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=-10.1 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 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 B3260C4338F for ; Mon, 16 Aug 2021 15:51:57 +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 5A5CD60F42 for ; Mon, 16 Aug 2021 15:51:57 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org 5A5CD60F42 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=nongnu.org Received: from localhost ([::1]:59516 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1mFeu4-0001by-Dp for qemu-devel@archiver.kernel.org; Mon, 16 Aug 2021 11:51:56 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:53250) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mFesq-0000MC-M9 for qemu-devel@nongnu.org; Mon, 16 Aug 2021 11:50:40 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:49672) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mFesl-0005qv-DC for qemu-devel@nongnu.org; Mon, 16 Aug 2021 11:50:39 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1629129034; 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: in-reply-to:in-reply-to:references:references; bh=Frw8xHDJU/5xB5dVvAU9JbBynGF0okN/90+Ms1LGAeM=; b=jLfzDcVP6a6TVOnMna8aG1mojRVMi6ZK5e4jYV+vW3vYur0YNbgaC/kr9zlo7vElTqTb9y x6sRO5lDcTcb87xCyr+F1AkqANiMNrXuPUGmt6+ajF9qqHFsH4xsFGYPyfLuXXh+nXmn7I BJAJO8U/FhZ2PV7Z+HWxp1ugjN5Mdo8= 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-104-86W--2zNOA6PsWlgM7pUSA-1; Mon, 16 Aug 2021 11:50:31 -0400 X-MC-Unique: 86W--2zNOA6PsWlgM7pUSA-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id D4A69108293E; Mon, 16 Aug 2021 15:50:30 +0000 (UTC) Received: from redhat.com (ovpn-113-125.phx2.redhat.com [10.3.113.125]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 2B02378C00; Mon, 16 Aug 2021 15:50:30 +0000 (UTC) Date: Mon, 16 Aug 2021 10:50:28 -0500 From: Eric Blake To: Nir Soffer Subject: Re: [PATCH] qemu-nbd: Change default cache mode to writeback Message-ID: <20210816155028.6rfmuxpdmc6yniit@redhat.com> References: <20210813205519.50518-1-nsoffer@redhat.com> MIME-Version: 1.0 In-Reply-To: <20210813205519.50518-1-nsoffer@redhat.com> User-Agent: NeoMutt/20210205-719-68949a X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 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=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=170.10.133.124; envelope-from=eblake@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -34 X-Spam_score: -3.5 X-Spam_bar: --- X-Spam_report: (-3.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.698, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham 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: Nir Soffer , Vladimir Sementsov-Ogievskiy , qemu-devel@nongnu.org, qemu-block@nongnu.org, qemu-stable@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Fri, Aug 13, 2021 at 11:55:19PM +0300, Nir Soffer wrote: > Both qemu and qemu-img use writeback cache mode by default, which is > already documented in qemu(1). qemu-nbd uses writethrough cache mode by > default, and the default cache mode is not documented. > > According to the qemu-nbd(8): > > --cache=CACHE > The cache mode to be used with the file. See the > documentation of the emulator's -drive cache=... option for > allowed values. > > qemu(1) says: > > The default mode is cache=writeback. > > So users have no reason to assume that qemu-nbd is using writethough > cache mode. The only hint is the painfully slow writing when using the > defaults. Oh, good catch. Unfortunately too late for 6.1 proper, but I'll add qemu-stable in cc and queue this through my NBD tree for 6.2. > Users can avoid the issue by using --cache=writeback[1] but the defaults > should give good performance for the common use case. > > [1] https://bugzilla.redhat.com/1990656 > > Signed-off-by: Nir Soffer > --- Reviewed-by: Eric Blake -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3266 Virtualization: qemu.org | libvirt.org