There is a known issue (bug #bz290821) in the device mapper which prevents reliable use of the cluster mirror in a configuration of 3 or more legs. This issue has not been seen in 2 leg mirror configurations. Customers should wait for the next kernel update to the 4.6 kernel before using 3 leg mirrors.
Actually this issue *has* been seen in 2 leg mirror configurations, just not as often.
reassigning to pkennedy
It appears that this content is included in the release notes here: http://www.redhat.com/docs/manuals/csgfs/release-notes/CS_4-RHEL4U6-relnotes.html