The default sync rate is normally defined in your drbd configuration (mostly /etc/drbd.conf)

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syncer {
    rate 100M; 
}

If you are setting up a virtual machine on drbd cluster you need to produce more I/O and your physical machines are under heavy load. You can stop some services or you can reduce the sync rate.

Set temporary sync rate:
You need to issue this command only on one of your nodes!

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root@drbd-node1:~# drbdsetup /dev/drbd0 syncer -r 10M

Enjoy the easy solution;)