I would say that you're doing something wrong if you're indexing data in Splunk and then deleting it 15 minutes later. Splunk indexes are not intended to hold mutable data, and it's pretty bad at it. (The delete command for example is really only for exceptional cases and doesn't actually remove data from disk.) If you need something that is always, you could consider using lookup files, or (when version 6.2 comes out). These are intended to supplement Splunk use cases with a mutable store. However, if this is your primary data, then Splunk may not be the right solution for this.
