Configure overcommit_ratio on PostgreSQL servers

[RobHooper]

unattended-upgrades on ocp22 has been failing reporting it is unable to allocate enough memory.
We have temporarily set overcommit_memory = 0 (the default value) to mitigate this.

PostgreSQL recommends overcommit_memory = 2 to help protect the service from accidental termination when memory resources are exhausted, however, this setting limits memory for all process causing issues particularly on smaller servers.

To resolve this we should increase overcommit_ratio from 50% to 90% / 100% allowing user space to access all available memory.

Further reading:

• https://www.postgresql.org/docs/current/kernel-resources.html#LINUX-MEMORY-OVERCOMMIT
• https://www.kernel.org/doc/Documentation/vm/overcommit-accounting

[jpmckinney]

Noting that Redis also warns:

WARNING Memory overcommit must be enabled! Without it, a background save or replication may fail under low memory condition. Being disabled, it can can also cause failures without low memory condition, see jemalloc/jemalloc#1328. To fix this issue add 'vm.overcommit_memory = 1' to /etc/sysctl.conf and then reboot or run the command 'sysctl vm.overcommit_memory=1' for this to take effect.