{"dc.va": "rack1", disk:"ssd", ssd: "installed" } {"dc.va": "rack2", disk:"raid"} {"dc.gto": "rack1", disk:"ssd", ssd: "installed" } {"dc.gto": "rack2", disk:"raid"} {"dc.va": "rack1", disk:"ssd", ssd: "installed" } MongoDB Documentation, Release 3.0.4 To target a read operation to a member of the replica set with a disk type of ssd , you could use the following tag set: { disk: "ssd" } However, to create comparable write concern modes, you would specify a different set of getLastErrorModes configuration. Consider the following sequence of operations in the mongo shell: conf = rs.conf() conf.settings = { "getLastErrorModes" : { "ssd" : { "ssd" : 1 }, "MultipleDC" : { "dc.va" : 1, "dc.gto" : 1 } } } 1. Create a replica set configuration object conf : 2. Redefine the getLastErrorModes value to configure two write concern modes: 3. Reconfigure the replica set using the modified conf configuration object: rs.reconfig(conf) Now you can specify the MultipleDC write concern mode, as in the following, to ensure that a write operation propagates to each data center. Changed in version 2.6: A new protocol for write operations (page 852) integrates write concerns with the write operations. Previous versions used the getLastError command to specify the write concerns. db.users.insert( { id: "xyz", status: "A" }, { writeConcern: { w: "MultipleDC" } } ) Additionally, you can specify the ssd write concern mode to ensure that a write operation propagates to at least one instance with an SSD. To reconfigure a replica set when a majority of members are available, use the rs.reconfig() operation on the current primary , following the example in the Replica Set Reconfiguration Procedure . This document provides the following options for re-configuring a replica set when only a minority of members are accessible: You may need to use one of these procedures, for example, in a geographically distributed replica set, where no local group of members can reach a majority. See Replica Set Elections (page 580) for more information on this situation. Reconfigure a Replica Set with Unavailable Members • Reconfigure by Forcing the Reconfiguration (page 639) • Reconfigure by Replacing the Replica Set (page 639) Chapter 9. Replication 638