How to Resolve ORA-00845: MEMORY_TARGET not supported on this system

If you are planning to install additional database by DBCA on a Linux server, you should notice not only the limitation of semaphores, but also the limitation of the shared memory, otherwise, you might see ORA-00845 like the following when trying to startup the second database after the first one is online:
SQL> startup;
ORA-00845: MEMORY_TARGET not supported on this system
The root cause came from the insufficient shared memory on the Linux server. So let's check the usage of the shared memory /dev/shm.
[oracle@primary01 ~]$ df -h
Filesystem            Size  Used Avail Use% Mounted on
...
tmpfs                1006M  419M  587M  42% /dev/shm
The above result originates from /etc/fstab, you can see the total shared memory mounted on /dev/shm is 1G by default, and the available shared memory is not enough to initiate another database. Next, let's check the static configuration in /etc/fstab
[root@primary01 ~]# cat /etc/fstab
...
tmpfs                   /dev/shm                tmpfs   defaults        0 0
...
We can explicitly increase the shared memory from the default value 1G to 2G by setting "size=2g".
[root@primary01 ~]# vi /etc/fstab
...
tmpfs                   /dev/shm                tmpfs   size=2g        0 0
#tmpfs                   /dev/shm                tmpfs   defaults        0 0
...
Let's see the dynamic result of the shared memory again.
[oracle@primary02 ~]$ df -h
Filesystem            Size  Used Avail Use% Mounted on
...
tmpfs                 2.0G  754M  1.3G  37% /dev/shm
Now, we can startup the second database normally.