vmware upgrade

how large is your VMware environment?
Obviously you should have your backups up to date.
do you backup your VMDK's

to do the upgrade offline. (in other words make sure all your VM's are shutdown.
make a bootable esxi 5.5 cd
boot the system, it will give you prompts, errors and warnings if any.
errors need to be resolved, warnings need to be investigated (if they will cause any issues).
once the upgrade is done. You'll need to load the new version of vsphere.  just go to http://your VMware server IP
Then download the new vsphere.
Once that's done, login to vpshere, go to console of each OS and upgrade VMTools.

Here is my plan:

5 servers are in a HA cluster. I am going to move all VMs from one server to other 4 servers. Then put the host in maintenance mode. exit out of cluster. upgrade the host.

Host is connected to SAN LUN which is VMFS4 datastore. what will happen after the host is upgraded to 5.5.

The first you will need to do is

1. Check HCL

2. Upgrade current vCenter Server, because vCenter Server 4.0 does not support 5.5.

3. Then upgrade a single Host. (add back to vCenter Server 5.5)

4. Repeat 3 above for all servers.

5. Once you have done ALL the servers, then you can upgrade to VMFS 5.x if you like

6. I would disconnect LUNs from ESXi when upgrading.

7. Just as a warning, some prefer to

a. install a new vCenter Server
b. install new ESXi 5.5 servers

e.g. no upgrades.

Please let me know how this works?

1. I am moving all the VMs from to another hosts. These VMs are VMFS4.
2. upgrading the host to ESXi5 and creating a new Datastore with VMFS5.
3. moving the VMs from VMFS4 datastore to new VMFS5 datastore. What is the impact and is there any steps i need to do before moving to VMFS5 data store?

Please explain me how this works?

I assume you mean VMFS3, there is no VMFS4.

VMs are unaffected by the different versions of VMFS.

But, ESXi 4.x, cannot read a VMFS 5 datastore.

1. No issues, is moving from ESXi 4.x to ESXi 5.5.

2. No issues here.

3. If you have vCenter Server, Migrate will work fine.

Here is our current structure.

1. We have two vcenter environment.  (vcenter 4 and vCenter5)

once we upgrade the host we are going to add this host to vCenter5 environment. existing vCenter4 environment will remain as it is. We are plan to move one by one to vCenter 5 environment.

Okay, make sure vCenter Server is at 5.5.