I'm wondering what the best data structure (for storing data on disk) is for storing immutable time-series data (99% of the data is truly immutable, the 1% is metadata that is separate from the immutable data). I've been looking at log-structured merge-trees in particular because of it's heavy use by Cassandra and the like.
I do not really see what immutable has to do with this.
You simply store it in the normal way and choose not to update it.
Your problem seems to be how to deal with a high insert rate, which unless you are google, amazon or facebook will be easily handled by any modern database.