MongoDB will flush data to the disk every 1 minute by default. Background flush refers to the time it takes for the data to persist to disk. It should not be more than 1 second for every one minute period.

Modifying the flush interval may help with background flush time; by writing to disk more frequently there will be less data to write and this could, in some cases, make writes faster.

The fact that background flush time gets affected by the write load means that if our background flush time starts getting too high, we should consider sharding our database to increase write capacity.