Re: [PATCH v3 7/8] ext4: Use rbtrees to manage PAs instead of inode i_prealloc_list

[Andreas Dilger]

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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