I am building an index in MongoDB for a large collections (~300GB) and the db terminal window keeps printing:
[DataFileSync] flushing mmaps took 10920ms for 151 files
The time that it takes to flush mmaps is usually around 10 seconds and sometimes higher. I'm running MongoDB on a high performance machine so RAM availability is not an issue.
What does the data file sync message mean?
Is it an issue??
The real problem with the flushing routine is that MongoDB seems to freeze while doing that.
Even simple queries from a terminal window on the same host have to wait for the flushing to finish in order to complete and ~10 seconds (up to 150s) is way too long. The routine happens (at irregular intervals of 1, 2, or 3 mins) even if no operations are run on the server and there are zero connections active.
I'm running MongoDB 2.2.1 2008R2+ on Windows Server 2008 R2.