ENTER SATA II
The developers of SAS may not have fully anticipated SATA II, a higher performance iteration of SATA that provides features that put it in direct competition with SAS in many areas. For example, SATA II supports hot-swapping. This is obviously important for enterprise SANs, as well as for smaller RAID arrays and NAS devices. Because less-expensive SATA II drives can be swapped into and out of arrays, this eliminates the need to go to SCSI (or SAS) in order to hot-swap drives.
Additionally, SATA II specifies a port multiplier that enables connections to up to 15 SATA or SATA II drives. However, four or five drives connected to a single port multiplier provides optimal performance. A port multiplier can be connected internally to a server and deliver as much as 6Tbytes of storage, assuming 15 400Mbyte drives on a single channel. However, there's no technical limit on the number of drive channels that can be installed on a server or used inside a SAN or NAS system.
SATA II also comes with a new external cabling specification that lets a server and SAN connect over distances of up to 2 meters. A single cable can carry four channels of data. Connecting one cable to a device with four port multipliers can theoretically provide 24Tbytes of storage using the above assumptions. Realistically, a SAN with 20 drives (five on each channel) can deliver 8Tbytes of storage.
Host redundancy can be achieved via a port selector. The port selector can be attached to two hosts so that when the primary host fails, the port selector can automatically switch control to the secondary host, thus enabling rapid failover.
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