Using nobarrier with ext4 and google persisten disks
Red Hat has a document on the use of barriers:
The use of nobarrier is no longer recommended in Red Hat Enterprise Linux 6 as the negative performance impact of write barriers is negligible (approximately 3%). The benefits of write barriers typically outweigh the performance benefits of disabling them. Additionally, the nobarrier option should never be used on storage configured on virtual machines.
ENABLING/DISABLING WRITE BARRIERS