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Mon, 18 Feb 2019 06:00:29 +0100 Subject: Re: [PATCH v3 1/9] btrfs: delayed-ref: Introduce better documented delayed ref structures To: Qu Wenruo , 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> <51a84783-2eaf-8f90-ec0d-8bc175279ef8@suse.de> From: Qu Wenruo Openpgp: preference=signencrypt Autocrypt: addr=quwenruo.btrfs@gmx.com; prefer-encrypt=mutual; keydata= mQENBFnVga8BCACyhFP3ExcTIuB73jDIBA/vSoYcTyysFQzPvez64TUSCv1SgXEByR7fju3o 8RfaWuHCnkkea5luuTZMqfgTXrun2dqNVYDNOV6RIVrc4YuG20yhC1epnV55fJCThqij0MRL 1NxPKXIlEdHvN0Kov3CtWA+R1iNN0RCeVun7rmOrrjBK573aWC5sgP7YsBOLK79H3tmUtz6b 9Imuj0ZyEsa76Xg9PX9Hn2myKj1hfWGS+5og9Va4hrwQC8ipjXik6NKR5GDV+hOZkktU81G5 gkQtGB9jOAYRs86QG/b7PtIlbd3+pppT0gaS+wvwMs8cuNG+Pu6KO1oC4jgdseFLu7NpABEB AAG0IlF1IFdlbnJ1byA8cXV3ZW5ydW8uYnRyZnNAZ214LmNvbT6JAVQEEwEIAD4CGwMFCwkI BwIGFQgJCgsCBBYCAwECHgECF4AWIQQt33LlpaVbqJ2qQuHCPZHzoSX+qAUCWdWCnQUJCWYC bgAKCRDCPZHzoSX+qAR8B/94VAsSNygx1C6dhb1u1Wp1Jr/lfO7QIOK/nf1PF0VpYjTQ2au8 ihf/RApTna31sVjBx3jzlmpy+lDoPdXwbI3Czx1PwDbdhAAjdRbvBmwM6cUWyqD+zjVm4RTG rFTPi3E7828YJ71Vpda2qghOYdnC45xCcjmHh8FwReLzsV2A6FtXsvd87bq6Iw2axOHVUax2 FGSbardMsHrya1dC2jF2R6n0uxaIc1bWGweYsq0LXvLcvjWH+zDgzYCUB0cfb+6Ib/ipSCYp 3i8BevMsTs62MOBmKz7til6Zdz0kkqDdSNOq8LgWGLOwUTqBh71+lqN2XBpTDu1eLZaNbxSI ilaVuQENBFnVga8BCACqU+th4Esy/c8BnvliFAjAfpzhI1wH76FD1MJPmAhA3DnX5JDORcga CbPEwhLj1xlwTgpeT+QfDmGJ5B5BlrrQFZVE1fChEjiJvyiSAO4yQPkrPVYTI7Xj34FnscPj /IrRUUka68MlHxPtFnAHr25VIuOS41lmYKYNwPNLRz9Ik6DmeTG3WJO2BQRNvXA0pXrJH1fN GSsRb+pKEKHKtL1803x71zQxCwLh+zLP1iXHVM5j8gX9zqupigQR/Cel2XPS44zWcDW8r7B0 q1eW4Jrv0x19p4P923voqn+joIAostyNTUjCeSrUdKth9jcdlam9X2DziA/DHDFfS5eq4fEv ABEBAAGJATwEGAEIACYWIQQt33LlpaVbqJ2qQuHCPZHzoSX+qAUCWdWBrwIbDAUJA8JnAAAK CRDCPZHzoSX+qA3xB/4zS8zYh3Cbm3FllKz7+RKBw/ETBibFSKedQkbJzRlZhBc+XRwF61mi f0SXSdqKMbM1a98fEg8H5kV6GTo62BzvynVrf/FyT+zWbIVEuuZttMk2gWLIvbmWNyrQnzPl mnjK4AEvZGIt1pk+3+N/CMEfAZH5Aqnp0PaoytRZ/1vtMXNgMxlfNnb96giC3KMR6U0E+siA 