My setup: I have 2 hosts, and 2 shards each. Host1 has 2 shards, and is the master of the replicas host2 has the secondaries of the 2 shards. . host1: shard1 (repset1),shard2 (repset2) host2: shard1 (repset1),shard2 (repset2) There's also a 3rd host that acts as arbitrer. I have 50 threads writing randomly to both shards (using a hash) via mongos with REPLICA_SAFE WriteConcern set on each insert. The questions: mongostat displays about 90% locked for both shards in host1 and about 1% locked on host2. Since I use REPLICA_SAFE which supposedly writes to both servers shouldn't the locks be the same? mongostat reports qr=30 for both shards of host1, and qw=0 always. Since I perform only writes how is this possible? Moreover on host2 all queues are reported 0. Faults are abut the same in all shards/hosts (arround 80). netIn/netOut on the secondaries (host2) are always about 200bytes/sec. Too low. mongos has 53 connections, host1's shards have 71 and 71 and host2's shards have 9 and 8. How is this? Please answer whatever you can. Thanks!