This article decribes how defragementing a 900 GB drive assisted in addressing major performance issues on a reporting server.
If you are relying on using 'best-practice' percentage-based thresholds when you are creating an index maintenance plan for a SQL Server that checks the fragmentation in your pages, you may miss occasional 'edge' conditions on larger tables that will cause severe degradation in performance. It is worth being aware of patterns of data access in particular tables when judging the best threshold figure to use.
This article describes how an index gets fragmented and the steps which a DBA can take to fix index fragmentation
Disk Fragmentation is one of the cause of degraded performance. Use this script for fragmentation analysis report.