Extremely slow resync rate of DRBD on dedicated gigabit

After @Nils comment, I started looking into how utilized the disks are. And noticed that I am getting a lot more reads than I used to before the system reconfiguration to DRBD. Futher research showed disk utilization at near 100%, and slow down of batch processes that were running at that time. Fixing MySQL config to increase buffer pool size to eliminate most of the reads looks like fixed the issue.

So the problem was that the drives were so busy, that they couldn't handle a lot of resync work that DRBD wanted to throw at them.

Try forcing a sync rate

drbdsetup /dev/drbd0 syncer -r 100M

You can also set that up for after reboot via the syncer {} in the config