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:
There are 2 parts to this that I would like to note. First, in order to get drbd, iscsi and multipath to play nicely together under systemd, I had to do the following:
Create /etc/systemd/system/drbd.service.d/override-after.conf with the following contents:
[Unit] After=systemd-modules-load.service network.target open-iscsi.service dev-mapper-bhqibm01\x2dr6\x2dsata01\x2dlun01.device Requires=dev-mapper-bhqibm01\x2dr6\x2dsata01\x2dlun01.device
I also had to edite each of the /etc/iscsi/nodes/_iqn_/_ip_/default files and set: