MongoDB script to backup replication set locally to a ubuntu 14.04 server

Replication in MongoDb is a simple process to setup and I configured it locally during an online course I completed via the 10gen website, obviously this will fail over without an issue to your other instance but depending upon what fails on your machine will it be of any use!

I'd advise working your way through the free course they have it as it will solve 90% of the queries you will have with evaluating the product (replica sets are part of this)

We had it my old company and did run it on several machines for full redundancy and automatic failover to another node, you'll need a witness too.

As an answer to your first question, both tools (by default) will just walk the _id index to fetch the data and then write it out to disk. So, yes, both tools will similarly impact your working set which is why I would generally recommend running them against a secondary (preferably a hidden secondary if possible). I'll echo Stennie in the comments here and recommend other backup methods if you are dealing with large amounts of data.

For the second question, I assume you are looking for a mongodump equivalent of the --fields option from mongoexport to only dump out specific fields. The query option can be used to filter results, but it cannot be used with a projection (to select the fields returned) - this is a feature request that is being tracked in TOOLS-28 but is not yet scheduled. As Stennie also mentioned, the other option here is to write a custom exporter that fits your needs (and again, I would still recommend running it against a secondary to protect your working set).