ACFS includes a kernel driver that loads on the system and is used to access local devices and provide read write access to them in a shared fashion. However in case of exadata the ASm storagevolumes are not physical devices on the database nodes but are on teh storage servers. Hence no kernel driver can access those volumes. To cater for this kind of a setup where the asm volumes most resemble network attahed storage than a local volume would be a major change to the ACFS driver and would not provide much additional benefit over the existing DBFS paradigm. This is likely why ACFS will not be supported in Exadata.
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As per MOS, ACFS is not supported on Exadata storage and there are no plans to provide it. You should look at DBFS or a NAS filer when using Exadata Database Machine. It might be due to some technical strategy or licence cost.