vSphere 4/5 Hosts with VMFS-5

[SuperSpike]

I have a question regarding running vSphere 4 and vSphere 5 hosts in a sort of "mixed mode" environment. Specifically, I am bringing a significant number of new hosts online in my data centers, along with a bunch of new storage. I want to install vSphere 5 on these hosts, and I want the new shared storage devices formatted with VMFS-5. My question is, even though the existing vSphere 4 hosts won't be able to access the VMFS-5 file systems, is there any harm in having both vSphere 4 & vSphere 5 hosts with visibility (via SAN zoning) to both VMFS-3 and VMFS-5 datastores?

I've read through http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=200381... as well as http://www.vmware.com/files/pdf/techpaper/VMFS-5_Upgrade_Considerations.pdf. While both documents explicitly say that any vSphere 4 host will lose access to any datastore upgraded from VMFS-3 to VMFS-5, neither doc deals with the issue of having both versions running concurrently. I won't be able to upgrade all of our vSphere 4 hosts to 5 in one fell swoop, so there will be some time when both host versions are in use.

I want to make this as simple on the storage side as possible, so am trying to avoid creating new host groups for the vSphere 5 hosts, since all hosts will ultimately be accessing all shared storage devices once the upgrade is 100% complete.

So basically, as long as I don't have a need for my vSphere 4 hosts to access my VMFS-5 datastores, is there any harm in having the storage devices themselves visible, or should I explicitly mask them?

[weinstein5]

There should be no issue as long as you make sure the VMs running on the ESXi 4 hosts are kept on the VMFS 3 datastores -

[Gleed]

Are you just wanting to manage the host from the same vCenter, or do you want to put them into the same cluster?

I've always recommended you avoid doing VMFS upgrades or adding any VMFS5 volumes until all your hosts are running 5.0.  This is more of a precaution as I'm not aware of any issues, but unless  you can confirm that HA has been tested with mixed VMFS versions I wouldn't try it.

Keep in mind that while running in a mixed mode you'll also want to avoid doing any virtual hardware upgrades.  Similar to how VMFS5 volumes cannot be accessed by a 4.x host, VMs running vHW 8 cannot run on a 4.x host.

[kooltechies]

There is no technical reason for not being able to do this (VMFS3 and VMFS5) coexisting. As you said you can always mask it for any unforseen scenarios.