1.. SPDX-License-Identifier: GPL-2.0 2 3High Level Design 4================= 5 6An ext4 file system is split into a series of block groups. To reduce 7performance difficulties due to fragmentation, the block allocator tries 8very hard to keep each file's blocks within the same group, thereby 9reducing seek times. The size of a block group is specified in 10``sb.s_blocks_per_group`` blocks, though it can also calculated as 8 \* 11``block_size_in_bytes``. With the default block size of 4KiB, each group 12will contain 32,768 blocks, for a length of 128MiB. The number of block 13groups is the size of the device divided by the size of a block group. 14 15All fields in ext4 are written to disk in little-endian order. HOWEVER, 16all fields in jbd2 (the journal) are written to disk in big-endian 17order. 18 19.. include:: blocks.rst 20.. include:: blockgroup.rst 21.. include:: special_inodes.rst 22.. include:: allocators.rst 23.. include:: checksums.rst 24.. include:: bigalloc.rst 25.. include:: inlinedata.rst 26.. include:: eainode.rst 27