Write caches on disks can result in filesystem corruption since XFS may be told a log transaction has reached the disk when in fact it is still in the disk cache.
A journal log assumes that the log transaction is updated on disk before updating the metadata in the filesystem, but this may not be true with write caching
XFS is able to issue write barriers if the underlying devices support it
Ensures that the log entry is written before any other data
Write barriers are enabled by default in XFS
Filesystem will attempt to determine if barriers are supported and will issue a warning to the syslog if they are not
The nobarrier option disables write barriers
Barriers should be disabled when using a RAID with battery backed controller cache (but only if the individual disk write caches are disabled)