Understand More About "disk update time" stat


Can anyone please help me to understand about the below 2 KPI’s in detailed.

disk update time => The time required to update data on disk.
disk commit time => The time required to commit data on disk.

I didn’t seen commit option in Couchbase, where this commit came into picture in stats. And in Couchbase there is no update, we have only append even we modify any document.