  Modern data centers are becoming increasingly equipped with RDMA-capable
NICs. These devices enable distributed systems to rely on algorithms designed
for shared memory. RDMA allows consensus to terminate within a few microsecond
in failure-free scenarios, yet, RDMA-optimized algorithms still use expensive
two-sided operations in case of failure. In this work, we present a new
leader-based consensus algorithm that relies solely on one-sided RDMA verbs.
Our algorithm is based on Paxos, it decides in a single one-sided RDMA
operation in the common case, and changes leader also in a single one-sided
RDMA operation in case of failure. We implement our algorithm in the form of an
SMR system named Velos, and we evaluated our system against the
state-of-the-art competitor Mu. Compared to Mu, our solution adds a small
overhead of approximately 0.6 microseconds in failure-free executions and
shines during failover periods during which it is 13 times faster in changing
leader.
