They've gone through a few engines. They have a post about the newest one which isn't quite out yet:

https://influxdb.com/blog/2015/10/07/the_new_influxdb_storag...

We use them for storing "permanent" data. They slice up the data into weekly files, so if you don't access it often, it's no biggie to store a lot of data. If you constantly access data across time, you may run into performance degradation from a caching point of view (having to pull in the files from past, blowing caches of new stuff constantly).