The article mentions Postgres using many lseek and read calls. Does anyone know why Postgres doesn't use pread to cut the number of context switches in half (if, as the author says, the number of context switches is such a bottleneck)? On systems where pread doesn't exist, presumably they could use an inline function that emulates pread with lseek and read.

Yes, mmap would presumably be preferable in most situations, but pread would seem to be universally preferable to an lseek and read. I assume the kernel keeps per-fd statistics on sequential reads anyway, so for long-lived fds, lseek presumably wouldn't provide any better read-ahead hinting than pread.