvMVulsVjD0bQ8yw-1 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 mimecast-mx01.redhat.com (Postfix) with ESMTPS id CE53410CE781; Tue, 9 Jun 2020 14:46:25 +0000 (UTC) Received: from [10.3.113.22] (ovpn-113-22.phx2.redhat.com [10.3.113.22]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 190738FF60; Tue, 9 Jun 2020 14:46:25 +0000 (UTC) Subject: Re: [PATCH] qcow2: Reduce write_zeroes size in handle_alloc_space() To: Vladimir Sementsov-Ogievskiy , Kevin Wolf , qemu-block@nongnu.org References: <20200609140859.142230-1-kwolf@redhat.com> <02e24dca-99da-873d-8425-09a07571e675@virtuozzo.com> From: Eric Blake Organization: Red Hat, Inc. Message-ID: <042f0b8f-dd51-acc3-8498-ac9a5532df15@redhat.com> Date: Tue, 9 Jun 2020 09:46:24 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.8.0 MIME-Version: 1.0 In-Reply-To: <02e24dca-99da-873d-8425-09a07571e675@virtuozzo.com> Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=207.211.31.81; envelope-from=eblake@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/06/08 23:42:34 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=0.001, 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_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_PASS=-0.001, URIBL_BLOCKED=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: "Denis V. Lunev" , anton.nefedov@virtuozzo.com, qemu-devel@nongnu.org, mreitz@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 6/9/20 9:28 AM, Vladimir Sementsov-Ogievskiy wrote: > 09.06.2020 17:08, Kevin Wolf wrote: >> Since commit c8bb23cbdbe, handle_alloc_space() is called for newly >> allocated clusters to efficiently initialise the COW areas with zeros if >> necessary. It skips the whole operation if both start_cow nor end_cow >> are empty. However, it requests zeroing the whole request size (possibly >> multiple megabytes) even if only one end of the request actually needs >> this. >> >> This patch reduces the write_zeroes request size in this case so that we >> don't unnecessarily zero-initialise a region that we're going to >> overwrite immediately. >> > > Hmm, I'm afraid, that this may make things worse in some cases, as with > one big write-zero request > we preallocate data-region in the protocol file, so we have better > locality for the clusters we > are going to write. And, in the same time, with BDRV_REQ_NO_FALLBACK > flag write-zero must be > fast anyway (especially in comparison with the following write request). > >>           /* >>            * instead of writing zero COW buffers, >>            * efficiently zero out the whole clusters >>            */ >> -        ret = qcow2_pre_write_overlap_check(bs, 0, m->alloc_offset, >> -                                            m->nb_clusters * >> s->cluster_size, >> -                                            true); >> +        ret = qcow2_pre_write_overlap_check(bs, 0, start, len, true); >>           if (ret < 0) { >>               return ret; >>           } >>           BLKDBG_EVENT(bs->file, BLKDBG_CLUSTER_ALLOC_SPACE); >> -        ret = bdrv_co_pwrite_zeroes(s->data_file, m->alloc_offset, >> -                                    m->nb_clusters * s->cluster_size, >> +        ret = bdrv_co_pwrite_zeroes(s->data_file, start, len, >>                                       BDRV_REQ_NO_FALLBACK); Good point. If we weren't using BDRV_REQ_NO_FALLBACK, then avoiding a pre-zero pass over the middle is essential. But since we are insisting that the pre-zero pass be fast or else immediately fail, the time spent in pre-zeroing should not be a concern. Do you have benchmark numbers stating otherwise? -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org