3.1.5 Oracle Linux 7 update 3 systems running UEK R3 are unable to boot if the /root partition is formatted using btrfs

Updates to the btrfs file system utilities that include new features, such as the skinny-metadata feature, are incompatible with UEK R3. If the /root partition is formatted using btrfs, a system running UEK R3 is unable to boot as it cannot mount the /root file system. The system is able to boot using UEK R4 or RHCK kernel types.

This problem is particularly important during installation, as it is possible to manually configure partitioning and the file systems from within the installer. When the file system is formatted within the installer, using btrfs, all of the newer features are enabled. If you select to downgrade the kernel to UEK R3 after installation, the system is unable to boot.

Systems that are already using btrfs with UEK R3 are unaffected when they are upgraded, as the file system would have been formatted with the new features disabled. (Bug ID 24840489)