[CentOS] ZFS Scrubbing (might be off topic)

Folks

I have a ZFS file system.  It seems to be scrubbing too often.  As I 
type, it's 5 hours into the process with 36 hours to go, and seems to 
be doing it several times a week on a slow drive.

I cannot find an option to control the frequency; crontab has no 
references.  Any clues?

David