4V7biIoyNoaN33t8m5FwEwd2FQDG9dAXWhG13zcm9gnk63BN3wyCQR+X5+jsfBaS4dvNzvQv h8Uq/YGjCoV1ofKYh3WKMY8avjq25nlrhzD/Nto9jHp8niwr21K//pXVA81R2qaXqGbql+zo Message-ID: <72de8715-1a20-94b1-496e-dddb0983da87@gmx.com> Date: Mon, 18 Feb 2019 13:00:24 +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: <51a84783-2eaf-8f90-ec0d-8bc175279ef8@suse.de> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit X-Provags-ID: V03:K1:6th5F7rLk+7XcCeojdNAZ+LgzzQ+eR3EyZByo+YE4uXfkqzXejh U1KDsxKl2S0m2fUrg31RyzXXcsmT81Z3Up0oUannfm5CNm+pH0YYxai4MW50ZeDSjPu178I ju3L5V5+eQTnuO3kWG+GTH2hc0ZYvyGEseT1HHh/zJz6WoNyfBdvi2FF/4T/HJb4k6z2hqe xFKDSxkt64UAzHe/8j4kg== X-UI-Out-Filterresults: notjunk:1;V03:K0:4pKVnpc3L1Q=:bmqXmJDWNi5buUCwRNRMUQ idgcIOaJDVA/6ibRE84nqAfi8Y+j43Dn7zTwFpxrNl9qg2/IS20lSI1ZCYzEyuqpIh6AgmjCM VQsZZelvM6OVzI0oUtu9BvYWfYIHlKmTBITXyAygd/2aN5n4SrD2q5nxLESbhLr46W9Du+4bP YxnWtNDO/qLlkve3xz7JyXnC+CLip0iwqOTISKooerUoFf3iB7kkEERWlhz9ZTNmknvz9I6St 8GPSfrvoIaSY4dMn/PZwbhY/ZmRDXhUwsAolyaSuUTZXvj4/86VN++9y7CQ5ePcH/2oSkqPHH Blm36yF2T2KY34lx4Y1wL1XCHN68eODkHQq4w9+JQ5eH38wjroPinnii+dA+V6D4OaR28hMw4 YxTnTuY9J1fwoSJyysiIqCoJZ1WeGgLhVZotndWZro4L7C1JaBX3EmAZ5aE5Z1+cyt6ST4EGZ /RtVAOXEsjYgTaFDsjGbMVoq03JP+dfCN1ATWz8b2gBCk4aiAxDiG1IqD7vZTPhI/EbvoTk/x BoW7lqahb9W0/XRRQT38JjEFqyGxBa8TpibWwvtfPyQx7I/RMhZX++bf7F8wyfI4XQKaERDvd /zgFfxc9M8WmbUIvX2w7mz9HpX8sQ/wFp7muY/dntD61snrnmg0kiONWIZzKx4iWL1d5RmHCW ptoGe1e0afOPWya1jFAM9K6sSAZwupCQm05hCdNBe60XBTnnQKsPUolQjABpfY6CIvKoiYrYU faiVbJSJ0H1lrtjdUXb4ZyrGTFK5kYZB4984wu+2Eiz6tadYHp/uyxoPlZ0Q1dEzWCHCRheuz hA4gtZvt03ppH0xB/VcpuQ7fMp0c1FT/8VdLiEw1uSLuFmwEG4+imEP4TVV+ZhJIfbkojxc86 CRohQpmR9OUdX0/J5Nj4yGBsok6lM9CONSS2RcSzSo3zVr9l4tR3ZBkcxYEEdSpZGtVQHiP2f 6USKFEPAX/w== Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On 2019/2/11 下午10:23, Qu Wenruo wrote: > [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. Is there any extra suggestion on the wording about the anti-initiative offset used in data backref? Thanks, Qu > > Thanks, > Qu >