[SOLUTION] ORA-00845 problem

By default /dev/shm is created to use half size of your server physical memory

If you see that MEMORY_TARGET not supported on this system​ while starting Oracle Database Instance in sqlplus window, please follow the instructions as below;

For this case, you should increase memory that using by Oracle as root user (e.g. size=10240m means 10 GB RAM)
-> umount tmpfs
-> mount -t tmpfs shmfs -o size=10240m /dev/shm

If you encounter any error like that "the system is using already", (e.g. process id of /dev/shm is "5265")
-> fuser -m /dev/shm
or
-> lsof /dev/shm
-> kill 5265

Ref: http://blog.yannickjaquier.com/linux/linux-memory-usage.html