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(2a02-a466-68ed-1-9386-40ea-f665-2d25.fixed6.kpn.net. [2a02:a466:68ed:1:9386:40ea:f665:2d25]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-ada5e2bf011sm1158127766b.105.2025.06.04.14.36.37 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 04 Jun 2025 14:36:38 -0700 (PDT) Message-ID: <3fea5116-8532-4076-a824-620dc4c5a627@gmail.com> Date: Wed, 4 Jun 2025 23:36:37 +0200 Precedence: bulk X-Mailing-List: linux-btrfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: Why does defragmenting break reflinks? To: Qu Wenruo , =?UTF-8?B?8J2VjfCdlZbwnZWd8J2VoPCdlZTwnZWa?= =?UTF-8?B?8J2Vl/CdlarwnZWW8J2Vow==?= , linux-btrfs@vger.kernel.org References: <9d74d71f-b65d-4f06-adb3-18f7698edb8a@velocifyer.com> Content-Language: en-US From: Ferry Toth In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Op 30-05-2025 om 02:27 schreef Qu Wenruo: > > > ๅœจ 2025/5/30 09:22, ๐•๐•–๐•๐• ๐•”๐•š๐•—๐•ช๐•–๐•ฃ ๅ†™้“: >> BTRFS-FILESYSTEM(8) says "defragmentingย  withย  Linux kernel versions < >> 3.9 or โ‰ฅ 3.14-rc2 as well as with Linux stable kernel versions โ‰ฅ >> 3.10.31, โ‰ฅ 3.12.12 or โ‰ฅ 3.13.4 will break up the reflinks of COW data >> (for example files copied with cp --reflink, snapshots or de- >> duplicated data)." Why does defragmenting not preserve reflinks and >> why was it removed? >> > > Defrag means to re-dirty the data, and write them back again, which will > cause COW. That sounds like an implementation choice. > And by nature this breaks reflinked data extents. With a different implementation this would not by nature be the case. Actually this makes defrag a very dangerous operation on disks with many snapshots (> 20). When you would defrag each snapshot suddenly your 5% full disk would be 100%.