The new DRBD release (9.3.1) exhibits a severe performance degradation for RDMA. Transfer rates plummet to a handful of MB/s, far below expectations for a 100 Gb/s Infiniband connection equipped with a single Mellanox ConnectX-5 card.
For example, copying 11400 MB starting out at 1.3 GB/s rapidly deteriorates afterward. The final 100 MB segments complete at just 2.5 MB/s and 1.1 MB/s:
# dd if=/dev/zero of=test bs=1M count=11400 status=progress oflag=direct
10686038016 bytes (11 GB, 10 GiB) copied, 8 s, 1.3 GB/s
# dd if=/dev/zero of=test bs=1M count=100 status=progress oflag=direct
104857600 bytes (105 MB, 100 MiB) copied, 43 s, 2.5 MB/s
# dd if=/dev/zero of=test bs=1M count=100 status=progress oflag=direct
104857600 bytes (105 MB, 100 MiB) copied, 94.2242 s, 1.1 MB/s