Good article but there are a few problems. Some of those with my code, I didn't realize I had changed the string column from text to a blob in the new schema and I added a note to include the deletes for the new index tables.
Here are some corrections. The string tables are only used for populating the UI and in fact only the metric_names is used, the tag_names and tag_values aren't used at all. In the new beta I don't populate the last two anymore.
When doing queries it reads the row_key_index only once. In the beta code it will still read the old row_key_index but as soon as you go beyond the 3 week window that query will return lightning fast as cassandra does a really good job of quickly telling you when data is not there.
Did you get a chance to run some performance tests?