MercuryFTP is a custom data transfer protocol created by Datto specifically for backing up Windows-based production machines. It can perform compression and encryption, and acts as a windowed protocol that allows writing of a second block of data before the first one has finished.
MercuryFTP also increases the efficiency of the differential merge process. To perform a differential merge, the iSCSI and Samba transport methods must read a block from a Datto appliance, compare it to what exists on the production machine, and update the backup set if it is different.
Unlike iSCSI and Samba, MercuryFTP queries the server on the Datto appliance for the hash of the block, which is much shorter to send over the network. Only if the hash doesn't match will the device then send the block. This process creates far less I/O for the same outcome.