[DPE-6124] optimise max map for VM deployments

[MiaAltieri]

Context

Solves issue #518 which reports 2024-11-28T21:05:16.089+00:00: vm.max_map_count is too low

This message comes from the fact that each network connection requires 2 mapped virtual memory areas. So mongodb is pointing out that if the server accepts all of the connections allowed, it will not have enough maps and possibly cause the mongod to crash.

MongoDB defines this value to be 2x the possible of soft ulimit. We could automatically set vm.max_map to 2x of soft ulimit. But I believe this is not advisable since each memory-mapped area consumes resources, and having too many can lead to increased memory usage and slower performance. Question - is there a programmatic way to determine at what point the number of memory mapped areas is detrimental? If so we could always consider decreasing the ulimt.

With that said I believe setting it to 262144 is a good spot to start and we could come back to these questions later in the future

Solution

We adopted an approach similar to Kafka here which is to set vm_max when possible. If it is not possible we log a warning and do not attempt to set it again

Testing

tested by hand in OpenStack deployment to verify that the following warning does not appear when connecting to Mongodb. But it was necessary to restart mongod to have the message go away.

   2024-11-28T21:05:16.089+00:00: vm.max_map_count is too low

know issue

vm.max_map_count is too low

is present until charmed-mongodb.mongod is restarted (despite the value being set before the installation of thecharmed-mongodb snap.)