using madvise for local.oplog.rs (or log.oplog.$main) may improve performance, as we know the access pattern. for example this could save waste of the file system cache if the system knows in some cases it won't be needing the data again

specifically:

(1) MADV_SEQUENTIAL might perform better in general

(2) MADV_DONTNEED might make sense on a replica set non-primary. on a non-primary, no one is reading its oplog, we are just writing to it.

if these were tested and performance was the same, I would be in favor of using, as they probably make caching smarter, if anything.

http://linux.die.net/man/2/madvise