Eventually you will upgrade your ESX/ESXi environment to ESXi5. With vSphere 5 VMware eliminated the ESX branch of their virtualization products. All future versions will run directly on the hypervisor platform ESXi. The normal upgrade path to vSphere 5 starts with the vCenter Upgrade to vCenter 5. From there you will tackle the different ESX and ESXi hosts.
For ESX hosts VMware offers a force migration tool that brings the specific host in question over to the ESXi world. I have done this several times at this point and the in-place force migration worked flawlessly. The formerly ESX 4.1 hosts rebooted after the force migration and came back as ESXi hosts – in a certain way this is kind of weird and probably feels like one of those transgender surgeries where the dude gets knocked out and wakes up being a dudette. 
Anyway, the process works great and you should definitely consider it as part of your upgrade path to ESXi 5.
One of the new features of vSphere 5 is the new or updates VMFS file system “VMFS 5”. With version 5 of their VMFS file system VMware has removed certain limitations and issues related to the older versions of their file system. As an example, the older VMFS versions were limited to a 2 TB LUN size. You were also tied to the initial block size that was chosen when a LUN was created. If you had to deploy a larger VM down the road you could not put a 1.1 TB VMDK on a LUN with a 2 meg block size. By default the new block size is 1 meg and that’s it. You do not need anything else even if you deploy very large VMDKs. By the way – if you are using pass-thru RDMs – VMFS 5 now supports RDMs up to a size of 60 TB. Quite impressive! Please keep in mind that existing RDMs are still limited to 2 TB in size. You still need to create new ones and migrate your data over if you want to take advantage of the new 60 TB limit.
An important requirement that needs to be in place before you can upgrade is the fact that each vSphere host needs to be running ESXi5 or higher. Only ESXi 5 vSphere hosts can read the VMFS5 file system. Existing LUNs will keep the block size that was set during the initial configuration. You will have to move all VMs off that LUN and recreate to take advantage of the 1 MB block size in VMFS 5. However, in-place upgrades work just fine on your existing LUNs and you can do this with all VMs up and running. No need to power off your virtual machines.
It is also important to know that VMware is putting away with those darn iSCSI reservation problems by switching over to ATS VAAI primitives. This enhancement improves performance in and around file locking that often was an issue with iSCSI reservations.
How does the migration to VMFS 5 work? Migrating your VMFS file system over to VMFS5 is extremely easy. In vCenter select any of the existing hosts, select the Configuration tab and then select storage. Select the LUN that you want to upgrade to VMFS 5. As shown on the screenshot the lower pane of the windows gives you the option to “Upgrade to VMFS-5” right from vCenter. If you are brave enough to continue (just kidding – go ahead and do it and reap the benefits) you will get prompted with a warning message telling you that all hosts that need to access that LUN need to be ESXi 5 or higher . From there it is all easy. Click “OK” and off it goes. vCenter will show the task for the upgrade in the task pane and then it will initiate a rescan of all hosts in that particular cluster. The entire process is done in under 1 minute. I am not sure if that is just the visible piece or if the upgrade process takes longer “under the hood”. In my testing I was pinging VMs that were sitting on the LUNs as well as I was having RDP (Remote Desktop) sessions open. I did not see any outage nor did I felt a “visible” performance impact. You can also do the migration via PowerCLI through the vmkfstools: “# vmkfstools -T /vmfs/volumes/<VMFS3datastorename>”
Conclusion: Upgrading your existing VMFS LUNs to the new VMFS 5 file system in VMware vSphere is easy and should be one of your first steps after upgrading your hosts to ESXi 5. However, I am a big friend of testing even the easy and safe things in my environment so that I see how things are behaving and therefore I recommend you do the same. VMFS5 offers great benefits and you should definitely look at the different specs for VMFS 5 and take advantage of them.
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