So, the question mostly is what are barriers ?
Speaking of journaled filesystems, if you really want to grant the integrity and consistency of the data, the kernel must be able to perform
some write operation before some other write operation. The problem that arise is that practically every hard disk now days sports some
form of write cache that could interfere with the sequence of writes. To avoid this problem some operation has been introduced (barriers)
meaning that all write operation before one specific point should occurs before the write operations after the barrier.
But .. not all hard disks look like to support the "barriers", hence the kernel message.
"Ordered data mode": continuing to speak of journaled filesystem, journaling all write operation may result in performance problems,
mostly because each write operation will result in a double write, once on journal, and once on a definitive location.
An alternate method of journaling is the so called "ordered data mode", this method really perform journalling only related to metadata,
and not the on the data itself, resulting in a good consistency but not as strong as a fully journaled filesystem, a tradeoff between
consistency and performance.
Commit interval is the maximum time the kernel will wait before to flush the cache to the hard disk.
(laptop-mode often increase this delay, resulting in power saving at price of probability to lost more data).