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 BEB2FC83009 for ; Tue, 28 Apr 2020 14:30:21 +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 89826206C0 for ; Tue, 28 Apr 2020 14:30:21 +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="F+JlJ+vI" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 89826206C0 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]:60906 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jTRFc-0004om-JD for qemu-devel@archiver.kernel.org; Tue, 28 Apr 2020 10:30:20 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:44908) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jTREP-000337-Ln for qemu-devel@nongnu.org; Tue, 28 Apr 2020 10:29:17 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.90_1) (envelope-from ) id 1jTRBA-0003XQ-G2 for qemu-devel@nongnu.org; Tue, 28 Apr 2020 10:29:05 -0400 Received: from us-smtp-delivery-1.mimecast.com ([205.139.110.120]:49673 helo=us-smtp-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 1jTRB9-0003X5-TM for qemu-devel@nongnu.org; Tue, 28 Apr 2020 10:25:44 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1588083942; 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=3sXYdwn3zxNdlMN3DkYHwlX93bwWqrgCPpWXcqDBqRM=; b=F+JlJ+vIX9BIBdsJk94HEgj5B7YTHH1VIN07XwNdPUAWbDspZDoCKMSABZSfxrDbxvYYBE V0pN/G2r/3bO4Y/1V9KIFcU8VHBOG+uVb5GmxANhJCwbgU/dMfNoG9RMPtmXvBXmtS6p/p 5xru7MVqD9QMT+wHTIwQOD6WmTnA0R8= 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-308-HOctJouIORu50O0_RkzVJw-1; Tue, 28 Apr 2020 10:25:38 -0400 X-MC-Unique: HOctJouIORu50O0_RkzVJw-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 41FE610B2EC3; Tue, 28 Apr 2020 14:25:23 +0000 (UTC) Received: from [10.10.116.80] (ovpn-116-80.rdu2.redhat.com [10.10.116.80]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 67B9B60D3D; Tue, 28 Apr 2020 14:25:22 +0000 (UTC) Subject: Re: [PATCH 1/4] block: Add bdrv_make_empty() To: Kevin Wolf References: <20200428132629.796753-1-mreitz@redhat.com> <20200428132629.796753-2-mreitz@redhat.com> <20200428140132.GF5789@linux.fritz.box> <20200428141641.GH5789@linux.fritz.box> From: Eric Blake Organization: Red Hat, Inc. Message-ID: <2c52cdc0-ba9b-081e-b593-fd3cf49dca12@redhat.com> Date: Tue, 28 Apr 2020 09:25:20 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0 MIME-Version: 1.0 In-Reply-To: <20200428141641.GH5789@linux.fritz.box> 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: 7bit Received-SPF: pass client-ip=205.139.110.120; envelope-from=eblake@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/04/28 02:16:38 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] X-Received-From: 205.139.110.120 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: qemu-devel@nongnu.org, qemu-block@nongnu.org, Max Reitz Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 4/28/20 9:16 AM, Kevin Wolf wrote: >> >> Yes. Although now I'm wondering if the two should remain separate or should >> just be a single driver callback where flags can include BDRV_REQ_ZERO_WRITE >> to distinguish whether exposing the backing file vs. reading as all zeroes >> is intended, or if that is merging too much. > > I don't think the implementations for both things are too similar, so > you might just end up having two if branches and then two separate > implementations in the block drivers. > Yeah, the more I think about it, the more two callbacks still make sense. .bdrv_make_empty may or may not need a flag, but .bdrv_make_zero definitely does (because that's where we want a difference between making the entire image zero no matter the delay, vs. only making it all zero if it is is fast). > If anything, bdrv_make_empty() is more related to discard than > write_zeroes. But we use the discard code for it in qcow2 only as a > fallback because in the most common cases, making an image completely > empty means effectively just creating an entirely new L1 and refcount > table, which is much faster than individually discarding all clusters. > > For bdrv_make_zero() I don't see an opportunity for such optimisations, > so I don't really see a reason to have a separate callback. Unless you > do know one? The optimization I have in mind is adding a qcow2 autoclear bit to track when an image is known to read as all zero - at which point .bdrv_make_zero instantly returns success. For raw files, a possible optimization is to truncate to size 0 and then back to the full size, when it is known that truncation forces read-as-zero. For NBD, I'm still playing with whether adding new 64-bit transactions for a bulk zero will be useful, and even if not, maybe special-casing NBD_CMD_WRITE_ZEROES with a size of 0 to do a bulk zero, if both server and client negotiated that particular meaning. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org