This site explains the key differences between vSphere 5.1 and vSphere 5.5. An overview comparison from all vSphere Versions is available here.
| vSphere 5.1 | vSphere 5.5 | |
| Released | September 2012 | September 2013 |
| Hypervisor | ESXi | ESXi |
| Physical CPUs per host | 160 | 320 |
| Physical RAM per host | 2 TB | 4 TB |
| vCPU per VM | 64 | 64 |
| vMEM per VM | 1 TB | 1 TB |
| VMDK Size | 2TB | 62 TB |
| VM Hardware Version | 9 | 10 |
| VMFS Version | 5.58 | 5.60 |
| Free ESXi Memory Limit | 32 GB | unlimited |
| Single Sign-On | SSO 1.0 built on identity management by RSA | SSO 5.5 completely rewritten from the ground up. Supports a multi-master model |
| vSphere Flash Read Cache | NO | YES |
| Virtual SAN | NO | YES |
| Reliable Memory Technology | NO | YES |
| vSphere Big Data Extensions | NO | YES |
| vSphere App HA | NO | YES |
| 40GBps NIC Support | NO | YES |
| Windows 2012 Cluster Support | NO | YES |
| PDL AutoRemove | NO | YES |
| vSphere Replication | Only one copy of a VM | Up to 24 replicas |
| vCSA Scale (vPostgres) | 5 Hosts 50 VMs | 100 Hosts 3000 VMs |
:cool: Nice..! and Thanks...