When you have a new primary, it starts up the balancer thread. Before starting regular balancing, it tries to recover any migrations that were abandoned when the previous primary stepped down. But the logging for this is poor.

The message "CSRS balancer thread is recovered" (in balancer.cpp) is confusing. Particularly when it doesn't actually recover anything, or it's the first term of the replica set and there was no previous primary. What is being recovered is never explained.

Perhaps adding counters to the recovery functions so that we know whether anything happened, and explaining what 'recovered' or 'recovering' actually mean.