Stability and impract of TCP Fast Open

Ops and Admin
,
#1

We have a sharded cluster where each shard has a 5-node replica set spread across 5 physical data centers. Our mongos instances are located on an app servers and can have 150ms latency because they are on the other side of the planet. I stumbled across --tcpFastOpenServer and thought it could significantly reduce connection setup latency while also reducing the number of needed pooled connections (since creating a new connection wouldn’t require a 3-way handshake).

Has anyone here turned this setting on? If so, do you have any data or anecdotes of how well it worked? Anyone have any unexpected negative results? I haven’t seen much on the forums discussing this.

#2

The documentation seems to indicate this is on by default if both the client and server support TFO. My db.serverStatus() output contains the following:

    tcpFastOpen: {
      kernelSetting: Long("1"),
      serverSupported: true,
      clientSupported: true,
      accepted: Long("0")
    },

Is there a way to see if TFO is actually already being used?