MongoDB initial sync is very slow

Question

We are synchronizing the new server with our MMAPv1 replica set, and since this one uses the wiredTiger storage engine, we must complete the initial synchronization.

Now the process inserts only a few thousand documents per hour. Is there any way to debug what is the bottleneck?

Both CPU and IO do not seem to be a problem, this is a sample of iostat:

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           1.99    0.05    0.52    4.01    1.00   92.43

The new server is synchronized with the secondary, which has similar indicators.

Answer 1

We had fastsync=truea configuration file inside. Using this parameter, the server replicated new data, not existing data.