Most file systems (such as XFS, Ext3, Ext4, or reiserfs) send write barriers to disk after fsync or during transaction commits. Write barriers enforce proper ordering of writes, making volatile disk write caches safe to use (at some performance penalty). If your disks are battery-backed in one way or another, disabling barriers can safely improve performance.
Sending write barriers can be disabled using the nobarrier mount option.
WARNING: Disabling Barriers Can Lead to Data Loss
Disabling barriers when disks cannot guarantee caches are properly written in case of power failure can lead to severe file system corruption and data loss.