POSTGRESQL error Could not create shared memory segment: Cannot allocate memory
The HINT seems pretty clear,


  This error usually means that PostgreSQL's request for a shared memory segment exceeded available memory or swap space, or exceeded your kernel's SHMALL parameter. You can either reduce the request size or reconfigure the kernel with larger SHMALL. To reduce the request size (currently 5616156672 bytes), reduce PostgreSQL's shared memory usage, perhaps by reducing shared_buffers or max_connections.


Redhat has really good docs on SHMALL. The sysctl method is the new method.

sysctl -w kernel.shmall=5616156672


See man sysctl. These changes do not persist through reboot. Pay attention to the bottom, to make the change permanent,

echo "kernel.shmall=5616156672" &gt;&gt; /etc/sysctl.conf


Also note the top which is great generic advice This parameter sets the total amount of shared memory pages that can be used system wide. Hence, SHMALL should always be at least ceil(shmmax/PAGE_SIZE).