I have just finished troubleshooting an issue where DRBD was causing a bottleneck for an application (ddumbfs) that has an open mmaped file. The issue was that ddumbfs issues fsync() calls periodically on its mmaped files, and when these were happening I/O would pause while the data was being replicated.
The issue is partially fixed by increasing the max-buffers and max-epoch to the maximum values, to allow the maximum number of operations to be in-flight at any given point in time: