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=-2.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, 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 4CCAAC433E0 for ; Tue, 9 Jun 2020 16:21:30 +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 11A92206A4 for ; Tue, 9 Jun 2020 16:21:30 +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="ADizKlDv" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 11A92206A4 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]:42196 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jih0D-0006Gk-1P for qemu-devel@archiver.kernel.org; Tue, 09 Jun 2020 12:21:29 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:46316) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1jigym-0005F5-LO for qemu-devel@nongnu.org; Tue, 09 Jun 2020 12:20:00 -0400 Received: from us-smtp-2.mimecast.com ([205.139.110.61]:29853 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 1jigyl-0007ji-7c for qemu-devel@nongnu.org; Tue, 09 Jun 2020 12:20:00 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1591719597; 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=SyhfPKerh7BFn3BTRSnCQaLrCiEcxl0Ns1kmXdVRSrg=; b=ADizKlDvzaYmDeAEL98yJU9nr7Ix/px1/WGT7Cl8gsx/QOS+f/PxJaprUFO9ClGvL8/cLX vmOmZXHPml1wnK4Q4CVUEWb1go/ERRX/WqJTZ+3xOEszZyMs8ZDSK3HnCxkVqL7YmxgVQg 4A4C7bf28yTUlk+3Z7DbcUBG4uiabAo= 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-220-KbuBlZEHN_CTJrRx1xXEHA-1; Tue, 09 Jun 2020 12:19:52 -0400 X-MC-Unique: KbuBlZEHN_CTJrRx1xXEHA-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 111BD1B18BC2; Tue, 9 Jun 2020 16:19:51 +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 65E6310190A7; Tue, 9 Jun 2020 16:19:50 +0000 (UTC) Subject: Re: [PATCH] qcow2: Reduce write_zeroes size in handle_alloc_space() To: Kevin Wolf References: <20200609140859.142230-1-kwolf@redhat.com> <02e24dca-99da-873d-8425-09a07571e675@virtuozzo.com> <042f0b8f-dd51-acc3-8498-ac9a5532df15@redhat.com> <20200609151810.GD11003@linux.fritz.box> From: Eric Blake Organization: Red Hat, Inc. Message-ID: Date: Tue, 9 Jun 2020 11:19:49 -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: <20200609151810.GD11003@linux.fritz.box> Content-Language: en-US X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 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=205.139.110.61; envelope-from=eblake@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/06/09 02:41:53 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: Vladimir Sementsov-Ogievskiy , qemu-block@nongnu.org, qemu-devel@nongnu.org, mreitz@redhat.com, anton.nefedov@virtuozzo.com, "Denis V. Lunev" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 6/9/20 10:18 AM, Kevin Wolf wrote: >>>> -        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? > > I stumbled across this behaviour (write_zeros for 2 MB, then overwrite > almost everything) in the context of a different bug, and it just didn't > make much sense to me. Is there really a file system where fragmentation > is introduced by not zeroing the area first and then overwriting it? > > I'm not insisting on making this change because the behaviour is > harmless if odd, but if we think that writing twice to some blocks is an > optimisation, maybe we should actually measure and document this. > > > Anyway, let's talk about the reported bug that made me look at the > strace that showed this behaviour because I feel it supports my last > point. It's a bit messy, but anyway: > > https://bugzilla.redhat.com/show_bug.cgi?id=1666864 > > So initially, bad performance on a fragmented image file was reported. > Not much to do there, but then in comment 16, QA reported a performance > regression in this case between 4.0 and 4.2. And this change caused by > c8bb23cbdbe, i.e. the commit that introduced handle_alloc_space(). > > Turns out that BDRV_REQ_NO_FALLBACK doesn't always guarantee that it's > _really_ fast. fallocate(FALLOC_FL_ZERO_RANGE) causes some kind of flush > on XFS and buffered writes don't. So with the old code, qemu-img convert > to a file on a very full filesystem that will cause fragmentation, was > much faster with writing a zero buffer than with write_zeroes (because > it didn't flush the result). Wow. That makes it sound like we should NOT attempt fallocate(FALLOC_FL_ZERO_RANGE) on the fast path, because we don't have guarantees that it is fast. I really wish the kernel would give us fallocate(FALLOC_FL_ZERO_RANGE|FALLOC_FL_NO_FALLBACK) which would fail fast rather than doing a flush or other slow fallback. > > I don't fully understand why this is and hope that XFS can do something > about it. I also don't really think we should revert the change in QEMU, > though I'm not completely sure. But I just wanted to share this to show > that "obvious" characteristics of certain types of requests aren't > always true and doing obscure optimisations based on what we think > filesystems may do can actually achieve the opposite in some cases. It also goes to show us that the kernel does NOT yet give us enough fine-grained control over what we really want (which is: 'pre-zero this if it is fast, but don't waste time if it is not). Most of the kernel interfaces end up being 'pre-zero this, and it might be fast, fail fast, or even fall back to something safe but slow, and you can't tell the difference short of trying'. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org