read --> disk blocks

there are two kinds of high-level accesses: sync, async
there are two kinds of low-level accesses: page cache and direct IO
sync infact uses async adding waiting on the async
sync, async both go through low-level accesses either through page cache or skipping page cache

generally unix filesystems use native unix read/write implementation. from files, control goes to mapping toward inode read_page method. that method converts file pages to disk blocks.