The daily indexing guideline per indexer increased from 250 GB/day to 300 GB/day, i.e a 20% increase in indexing volume. This is a result of the optimizations made in 6.3. In order to get a good understanding of some of the improvements and the effect it will have on resource utilization, I recommend going through the session notes from "Harnessing Performance and Scalability in Splunk 6.3" and Splunk docs. Please find the links below
https://conf.splunk.com/session/2015/conf2015_ANekkanti_SPal_ATameem_Splunk_SplunkClassics_Harnessing63PerformanceAnd_a.pdf
http://docs.splunk.com/Documentation/Splunk/6.3.0/Capacity/Parallelization
http://docs.splunk.com/Documentation/Splunk/6.3.0/Indexer/Pipelinesets
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