Unknown VMs, missing datastores

Symptoms of the Issue: 
All the VMs are listed as unknown in a host or cluster ex: unknown, unknown 1, unknown 2
When you select a VM in the left pane under the summary tab you will not see the datastore containing the VM.
You will be able to see the LUN presented in the configuration>storage but through command line too you will not be able to see the LUN presented to the host or cluster.
What worked for me:
esxcfg-volume -M "VMFS UUID" and service mgmt-vmware restart
What caused it :
You moved all the data from the EVA 4000 to EVA 64000.
OR
After A normal Reboot you face this problem
OR
LUNs are detected as snapshots
Why :

when you unpresent and represent an LUN from the Command VIEW side the LUNs are getting an new NAA.XXXXXX Numbers, so the ESX couldn’t find that again. The datastore is then displayed in grey and is not accessable.

Also even when you remount the Datastores back to the ESX Host, the generated UUID for the particular LUN has been changed and this would result in “unable to power on the VM” because the full path for the swap file

is including the UUID which is after the remount different:

http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1011387

Unable to install virtual center agent or hosts disconnected

Issue: While connecting 3.x hosts to the virtual center 2.5 you get an error saying unable to install virtual center server agent and/or incorrect username and password.
Set up: Virtual Center Server 2.5 Update x
Esx 3.x
When customer tries to add the existing esx 3.x host it fails with an error  “Unable to access file.
C:/program <file:///C:/program> files\VMware\Infrastructure\VirtualCenter Server\upgrade\vpx-upgrade-esx-7-linux-104215.sig.sig”

1. check whether you have the vpxa agent installed or not and if it is installed then what is the version of it.
2. rpm -qa grep vpxa
3. If the above command returns no output then skip to the step 6 and if it returns something like "VMware-vpxa-2.5.0-xxxxxx" then follow the next steps too.
4. service mgmt-vmware stop && service vmware-vpxa stop
5. now remove the existing vpxa agent (virtual center server agent) by the command                   rpm -e VMware-vpxa-2.5.0-xxxxxx
6.  In the vcenter server go to C:\Program Files\VMware\Infrastructure\VirtualCenter Server\upgrade and copy                                                                                                                       vpx-upgrade-esx-7-linux-xxxxxx and                                                                           vpx-upgrade-esx-7-linux-xxxxxx.sig                                                                                         to any directory of the troubled esx host and run the command                                                    sh vpx-upgrade-esx-7-linux-xxxxxx                                                                                          to install the vpxa agent (virtual center server agent).                                                                  Note:- xxxxxx should be the same as the build number of the vCenter server. If you do not find the above files then skip the next step.
7. If the above files are missing or still you are facing the same problem then download and install any Virtual Center 2.5 with a build number higher than the existing virtual center. example:- If your virtual center was 2.5 update 3 then you install virtual center 2.5 update 4 or above. Issue Resolved.
Thanks to :- http://vikashkumarroy.blogspot.com/2009/07/not-able-to-add-esx-host-into-virtual.html

VIC 4.U1 Unable to launch p2v

Issue:- vpxclient.exe runtime error while trying to p2v or import a VM.Microsoft Visual C++ Runtime Library error message is displayed during import or export of a machine.
vSphere Client :  4.0 update 1 build 208111
vCenter Server : 4.0 update 1 build 208111
ESX                     : 4.0 update 1
Resolution:
What worked for me:
Update the vSphere Client to update 2.

What did not work but may work for you:
http://michaelellerbeck.com/2009/11/16/vpxclient-exe-runtime-error-vsphere-vcenter-import-crashes/
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1011329

command to know the driver version of the NIC

Issue: - customer wants to know the bnx2 driver version for the NIC.
Resolution: - rRun the command ethtool -i vmnic0