If we need to share a virtual disk in a read-only fashion between VMs, that's basically an ISO isn't it? We can drop the ISO on VMFS or NFS and map it to as many VMs as we need but that's not really for a working data set. If we need to share a virtual disk between cluster members, there is the multi-writer flag. If you work with Oracle RAC in a vSphere environment then you know about that one. There is the physical mode RDM to share quorum and data between cluster nodes (Is MSCS the final offender there?) and of course NFS to sharing files but what about specifically VMDKs?
Since v5.1, vCloud Director provides this capability in "Independent Disks" and allows the creation of a new disk within an Org vDC. As the name would imply, this disk will then be independent of a vApp and be read-write to any given VM. The capacity is given in the API call, but what about performance and placement? You can also attach a storage profile to help define performance and chargeback.
In this particular customer's use case, it is able to be a transitional working set between VMs and vApps within that Org vDC. To take the discussion to a new-level, they are also working with Cloud Foundry and creating vBlobs and wanted a similar storage construct that they can move around independent of a given VM's or vApp's lifecycle.
I realize that vCAC is "the new hotness" and vCD is "old and busted" but there are still several features of vCD that come in handy on a regular basis. Such as saving vApps back to the catalog, spinning up tenant networks automatically (vCAC 6.0 + NSX is getting there), vCloud Connector for defining a Pub/Sub model for your vDC catalog. Also I like vCD's extensible object metadata repository. Try it, you can store anything about your VMs and use those tags to programmatically identify any sets of objects within your vDC that you want. This reminds me I need to learn more about vCAC's custom properties and wading through the latest vCAC 6.0 SDK...and try to be a vCO ninja this year:-) Oh well, maybe a green belt at least?
vCD 5.1 Independent disk links
vCD 5.5 API link (see pages 80 and 112 for independent disk info
SCSI Multi-writer flag usage
Get and set vCloud Director metadata