In userland, we tend to think of streams of bytes, as provided by original Unix and as all the docs teach us to treat them - that read(), write() are the primitives and they do byte-aligned reads and writes.

Except the actual Linux VFS has, as its core primitive, mmap() + pagein/pageout mechanism, with read() and write() being simulated over the pagecache which treats the files as mmap()ed memory regions. It's how IO caching is done on Linux, and it's source of various issues for ZFS and people using different architectures because for a long time (changed quite recently, afaik) Linux VFS only supported page-sized or smaller filesystem blocks. Which is a bit of a problem if you're a filesystem like ZFS where the file block can go from 512b to 4MB (or more) in the same dataset, or VMFS which uses 1MB blocks.