I was working on testing some 11g streams configurations today. I needed to startup 3 databases instances on the same server. I was using AMM (Automatic memory management). When i was trying to startup the 3rd database, i kept getting the error message “ORA-00845: MEMORY_TARGET not supported on this system”. I also had error messages in the alert log.
This is because, the space allocated for /dev/shm is not sufficient to allocate the SGA+PGA’s for all the 3 database instances (When using the initialization parameter memory_target). The space allocated needs to be >= the total SGA+PGA size of all the 3 instances together.
You can increase the space allocated using the command “mount -t tmpfs shmfs -o size=2000m /dev/shm” ( I had 3 instances 600mb each SGA+PGA). You can persist this allocation across reboots by adding it to the /etc/fstab.