18c2ecf20Sopenharmony_ci.. SPDX-License-Identifier: GPL-2.0 28c2ecf20Sopenharmony_ci 38c2ecf20Sopenharmony_ciHigh Level Design 48c2ecf20Sopenharmony_ci================= 58c2ecf20Sopenharmony_ci 68c2ecf20Sopenharmony_ciAn ext4 file system is split into a series of block groups. To reduce 78c2ecf20Sopenharmony_ciperformance difficulties due to fragmentation, the block allocator tries 88c2ecf20Sopenharmony_civery hard to keep each file's blocks within the same group, thereby 98c2ecf20Sopenharmony_cireducing seek times. The size of a block group is specified in 108c2ecf20Sopenharmony_ci``sb.s_blocks_per_group`` blocks, though it can also calculated as 8 \* 118c2ecf20Sopenharmony_ci``block_size_in_bytes``. With the default block size of 4KiB, each group 128c2ecf20Sopenharmony_ciwill contain 32,768 blocks, for a length of 128MiB. The number of block 138c2ecf20Sopenharmony_cigroups is the size of the device divided by the size of a block group. 148c2ecf20Sopenharmony_ci 158c2ecf20Sopenharmony_ciAll fields in ext4 are written to disk in little-endian order. HOWEVER, 168c2ecf20Sopenharmony_ciall fields in jbd2 (the journal) are written to disk in big-endian 178c2ecf20Sopenharmony_ciorder. 188c2ecf20Sopenharmony_ci 198c2ecf20Sopenharmony_ci.. include:: blocks.rst 208c2ecf20Sopenharmony_ci.. include:: blockgroup.rst 218c2ecf20Sopenharmony_ci.. include:: special_inodes.rst 228c2ecf20Sopenharmony_ci.. include:: allocators.rst 238c2ecf20Sopenharmony_ci.. include:: checksums.rst 248c2ecf20Sopenharmony_ci.. include:: bigalloc.rst 258c2ecf20Sopenharmony_ci.. include:: inlinedata.rst 268c2ecf20Sopenharmony_ci.. include:: eainode.rst 278c2ecf20Sopenharmony_ci.. include:: verity.rst 28