Q. If I do not have RDMA capable NICs would I ever select the SMB Live Migration option in Windows Server 2012 R2?
A. Windows Server 2012 R2 introduced two new options for how Live Migration traffic is sent; Compression and SMB. Compression is the default and still uses TCP/IP to transmit the data however the data is compressed which results in less data sent over the network at the expense of CPU cycles to compress and decompress the data however this results in faster migration times. The SMB option does not use compression and instead uses SMB to send the data over the network. If you have RDMA capable network adapters then selecting SMB will result in SMB Direct being used which leverages the RDMA capability of the NICs meaning very low CPU use on the host and fast transfer of the data.
What about if you don't have RDMA capable NICs, would you ever use SMB? The answer is it depends but likely yes. SMB is the best choice when you want to ensure all available network bandwidth is being used which is especially relevant if using multiple NICs for Live Migration. If using either the TCP/IP or Compression option and you have multiple NICs either stand-alone or in a team then concurrent Live Migrations between two hosts would only use one set of NICs because of the algorithms used for load balancing. If however you use SMB then all available NICs will be used, establishing multiple paths, even for a single Live Migration because of the SMB Multichannel feature. Therefore if you want to maximize the utilization of higher capacity networks (generally 10 Gbps or above but possibly less) then use SMB instead of TCP/IP or Compression. The reason for ever using Compression instead of SMB is that Compression does compress the data which is more important when you have smaller amounts of bandwidth available for the Live Migrations than using multiple paths hence the guideline for SMB at 10 Gbps and above and Compression below that.