Add runbook for MongoDBVirtualMemoryUsage

[b3n3w]

Hello,

first of all, thanks for this collection of alerts!.
I've tested the set of percona mongoDB alerts in my current setup and could see, that the "MongoDBVirtualMemoryUsage" is always firing.
This means, that the virtual memory is taking up more than three times the space as the resident memory -> high memory usage.

Do you have any idea how to handle this ? Currently it is super noisy for me.

[samber]

I have no mongo instance for testing this rule. Can you write a new query?

[dosmanak]

Hello. I observe this alert on one secondary in 3 nodes cluster. The server has plenty of RAM and there is not much load at the moment. Yet after some batch operation, this alert remained active whole day.
It seems the mongo does not free memory if not under pressure.

[dosmanak]

Here is probably the source for metrics in mongo docs.

The value of mem.resident is roughly equivalent to the amount of RAM, in mebibyte (MiB), currently used by the database process. During normal use, this value tends to grow. In dedicated database servers, this number tends to approach the total amount of system memory.

I suggest to delete this alert.

[samber]

done