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Fibre channel I/O for storage will be faster than Ethernet because it is focused on storage I/O and does not have the extra overheads and non-data transmissions. FC also is deterministic, meaning that the response time should be relatively consistent. Ethernet may be variable because it has the option to discard packets if it gets busy, requiring the receiver to detect missing packets of information and request them to be retransmitted.
FC drives are faster than SATA drives (at least all the current generations that I've seen) for a couple of reasons. The first is that the cost savings for ATA is accomplished primarily by having a single processor to handle command and servo positioning. In FC there are two separate processors. This allows the FC drives to usually spin faster and handle servo problems induced by vibration or wear without sacrificing performance (ATA will use more of the processor power for servo control in that case rather than handling data transfer). FC is a more hardware intensive interface and is more costly, but also provides more performance.
NAS is remote file serving so there are more elements in the data path than a SAN, which effectively gives you direct block I/O. Implemented correctly, a SAN should always be faster to transfer data than a NAS implementation. The question always is: What are your requirements? NAS may fit your requirements effectively.
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