This is going to be a bunch of small points. There is sadly no single answer to your question, however.
MongoDB allows the OS kernel to handle memory-management. Aside from throwing as much RAM as possible at the problem, there are only a few things that can be done to 'actively manage' your Working Set.
The one thing that you can do to optimize writes is to first query for that record (do a read), so that it's in working memory. This will avoid the performance problems associated with the process-wide Global Lock (which is supposed to become per-db in v2.2)
There is no hard-and-fast rule for RAM vs SSD ratio, but I think that the raw IOPS of SSDs should allow you to go with a much lower ratio. Off the top of my head, 1:3 is probably the lowest you want to go with. But given the higher costs and lower capacities, you are likely going to need to keep that ratio down anyway.
Regarding 'Write vs Reading phases', am I reading correctly that once a record is written, it is seldom updated ("upserted")? If that is the case, it may be worthwhile to host two clusters; the normal write cluster, and read-optimized cluster for "aged" data that hasn't been modified in [X time period]. I would definitely enable slave-read on this cluster. (Personally, I'd manage that by including a date-modified value in your db's object documents.)
If you have the ability to load-test before going into Prod, perf monitor the hell out of it. MongoDB was written with the assumption that it would be often be deployed in VMs (their reference systems are in EC2), so don't be afraid to shard out to VMs.
A connection consists of a source IP:Port and a destination IP:Port. Packets from the source IP:Port have to traverse the OUTPUT chain. This happens even when you are connecting to the loopback interface so as you have discovered you need to allow outgoing connections to 127.0.0.1.
It is normal not to block the loopback interface as many services use it and doing so can cause problems.
Best Answer
You can create another connection from the destination server and use the
save()function.For example: