HP P420 Bandwidth Limitation

I have an HP DL360p G8 with P420g with 1G Cache and 4x Seagate Constellation 3TB as Raid 5 and 10Gbps UPLINK to my datacenter, As I know p420 is 6Gbps so its for the whole of the interfaces? or its 6Gbps Per Interface/hard disks?

Generally, SAS or SATA use point-to-point links, so the specified 6 Gbit/s are per lane/directly connected disk.

A Constellation ES.3 drive is specified at 175 MB/s for sequential access. Four disks in RAID 5 should be able to get you a sequential throughput of ~500 MB/s - that's nothing the P420 bottlenecks.

If that's not enough you'd either need to add more harddisks or replace them with SSDs (SSDs would also increase the number if IOPS by several orders of magnitude, but they have a limited life for large writing amounts).

And while you're at it, consider using RAID levels 6 or 10 for better resilience. RAID 5 has a single drive redundancy. In case of a drive failure, a single previously undetected read error kills your rebuild.