my.sharepoint.com.au My site use ContentDB_Mysite.
but my.sharepoint.com.au/private/nameA use different content DB because the my site was created by powershell without specify the content db.
So internally get any content db from sql server using round robin method.
However I want to move all these content db to ContentDB_Mysite.
there is powershell command Move-SPSite.
Move-SPSite <http://ServerName/Sites/SiteName> -DestinationDatabase <DestinationContentDb>