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Mon, 11 Feb 2019 14:23:36 +0000 (UTC) Subject: Re: [PATCH v3 1/9] btrfs: delayed-ref: Introduce better documented delayed ref structures To: Nikolay Borisov , linux-btrfs@vger.kernel.org References: <20190211051653.3167-1-wqu@suse.com> <20190211051653.3167-2-wqu@suse.com> <216f4acf-faf9-6ca3-c9f2-68a9eaa763fa@suse.com> <77f7ed38-aca5-f799-08eb-e8a3100fe57c@suse.de> <8e4f62e1-f0ab-5f1f-6fc6-684d3e53b8b8@suse.com> From: Qu Wenruo Openpgp: preference=signencrypt Autocrypt: addr=wqu@suse.de; prefer-encrypt=mutual; keydata= mQENBFnVga8BCACyhFP3ExcTIuB73jDIBA/vSoYcTyysFQzPvez64TUSCv1SgXEByR7fju3o 8RfaWuHCnkkea5luuTZMqfgTXrun2dqNVYDNOV6RIVrc4YuG20yhC1epnV55fJCThqij0MRL 1NxPKXIlEdHvN0Kov3CtWA+R1iNN0RCeVun7rmOrrjBK573aWC5sgP7YsBOLK79H3tmUtz6b 9Imuj0ZyEsa76Xg9PX9Hn2myKj1hfWGS+5og9Va4hrwQC8ipjXik6NKR5GDV+hOZkktU81G5 gkQtGB9jOAYRs86QG/b7PtIlbd3+pppT0gaS+wvwMs8cuNG+Pu6KO1oC4jgdseFLu7NpABEB AAG0F1F1IFdlbnJ1byA8d3F1QHN1c2UuZGU+iQFUBBMBCAA+AhsDBQsJCAcCBhUICQoLAgQW AgMBAh4BAheAFiEELd9y5aWlW6idqkLhwj2R86El/qgFAlnVgp0FCQlmAm4ACgkQwj2R86El /qilmgf/cUq9kFQo577ku5gc6rFpVg68ublBwjYpwjw0b//xo+Wo1wm+RRbUGs+djSZAqw12 D4F3r0mBTI7abUCNWAbFkYZSAIFVi0DMkjypIVS7PSaEt04rM9VBTToE+YqU6WENeJ57R2p2 +hI0wZrBwxObdsdaOtxWtsp3bmhIbdqxSKrtXuRawy4KnQYcLuGzOce9okdlbAE0W3KHm1gQ oNAe6FX8nC9qo14m8LqEbThYH+qj4iCMlN8HIfbSx4F3e7nHZ+UAMW+E/lnMRkIB9Df+JyVd /NlXzIjZAggcWsqpx6D4wyAuexKWkiGQeUeArUNihAwXjmyqWPGmjVyIh+oC6LkBDQRZ1YGv AQgAqlPrYeBLMv3PAZ75YhQIwH6c4SNcB++hQ9TCT5gIQNw51+SQzkXIGgmzxMIS49cZcE4K Xk/kHw5hieQeQZa60BWVRNXwoRI4ib8okgDuMkD5Kz1WEyO149+BZ7HD4/yK0VFJGuvDJR8T 7RZwB69uVSLjkuNZZmCmDcDzS0c/SJOg5nkxt1iTtgUETb1wNKV6yR9XzRkrEW/qShChyrS9 fNN8e9c0MQsC4fsyz9Ylx1TOY/IF/c6rqYoEEfwnpdlz0uOM1nA1vK+wdKtXluCa79MdfaeD /dt76Kp/o6CAKLLcjU1Iwnkq1HSrYfY3HZWpvV9g84gPwxwxX0uXquHxLwARAQABiQE8BBgB CAAmFiEELd9y5aWlW6idqkLhwj2R86El/qgFAlnVga8CGwwFCQPCZwAACgkQwj2R86El/qgN 8Qf+M0vM2Idwm5txZZSs+/kSgcPxEwYmxUinnUJGyc0ZWYQXPl0cBetZon9El0naijGzNWvf HxIPB+ZFehk6Otgc78p1a3/xck/s1myFRLrmbbTJNoFiyL25ljcq0J8z5Zp4yuABL2RiLdaZ Pt/jfwjBHwGR+QKp6dD2qMrUWf9b7TFzYDMZXzZ2/eoIgtyjEelNBPrIgOFe24iKMjaGjd97 fJuRcBMHdhUAxvXQF1oRtd83JvYJ5OtwTd8MgkEfl+fo7HwWkuHbzc70L4fFKv2BowqFdaHy mId1ijGPGr46tuZ5a4cw/zbaPYx6fJ4sK9tSv/6V1QPNUdqml6hm6pfs6A== Message-ID: <51a84783-2eaf-8f90-ec0d-8bc175279ef8@suse.de> Date: Mon, 11 Feb 2019 22:23:30 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.5.0 MIME-Version: 1.0 In-Reply-To: <8e4f62e1-f0ab-5f1f-6fc6-684d3e53b8b8@suse.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org [snip] >>> Looking at the dev >>> docs and the description for 'offset' field in btrfs_file_extent_item I >>> can sort of deduce that this field will only be different than null if >>> this reference is for an extent which is shared between 2 snapshots. >> >> Don't forget reflink and data CoW. >> >> Like this: >> >> item 6 key (257 EXTENT_DATA 0) itemoff 15813 itemsize 53 >> generation 6 type 1 (regular) >> extent data disk byte 13631488 nr 1048576 >> extent data offset 0 nr 4096 ram 1048576 >> item 7 key (257 EXTENT_DATA 4096) itemoff 15760 itemsize 53 >> generation 7 type 1 (regular) >> extent data disk byte 14680064 nr 4096 >> extent data offset 0 nr 4096 ram 4096 >> item 8 key (257 EXTENT_DATA 8192) itemoff 15707 itemsize 53 >> generation 6 type 1 (regular) >> extent data disk byte 13631488 nr 1048576 >> extent data offset 8192 nr 1040384 ram 1048576 >> >> EXTENT_DATA items at 0 and 8K offset are original from one larger >> extent, EXTENT_DATA item at 4K offset is newly written one. > > Okay this makes sense, however if we take item 8 being inserted then > according to the comments, the 'offset' member for this data ref will be > 0 since 8k (from key.offset) - 8k (from btrfs_file_extent_offset)? WHy > is that, shouldn't the offset here be 8k rather than 0? To avoid creating a new data backref item. I don't like this idea too, it makes btrfs check, especially lowmem mode, pretty slow. If I'm going to re-design the on-disk format, this is definitely going to disappear. But the design is already here for a long long time, even it caused problems before, we still need to follow the behavior. Thanks, Qu