[tytso]

Um... Oracle (and other enterprise databases like DB2) don't use mmap. They use Direct I/O. Oracle does have anonymous (non-file-backed) memory which is mmap'ed and shared across various Oracle processes, called the Shared Global Area (SGA), but it's not used for I/O.

[hyc_symas]

Fwiw, I wrote a Direct I/O patch for BerkeleyDB but withdrew it later because it didn't ever improve I/O perf or memory footprint.