Does MongoDB journaling guarantee durability?

Question

Even if journaling is on, is there still a chance to lose writes in MongoDB?

\"By default, the greatest extent of lost writes, i.e., those not made to the journal,

Answer 1

Journaling is for keeping the data on a particular mongod in a consistent state, even in case of chainsaw madness, however with client settings through writeconcern it can be used to force out durability. About write concern DOCS.

There is an option, j:1, which you can read about here which ensures that the particular write operation waits for acknowledge till it is written to the journal file on disk (so not just in the memory map). However this docs says the opposite. :) I would vote for the first case it makes me feel more comfortable.

If you run lots of commands with such option mongodb will adapt the size of the commit interval of the journal to speed things up, you can read about it here: DOCS this one you also mentioned and as others already said that you can specify an interval between 2-300ms.

Durability is much more ensured in my opinion over the w:2 option while if the update/write operation is acknowledged by two members in a replicaset it is really unlikely to lose both in the same minute (datafile flush interval), but not impossible.

Using both options will cause the situation that when the operation is acknowledged by the database cluster it will reside in memory at two different boxes and on one it will be in a consistent recoverable disk place too.