As a corollary, it's not possible to read a single byte from a file (with regular usermode I/O). When you call e.g. `read(fd, buf, 1)` the Linux kernel will rea

As a corollary, it's not possible to read a single byte from a file (with regular usermode I/O). When you call e.g. `read(fd, buf, 1)` the Linux kernel will read an entire 4096 page into the page table cache, and then copy 1 byte from that into your buffer. This might sound inefficient but in practice consecutive reads are so much more efficient than random reads that the difference between reading 1 byte and 4096 bytes is negligible when the data is stored consecutively. Consequently, when designing on-disk data structures that are accessed randomly (like Btree pages in a database, for example), it usually doesn't make sense to make them smaller than 4 KB. (It is possible to bypass the page table cache using O_DIRECT, at least for some filesystems, but that has its own size and alignment restrictions; typically you can only read 512 byte blocks.)