QoS definitions define the link attributes and bandwidth limiting for replications using that link.
You must specify a QoS definition as part of a replication. If you do not have a QoS definition, click on Create QoS Definition to create one.
In a synchronous replication, each write is committed to the local Compellent and at the same time (synchronously) to the remote Compellent. Both are completed before the write is acknowledged to the server. Reads are local.