Few KB articles which I went through and feel very useful and great.
Keeping this for my Future reference.
Recreating a missing virtual machine disk (VMDK) descriptor file
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1002511
Unmounting a LUN or Detaching a Datastore/Storage Device from multiple ESXi 5.x hosts
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2004605
Investigating virtual machine file locks on ESXi/ESX
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=10051
Methods of upgrading to ESXi 5.1
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2032757
Upgrading to vCenter Server 5.1 best practices
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2021193
Installing or upgrading to ESXi 5.1 best practices
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2032756