bcachefs checksums (and compresses) at extent granularity, not block; encoded extents (checksummed/compressed) are limited to 128k by default.

This is a really good tradeoff in practice; the vast majority of applications are doing buffered IO, not small block O_DIRECT reads - that really only comes up in benchmarks :)

And it gets us better compression ratios and better metadata overhead.

We also have quite a bit of flexibility to add something bigger to the extent for FEC, if we need to - we're not limited to a 32/64 bit checksum.