Oracle startup not possible – ORA-00845: MEMORY_TARGET not supported on this system – but memory size seems to be fine

You might be using Automatic Memory Management (AMM).

AMM uses two initialization parameters:

• MEMORY_TARGET
• MEMORY_MAX_TARGET

The shared memory file system should have enough space to accommodate the MEMORY_TARGET and MEMORY_MAX_TARGET values.

To verify:

SQL> show parameter memory

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
hi_shared_memory_address             integer     0
memory_max_target                    big integer 6096M
memory_target                        big integer 6096M
shared_memory_address                integer     0

In UNIX/Linux, you will have to set the shared memory file system accordingly.

Verify:

df -h /dev/shm

Set:

mount -t tmpfs shmfs -o size=<some_value_in_number_with_size> /dev/shm

For example,

mount -t tmpfs shmfs -o size=4096m /dev/shm