Hi,
Which version of ONTAP are you planning on deploying? Will the vserver allow protocols be restricted to CIFS only? Will the clients access the data using DFS? How many CIFS shares do you plan to provision? What FAS model are you using? As general guide you can check the minimum\maximum guides (mostly hardware model dependant).
In 9.3 you can select which volumes within your vserver will be replicated. For CIFS there are several options you might want to consider, volume count, volume capacity, file count, network bandwidth etc. I think it depends on your infrasturcutre, if you have a lot of data you might consider splitting the load amongst multiple vservers rather than a single vserver with a large number of volumes. In my experience the higher the volume count, the longer it will take to failover to DR as there are limits on concurrent volume snapmirror transfers. It really depends on your environment and your RTO\RPO, this is probably the most important point to consider.
/Matt
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