EXPERT RESPONSE
It is possible, but the problem with most implementations that allow this is that there is no way to manage the allocation of the controller resources for the varying performance disk drives. This means that the amount of cache allocated, processor resources or internal ports are managed as if the drives were all of the same type. Usually customers want to implement storage tiers for cost reasons and put less demanding or valuable data on less performing disks. If the disks are mixed and the storage system can't control the internal resource allocation, there may be cases where the performance needed for some data is being sacrificed because of usage of those resources (especially cache) for less demanding data.
Another issue is controlling the placement of the data -- putting it on the SATA or FC drives for example. If that's a manual process (not done with some software automation), the administrative cost of doing that may outweigh the savings from using cheaper drives.
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