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 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 smtp.lore.kernel.org (Postfix) with ESMTPS id BA43FC61DA3 for ; Tue, 21 Feb 2023 15:59:01 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1pUV2b-0005vQ-GN; Tue, 21 Feb 2023 10:58:53 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1pUV2Z-0005uz-VR for qemu-devel@nongnu.org; Tue, 21 Feb 2023 10:58:51 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1pUV2Y-0006d7-4M for qemu-devel@nongnu.org; Tue, 21 Feb 2023 10:58:51 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1676995128; 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=u21ijLjeelZIuES8MOORs4NB0OopgnuzS0DeoRzVrLQ=; b=KH9zT4uhLVbRMhGp8B/xAahYFNLpoRhDd1cZYyrns6BN14RwKLaWeO4cHi8PfoR4z4xgDN +xLcEDEEyJIAUh0/7+6sUYKRbXlwIYQxVP6xmTNy8nuTVdVkVcYUJGHCTJXEjPoSGOsZr3 q/eqsTAATDsLlI5Ac2Qiq23uHDZZ9LA= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-12-F0OwgQCcMza2EhtyZycmWQ-1; Tue, 21 Feb 2023 10:58:47 -0500 X-MC-Unique: F0OwgQCcMza2EhtyZycmWQ-1 Received: from smtp.corp.redhat.com (int-mx10.intmail.prod.int.rdu2.redhat.com [10.11.54.10]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id DB721802314; Tue, 21 Feb 2023 15:58:46 +0000 (UTC) Received: from redhat.com (dhcp-192-225.str.redhat.com [10.33.192.225]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 0900E492B00; Tue, 21 Feb 2023 15:58:45 +0000 (UTC) Date: Tue, 21 Feb 2023 16:58:44 +0100 From: Kevin Wolf To: Zhiyong Ye Cc: mreitz@redhat.com, qemu-block@nongnu.org, qemu-devel@nongnu.org Subject: Re: Questions about how block devices use snapshots Message-ID: References: <90855f8f-76ce-0a5f-3156-e69b157342c9@bytedance.com> <12bfc9a0-45e0-21f2-3d50-988ea2ad80c8@bytedance.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <12bfc9a0-45e0-21f2-3d50-988ea2ad80c8@bytedance.com> X-Scanned-By: MIMEDefang 3.1 on 10.11.54.10 Received-SPF: pass client-ip=170.10.133.124; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-124.mimecast.com 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_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: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Am 21.02.2023 um 14:27 hat Zhiyong Ye geschrieben: > > Hi Kevin, > > Sorry to bother you again. > > I intend to use this approach for snapshots of block devices, which, as you > say, requires a lot of disk space to store snapshot data. So, to save disk > space, after each successful external snapshot creation, I want to shrink > the block device that stores the backing_file image to the size that qcow2 > data actually occupies, since it has become read-only. But there is no way > to get the actual size of qcow2 when it is stored in a block device. > > Qemu-img info can easily get the actual size of qcow2 when it is stored in a > file using the fstat function, but this will fail and return 0 for block > devices. Therefore, it is necessary to implement the method of getting data > occupancy inside qcow2. I think there may be two possible ways to do this: > > - Add a cluster count field @nb_clusters in the BDRVQcow2State for each new > cluster allocated and the actual size occupied by qcow2 is: nb_clusters * > cluster_size. > - Iterate through the refcount block to find the value with the largest host > offset, and this is the actual size occupied by qcow2. > > Since I'm not very familiar with qcow2, may I ask if you have any advice on > getting the actual size when using qcow2? I think what you need is the 'image-end-offset' field from 'qemu-img check --output=json'. Kevin