HP 349239-B21 Serial ATA technology, 2nd edition - Page 5

SATA Performance The SATA specification was released in three parts: SATA 1.5 Gb/s, SATA 1.5 Gb/s with extensions, and SATA 3.0 Gb/s. The 10-year roadmap for Serial ATA allows it to scale eventually to 6.0 Gb/s. SATA 1.5 Gb/s The SATA specification was introduced with a maximum theoretical bandwidth of 1.5 Gb/s, or 150 MB/s, factoring 20 percent encoding overhead (see "Signal integrity"). SATA 1.5 Gb/s focuses on increasing the bandwidth and mitigating the design problems associated with the parallel ATA architecture. SATA 1.5 Gb/s is designed to replace parallel ATA, mainly in desktop PCs (non-hot plug). Serial ATA is not hardware compatible with legacy Ultra ATA; however, it is fully compliant with the ATA protocol and, thus, software compatible with existing ATA drivers. SATA 1.5 Gb/s (with extensions) SATA 1.5 Gb/s (with extensions) addresses the needs of the lower cost server and non-mission critical enterprise storage markets. SATA 1.5 Gb/s (with extensions) enhances SATA 1.5 Gb/s with features such as native command queuing, out-of-order execution and delivery, and data scatter/gather lists (each described below). Any or all of these optional extensions can be implemented. Native command queuing enables a hard drive to take multiple requests for data from the processor and rearrange the order of those requests to maximize throughput. SATA 3.0-Gb/s hard drives will be able to queue and execute requests without any assistance from the CPU. Out-of-order execution and delivery keeps execution resources as busy as possible. In the native command queuing model, this feature allows the last half of the data requested by a command to be delivered and executed before the first half of the data. Out of order data delivery within commands requires support for non-zero buffer offsets. A data scatter/gather list is a data structure that assists the direct memory access (DMA) engine in locating memory regions that comprise the complete transfer buffer. This assistance is beneficial because virtual memory mapping mechanisms may scatter the buffer across several noncontiguous, physical memory pages. SATA 3.0-Gb/s The SATA 3.0 Gb/s specification doubles the previous data transfer rate and adds other extensions to improve the capabilities of SATA devices for server and networked storage applications. Theoretically, with the use of port multipliers, each port on a SATA 3.0 Gb/s host controller can connect up to 15 SATA drives. SATA devices SATA devices include initiators (SATA controllers), port multipliers, and targets (SATA drives) as shown in Figure 5. Port multipliers connect initiators to targets in a SATA domain. SATA devices in a domain do not need to be assigned an ID. All SATA devices have unique worldwide names (SATA addresses) assigned at manufacturing to simplify identifying initiator devices, port multipliers, and target devices. These devices, and the cabling that connect them, are described in this section. 5