As all SRX already in dual partioning , do I need to mention "PARTION" keyword in "reqest system software add \var\tmpfilename.tgz no-copy reboot" command?
ANS: NO, 12.1X44 will be already running with dual partition. You can use "show system snapshot media internal" to verify the same.
Which partion will be having newer image of JUNOS?
ANS: Newer version will go to Primary partition and Current 12.1XC44 will go to backup
How can I make both partion having same JUNOS snapshot? I know " request system snapshot slice alternate" will help but still want expert inputs.
ANS: You are right, run " request system snapshot slice alternate " after upgrade and reboot
If "reqest system software add \var\tmpfilename.tgz no-copy reboot" fails then how device will come up?
ANS: If upgrade fail device wont reboot. If there are any issues during reboot, device will try booting from backup partition which is 12.1X44