Just a couple of minor corrections: A ZFS physical block also records a variable, contiguous range of bytes on-disk, not unlike what you describe with bcachefs

Just a couple of minor corrections: A ZFS physical block also records a variable, contiguous range of bytes on-disk, not unlike what you describe with bcachefs extents (although, in ZFS the "contiguous" part can get more complicated due to gang blocks and RAID-Z, I think). That said, if you don't have compression enabled, then in ZFS the physical size of the block usually does correspond to the logical size, regardless of how much data was written to it (except for files smaller than one logical block size). Also, if I remember correctly, a ZFS logical block usually only maps to one ZFS physical block, although it can map to up to 3 physical blocks, each of them having the same data (the actual number depends on how many redundant physical copies the logical block is configured to have on top of the ZFS pool redundancy). But in any case, thank you for your long explanation. Some details still escape me, but I can consult the architecture document you linked if I have more doubts.