[Updated] This is a very short post as I only learnt about this recently myself. I thought it was only available in vSphere 5.5 but it appears to be in vSphere 5.1 too. Anyhow Storage DRS now has a new setting that allows you to configure the default VM affinity setting. Historically, VMDKs from the same virtual machine were always kept together on the same datastore by default; you had to set a VMDK anti-affinity rule to keep them apart. Now you can set a default for this option, which can either be to keep VMDKs together on the same datastore or to keep VMDKs apart on different datastores.
In the Storage DRS Runtime Settings in vSphere 5.5, under the Advanced Options, there is now a Default VM Affinity check box. When the check box is selected, it keeps VMDKs together on the same datastore. If the check box is not selected, VMDKs are placed on different datastores.
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