Recently I saw an ESATA RAID tower advertised by NewEgg. It contains 4 SATA drives and connects to the system via a single ESATA port. (price was pretty nice). My question is this.
In a typical system the SATA controller will sit directly on the PCI bus. In this external RAID controller, it seems to me that the communication from the SATA controller would have to go through some sort of protocol conversion to get passed via another layer of SATA to get back to the system. In essence, serial to parallel, back to serial, then again to parallel. Is this thinking correct, or am I all wet on this? If I am correct, wouldn't that likely introduce added latency and degrade performance relative to a RAID controller right on the bus?
You are probably right, but the speed impact may be small. By the way, the interface between your mobo and the external unit is not via the regular mobo SATA controllers. It is via a separate eSATA controller chip. However, that chip probably sits on the PCI bus anyway, so the impact is the same.
One important thing you should check. Some external chassis for multiple SATA drives use a single eSATA connection and depend on a feature called "port multiplier support" both at the external chassis' end and at the mobo eSATA controller end. Obviously the case makers will have included their half of this, but you should check whether your mobo eSATA port does have this feature, too.
On this particular case, though, I'm not sure. If the communication between mobo eSATA port and external case controller is made to look like a simple single external HDD, and all the multiple-drive work of a RAID array is handled entirely within the case's RAID controller, maybe Port Multiplier Support is not needed.