I/O Performance throttled down consistently in every minute with the same pattern. (Windows Clustering 2012 R2 + SQL Clustering 2012 R2)

Dear Expert...

I just done installation of Windows Clustering 2012 R2 + SQL Clustering 2012 R2. However, when i do I/O performance test. the result is not perfect. As per graph below, the I/O seems to have throttled down consistently in every minute with the same pattern. Any idea?

Note: The cluster is connect to SAN storage. The SAN Storage tested fine with other standalone server.

Many Thanks!

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