Kristina Chodorow explains the MongoDB
safe and fsync are not the same, here’s a rundown of the options:
safe => false: do not wait for a db response
safe => true: wait for a db response
safe => num: wait for that many servers to have the write before returning
fsync => true: fsync the write to disk before returning.
fsync => true implies
safe=>true, but not visa versa.
safe=>true and the write could be in successfully applied to a mmapped file but not yet written to disk.
Related to MongoDB durability.
Original title and link for this post: MongoDB safe and fsync Explained (published on the NoSQL blog: myNoSQL)