Will using summary indexes impact my total indexing volume and my license?
Yes. Summary indexes do count toward your total daily indexing volume. From a license perspective summary indexes are no different than the main index. The only indexed data that does not count towards your license are Splunk's own log files.
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which is true?
Do they or do they not count against volume?
This is no longer true.
I downvoted this post because this is plain ol' wrong.
I downvoted this post because this is old and now wrong information
As of version 4.0.10 and 4.1, summary indexed data DOES NOT count against your license!