How difficult is it to learn DB2 on mainframe if you know DB2 Linux?
Mainframe..... VM? VSE? MVS? zLinux?
Do you know the mainframe? Know the differance between MVS, OS390 and z/OS? (there is none). Can you write JCL, see if a job went all-right? Ever worked with TSO/ISPF? Ever worked with static-SQL? Ever heard of CICS and/or IMS/DC? What will be your function: DBA or application programmer?
To learn the mainframe will be a bigger challenge than DB2 itself.