I found this.

Old environment HP BL460c G6, in enclosure with Virtual connect, ESXi 5.1 installed, everything works fine regarding connections to P2000 G3

New environment. HP BL460c Gen8 v2, in enclosure with Cisco FEX and brocade san switch. ESXi 5.1 installed, connections to P2000 goes on and off. I have 8 LUN's attached the hosts and 7 of those goes on and off (vmfs 5.58). 1 LUN with older vmfs i stable. The attached LUN's stays online if I do this. In vcenter, on the host, configuration, advanced settings, vmfs 3, Set "VMFS3.hardwareacceleratedlocking to 0.

I then tried to install ESXi 5.5. In this version I cannot see P2000 LUN's at all, until hardwareacceleratedlocking is disabled, then connections goes online.

Having P2000, P6500 and 3Par in our environment, I'm not sure yet whether this setting is relevant for us.

NB. we have an other retired P2000 G3, which I tried to firmware upgrade to latest level (new firmware came in march 2014).

After this firmware upgrade I can attach LUN's to ESXi 5.5 host, without disabling hardwareacceleratedlocking feature.