Question

I have safe mode turned off in my MongoDB database because none of the data being written is absolutely 100% mission critical and the gain in insertion speed is very important, but I would really prefer if all of the data is written to the database.

My understanding is that with journaling turned on and safe mode turned off, if the server crashes in the 100ms between when a write request is received and the data is output to the journal, the data can be lost.

If the data is successfully written to the journal, is it a pretty safe bet, even if the database is lagging due to heavy load, that the data will end up in the database when the database catches up and is able to process what’s in the journal? Or is my understanding of what the journal does flawed? Are there any other circumstances under which inserted data may be lost?

What happens if I update a document a fraction of a second before another process attempts to read it, but the changes haven’t been committed to the collection yet? Will the read block until the insert has completed?

Answer 1

• The read will only be blocked until the insert has completed if the read is requested on the same connection as the write. There is no guarantee that once the data is written, it will be immediately visible to other connections unless proper getLastError is used.
• Data is processed and put in memory mapped region of data before journaling. However, it may not be “fsynced” to disk as often as journaling. This means that even though the load is high, the data should be eventually updated and become visible to other connections. Journaling data is only used to restore durability when mongod instance unexpectedly crashes.
• Your data may be lost due to network interruption, disk corruption, index dup, etc.