On Feb 10, 2023, at 7:37 AM, Jan Kara <jack@xxxxxxx> wrote: > So I belive mballoc tries to align everything (offsets & lengths) > to powers of two to reduce fragmentation and simplify the work for > the buddy allocator. If ac->ac_b_ex.fe_len is a power-of-two, the > alignment makes sense. But once we had to resort to higher allocator > passes and just got some random-length extent, the alignment stops > making sense. In addition to optimizing for the buddy allocator, the other reason that the allocations are aligned to power-of-two offsets is to better align with underlying RAID stripes. Otherwise, unaligned writes will cause parity read-modify-write updates to multiple RAID stripes. This alignment can also help (though to a lesser degree) with NAND flash erase blocks. Cheers, Andreas
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