On long-running backups I've noticed mongodump commands stall at a certain point into the backup. The mongodump process remains running with low CPU/mem usage just doing nothing, and the mongodb-consistent-backup tool sits waiting for it to complete.

This seems to be due to the use of subprocess.Popen() with PIPE-based output in Common/LocalCommand.py. Seeing .communicate() is only called after the command has completed, this makes the PIPE buffer a lot of data and eventually fill.

This issue is to track the problem and resolution: frequently emptying the buffer from Popen() processes so the buffer does not fill.