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 55E5BC61DA4 for ; Thu, 23 Feb 2023 07:35:56 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1pV68Y-0004AA-Ir; Thu, 23 Feb 2023 02:35:31 -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 1pV68V-00048H-A3 for qemu-devel@nongnu.org; Thu, 23 Feb 2023 02:35:27 -0500 Received: from mail-pj1-x102f.google.com ([2607:f8b0:4864:20::102f]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.90_1) (envelope-from ) id 1pV68S-0003Vj-Me for qemu-devel@nongnu.org; Thu, 23 Feb 2023 02:35:27 -0500 Received: by mail-pj1-x102f.google.com with SMTP id k21-20020a17090aaa1500b002376652e160so620943pjq.0 for ; Wed, 22 Feb 2023 23:35:17 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=bytedance-com.20210112.gappssmtp.com; s=20210112; h=content-transfer-encoding:in-reply-to:from:references:cc:to:subject :user-agent:mime-version:date:message-id:from:to:cc:subject:date :message-id:reply-to; bh=B6xtKmdESYgNFz3bWpMLwm4JS7exfxiWGZJa7AcnoYg=; b=lWWiXnzBbb67cb1cmLC3rck33Jp89T0nZfYS2tWg90iQd2uGjM/NNfMZESjL+C7wGg o7S8V+YPaA5cC8ctZW5Lb3/oQsgVKn/O3N0bmwYb3OYdjv4FEeEaTC8dffuWXEeI3FpI KeMzOO77Hb8BjTICQOzwIlfXZo9Y58vuQiYBLchQcuSD+xMytENSHWfLYLRmArMgR27e Blgx9ytkNFZHEaaxIsGfvo2brbnQvqiSQDhXlR2VTqi62r41G8At2GWp2ZIS/+5Gidnm XZuIHLkTnkN4BRFIgRXnSo1Hy67/Je7/evhzxdtLNYY9hn9A/F6wsGPpPEKK7Jy3kgHI NPGw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=content-transfer-encoding:in-reply-to:from:references:cc:to:subject :user-agent:mime-version:date:message-id:x-gm-message-state:from:to :cc:subject:date:message-id:reply-to; bh=B6xtKmdESYgNFz3bWpMLwm4JS7exfxiWGZJa7AcnoYg=; b=4q35pzjJ2Kyp22HrXf2nuxtm5xHvTZBZ1nM5pdjpZVO8ZP5vKeShTzvLRYbLoH89zq s/E+4LD2UJTqYuC3ARw+xC3quJtjEmYWj7Bdmvwg1Qgf+8h1kuhgNNWiu67PSo0avfq4 6/qo7T2qNgOf04B8jJCwz6mVNNFxy2YoVtNTRbBxVJgbPee8iZMh3i6F6KQFP/iyI4ox /kuqyoz628LEzsFhhYr1jQuJGZL/weKuXUx15sbEQA9Ygh29TdJJqChrrBrpYaq5YfLY KcUkAocIFe3FnKVC25r1VhIrp6jVEZy0ikM/Fwe8y74XuIlT9wzk4ROfy50ZaitX6M0y rIrg== X-Gm-Message-State: AO0yUKVGcQ26ZCJrnMmUB4NlG4y1OHxMRM+hyAv4qgnnWf8BV+Kubh4V +85M4C/cGUHBK5ABUIrVVMEuOA== X-Google-Smtp-Source: AK7set+S77lWad8ckuMzggiUlQihO9b/5+Q03wOv5Nece8UaAWHAfXIQBCrVJmc6qP1ctjdNCSsZVg== X-Received: by 2002:a17:902:e80f:b0:19a:972a:7cb3 with SMTP id u15-20020a170902e80f00b0019a972a7cb3mr14356514plg.60.1677137716777; Wed, 22 Feb 2023 23:35:16 -0800 (PST) Received: from [10.4.116.91] ([139.177.225.228]) by smtp.gmail.com with ESMTPSA id jf19-20020a170903269300b0019b0937003esm3414629plb.150.2023.02.22.23.35.14 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 22 Feb 2023 23:35:16 -0800 (PST) Message-ID: <2a5594e9-11b4-2ce8-c09c-3d75a0c87732@bytedance.com> Date: Thu, 23 Feb 2023 15:35:12 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:102.0) Gecko/20100101 Thunderbird/102.6.1 Subject: Re: Questions about how block devices use snapshots To: Kevin Wolf Cc: mreitz@redhat.com, qemu-block@nongnu.org, qemu-devel@nongnu.org References: <90855f8f-76ce-0a5f-3156-e69b157342c9@bytedance.com> <12bfc9a0-45e0-21f2-3d50-988ea2ad80c8@bytedance.com> From: Zhiyong Ye In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=2607:f8b0:4864:20::102f; envelope-from=yezhiyong@bytedance.com; helo=mail-pj1-x102f.google.com X-Spam_score_int: -19 X-Spam_score: -2.0 X-Spam_bar: -- X-Spam_report: (-2.0 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, NICE_REPLY_A=-0.102, RCVD_IN_DNSWL_NONE=-0.0001, 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 Hi Kevin, Thank you for your reply and this method works. May I ask if this 'image-end-offset' field can be shown in the qemu-img info too? Because it is also a very useful information whether qcow2 is placed on a file or a block device. Regards Zhiyong On 2/21/23 11:58 PM, Kevin Wolf wrote: > 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 >