Hi guys

Our sharepoint 2010 farm have 8 WFEs,  2 Apps and DB runs on clustering. The traffic between WFE, APP and Database Server is so high. How to investigate what factors causes this phenomenon?  

Thank you very much for any suggestion!

On the SQL server you can execute exec sp_who2 in order to identify the running SPID and impacted db.

Then using SQL profiler you can monitor the database in order to extract the stored procedures and identify what SharePoint component is impacting your platform.

Also you can check the timer jobs, full crawl, incremental crawl.

You can implement performance counters on SQL and SPS servers in order to monitor the server activity.

Here you have a complete guide

http://www.sharepointboco.com/troubleshooting-performance-issues-sharepoint-2010/