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Note that if you are on centos 6, the openssh version isn't new enough to support this feature.

this is not true

Yes it is. ControlPersist was introduced in openssh 5.6.

Centos 6 ships with a patched version of 5.3.

Both ssh and sshd I guess? Do both ends need to support the feature?

ah ok. ControlMaster and ControlPath are both supported though.