From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 11B49168BD for ; Fri, 3 Oct 2025 07:44:47 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=198.137.202.133 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759477489; cv=none; b=A928rdxawxJ9YyxvOnac8U/fZmmx8v5nHEbw6DdmZqe8Rg7Vy1trh5wsXObvQmBFBXxTMoFdIUct0ZZu4wX5GBEN5/fNS9Vy03UA3gKrwvgpTLnT0W9/Lk8ZHntQTdp4lV6xu9h9BMO5x/DIKUvdMBpK+xvIdCBFX+zAzMRpYFg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759477489; c=relaxed/simple; bh=zqKwVg4TyJQ0VPSRdYLvYYEPcaTseRKiw01RzpN1HWI=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=jBYxjJVcTmCMBYYyzi+w7oYaF4rqGQGPzrMewvASCIl3xfIPO61U2hUy5Y/z93OCyp/kkMaQKtV6C2jGAA/5Qv70Wgezw71z9lxgOqoUBjTWI4i3QFQXvW/Sy8b5Scq0BRM/IPjWU2r4wbuaqrqmlxW3IbxMm85C8Mlp8VYIvVw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=bombadil.srs.infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=R+A7V21m; arc=none smtp.client-ip=198.137.202.133 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=bombadil.srs.infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="R+A7V21m" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20210309; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=mlC2HfUdpw7RLVWO6Riy+j8Rit5Id5O1aXFoYUcuHho=; b=R+A7V21mQB8j/Vm5K+iIhtXJ/j I0+i12fT71A6bKwYAPVsn3i9K2GqK9fE+2N8bCppJI/0Sq3yUzLXB6/JBvW6nEYI2PkEYExQsDbnF GxL+aFtF5zvIzPTIcVnuQtPxr2Qbu6Db4NzoU2I596uusw7uuLagGGr2ewpKtbwjWJEbUHC1HdkPW D/lJvVNm/6ZxBrByiaXlvpj9E/jEwqxVPqdlWzFJUpVPS5+MgrdYR68MgF2NW6znnsoBo/VLWu/+5 tWtUpXA4w94hblFQEJZ/WEtMrg/4eaJ8dyHEKZvwCkhDHa/phIU+B9Ud8VIXuwwfl6RJa/ArNmS7E q7XvccdA==; Received: from hch by bombadil.infradead.org with local (Exim 4.98.2 #2 (Red Hat Linux)) id 1v4aT9-0000000BpwS-325H; Fri, 03 Oct 2025 07:44:47 +0000 Date: Fri, 3 Oct 2025 00:44:47 -0700 From: Christoph Hellwig To: Demi Marie Obenour Cc: Christoph Hellwig , Chris Laprise , linux-btrfs@vger.kernel.org Subject: Re: Can the output of FIEMAP on BTRFS be used to check if a file and its reflink copy might have diverged? Message-ID: References: <38f350b0-9a71-4627-9d36-57bf2f85e67a@gmail.com> <693793db-3431-48b3-8913-aadb86bc4ebc@gmail.com> Precedence: bulk X-Mailing-List: linux-btrfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <693793db-3431-48b3-8913-aadb86bc4ebc@gmail.com> X-SRS-Rewrite: SMTP reverse-path rewritten from by bombadil.infradead.org. See http://www.infradead.org/rpr.html On Mon, Sep 29, 2025 at 07:55:22PM -0400, Demi Marie Obenour wrote: > Can two extents have the same offset on disk but different logical contents? > For XFS that seems impossible unless a realtime device is involved, which > is not the case here. Only with the RT device. But again that is insider knowledge and not an API exposed to applications. More importantly the offset can change underneath without any notice to the application. > Is it easier if one requires the filesystem to be read-only? Taking a > device-mapper snapshot (thin or CoW) before the backup is not too onerous, > at least if the filesystem is already on an LVM LV. At least that locks out other changes. But it is a rather opaque setup.