SAS is a next-generation SCSI interface. Like SATA, it overcomes the severe limitations posed by a parallel interface architecture, and even greater problems resulting from awkward cabling and touchy connectors.
The SAS specification was built with SATA compatibility in mind. The cabling and connectors for SAS and SATA are identical, for instance. In fact, a SATA drive can be connected to a SAS interface and run. When a SATA drive is connected to a SAS connector, logic inside the SAS connector allows SATA commands to pass through to the system transparently. It doesn't work the other way around, however. Even though a SAS drive can be physically connected to a SATA interface, the drive won't work because the SATA interface doesn't support SAS.
The original idea behind providing SAS with the ability to connect to SATA devices was to enable storage device manufacturers (or even enterprise users with SAS installed on their servers) to use either SATA or SAS drives. The theory is that in some networks, where high performance and reliability aren't essential, a SATA drive could be used. Where higher performance or increased reliability is required, the SATA drive can be removed and replaced with a SAS drive.
A NAS manufacturer may offer two different NAS systems: a lower priced version with SATA drives, and a higher priced version with SAS. The buyer would be offered the option of upgrading the NAS system by adding more drives, or by replacing the SATA drives with SAS drives.
However, as with the relationship between PATA and parallel SCSI, the SAS silicon is considerably more complex than SATA and thus costs more to create. This cost is extended into a higher price for SAS drives and a higher price for SAS controllers and the devices they're installed on. SAS does have some advantages, however: It has a larger command set than SATA, providing better communications between the drive and the unit requesting data. It's also designed to transfer data more rapidly than SATA drives. Moreover, SCSI is often installed inside SANs that feature FC connections to servers, so FC SANs could conceivably use SAS drives instead of the even more expensive FC drives. In most cases, however, they're not--they're using SATA drives.
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