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[109.252.55.124]) by smtp.gmail.com with ESMTPSA id z3-v6sm4843457ljb.71.2018.10.09.10.25.45 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 09 Oct 2018 10:25:45 -0700 (PDT) Subject: Re: CoW behavior when writing same content To: Chris Murphy , "Gervais, Francois" Cc: "linux-btrfs@vger.kernel.org" References: From: Andrei Borzenkov Openpgp: preference=signencrypt Autocrypt: addr=arvidjaar@gmail.com; prefer-encrypt=mutual; keydata= xsDiBDxiRwwRBAC3CN9wdwpVEqUGmSoqF8tWVIT4P/bLCSZLkinSZ2drsblKpdG7x+guxwts +LgI8qjf/q5Lah1TwOqzDvjHYJ1wbBauxZ03nDzSLUhD4Ms1IsqlIwyTLumQs4vcQdvLxjFs G70aDglgUSBogtaIEsiYZXl4X0j3L9fVstuz4/wXtwCg1cN/yv/eBC0tkcM1nsJXQrC5Ay8D /1aA5qPticLBpmEBxqkf0EMHuzyrFlqVw1tUjZ+Ep2LMlem8malPvfdZKEZ71W1a/XbRn8FE SOp0tUa5GwdoDXgEp1CJUn+WLurR0KPDf01E4j/PHHAoABgrqcOTcIVoNpv2gNiBySVsNGzF XTeY/Yd6vQclkqjBYONGN3r9R8bWA/0Y1j4XK61qjowRk3Iy8sBggM3PmmNRUJYgroerpcAr 2byz6wTsb3U7OzUZ1Llgisk5Qum0RN77m3I37FXlIhCmSEY7KZVzGNW3blugLHcfw/HuCB7R 1w5qiLWKK6eCQHL+BZwiU8hX3dtTq9d7WhRW5nsVPEaPqudQfMSi/Ux1kc0mQW5kcmVpIEJv cnplbmtvdiA8YXJ2aWRqYWFyQGdtYWlsLmNvbT7CZQQTEQIAJQIbAwYLCQgHAwIGFQgCCQoL BBYCAwECHgECF4AFAliWAiQCGQEACgkQR6LMutpd94wFGwCeNuQnMDxve/Fo3EvYIkAOn+zE 21cAnRCQTXd1hTgcRHfpArEd/Rcb5+SczsBNBDxiRyQQBACQtME33UHfFOCApLki4kLFrIw1 5A5asua10jm5It+hxzI9jDR9/bNEKDTKSciHnM7aRUggLwTt+6CXkMy8an+tVqGL/MvDc4/R KKlZxj39xP7wVXdt8y1ciY4ZqqZf3tmmSN9DlLcZJIOT82DaJZuvr7UJ7rLzBFbAUh4yRKaN nwADBwQAjNvMr/KBcGsV/UvxZSm/mdpvUPtcw9qmbxCrqFQoB6TmoZ7F6wp/rL3TkQ5UElPR gsG12+Dk9GgRhnnxTHCFgN1qTiZNX4YIFpNrd0au3W/Xko79L0c4/49ten5OrFI/psx53fhY vLYfkJnc62h8hiNeM6kqYa/x0BEddu92ZG7CRgQYEQIABgUCPGJHJAAKCRBHosy62l33jMhd AJ48P7WDvKLQQ5MKnn2D/TI337uA/gCgn5mnvm4SBctbhaSBgckRmgSxfwQ= Message-ID: <7ea431d5-2966-f0e5-9dec-882e46743a69@gmail.com> Date: Tue, 9 Oct 2018 20:25:44 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org 09.10.2018 18:52, Chris Murphy пишет: > On Tue, Oct 9, 2018 at 8:48 AM, Gervais, Francois > wrote: >> Hi, >> >> If I have a snapshot where I overwrite a big file but which only a >> small portion of it is different, will the whole file be rewritten in >> the snapshot? Or only the different part of the file? > If you overwrite the whole file, the whole file will be overwritten. > Depends on how the application modifies files. Many applications write > out a whole new file with a pseudorandom filename, fsync, then rename. > >> >> Something like: >> >> $ dd if=/dev/urandom of=/big_file bs=1M count=1024 >> $ cp /big_file root/ >> $ btrfs sub snap root snapshot >> $ cp /big_file snapshot/ >> And which portion of these three files is different? They must be identical. Not that it really matters, but that does not match your question. >> In this case is root/big_file and snapshot/big_file still share the same data? > > You'll be left with three files. /big_file and root/big_file will > share extents, How comes they share extents? This requires --reflink, is it default now? > and snapshot/big_file will have its own extents. You'd > need to copy with --reflink for snapshot/big_file to have shared > extents with /big_file - or deduplicate. > This still overwrites the whole file in the sense original file content of "snapshot/big_file" is lost. That new content happens to be identical and that new content will probably be reflinked does not change the fact that original file is gone.