I've been doing some defrag related tests, and for that I needed to be able to create file with same set of block numbers (i.e. extents), for at least two times. May I know if there is any randomness in Ext4 allocator, and if there is any, can I disable it for the purpose of getting repeatable block-allocation patterns. Here are experiment details - For a 100K file (created using dd if=/dev/zero of=/mnt/f100k bs=4K count=100, oflag=direct) I got extent info in one run as this - File size of /mnt/file400k is 409600 (100 blocks of 4096 bytes) ext: logical_offset: physical_offset: length: expected: flags: 0: 0.. 15: 34816.. 34831: 16: 1: 16.. 99: 33824.. 33907: 84: 34832: last,eof while in the the second run I got somewhat different runs - File size of /mnt/file400k is 409600 (100 blocks of 4096 bytes) ext: logical_offset: physical_offset: length: expected: flags: 0: 0.. 0: 34816.. 34816: 1: 1: 1.. 15: 34320.. 34334: 15: 34817: 2: 16.. 99: 33824.. 33907: 84: 34335: last,eof Each run beings with a mkfs.ext4 with lazy inode/journal initialization disabled. Thanks, -- Joshi
