Store huge json data of indefinite size with key

I need to store the geo path data comprising of geo-points which should be indexed by unique key. For example: The path traveled by vehicle indexed by its trip id. This path can be of indefinite length.

As of now, I am thinking to store the path in the form of JSON object. The options that I have in my mind are Riak and MongoDB. I want to go with open-source technology. It will be nice if it supports clustering. In case one node goes down, we won't have any downtime in our application.

MySQL is currently our source of raw data (which we will be anyhow moving to the NoSQL DB but not as of now). But with the huge amount of data (2 million geo-point entries per day), it takes MYSQL a lot of time to filter the data based on timestamp. MySQL will still be our primary data source. The solution I am looking for will act as a cache for faster path retrieval based on id.

In current MySQL schema, the fields I have are:

system_timestamp,
gps_timestamp,
speed,
lat,
lot

This table store all the geo-points of the vehicle whether vehicle is on trip or not. Here trip is based on whether driver wants to track the movement or not. If he want to track the movement, we generate a unique trip id and associate it to the driver along with the trip's start time and the end time. Later for displaying the path based on trip id, we use the start & end time of the trip to filter the data from the raw table.

I want to store the trip path into secondary database as a cache so that it's retrieval will be fast.

Which database should be my ideal choice? What other options do I have?