Serial ATA 2

Serial ATA II (SATAII or SATA-2) is backward compatible with the original SATA, and so with parallel ATA.
SATAII introduced the following extensions.

Data transfer rate

The Data transfer rate increased to 3 Gb/sec (or 300 MB/sec) initially. and is planned to increase to 6 Gb/sec or 600 MB/sec) eventually.

Port multiplier

The updated Port Multiplier specification basically makes it easier to add a number of devices to a SATA cable by using a simple hub for port expansion. It allows Port Multipliers to notify the host if a device has been plugged or unplugged from a port, eliminating the process of host polling to determine where devices have been added or removed. Port multipliers provide similar functionality to SAS expanders, though the architecture is different. In theory, a Port Multiplier can have a maximum of 15 connected devices, but PMs cannot be cascaded or nested. Therefore, for a given SATA domain, there can be a maximum of 15 target devices. The 16th port is used as a reserved control port that can be used to collect information such as the number of device ports connected.

The PM drivers contain address information within their data frames, to ensure that they can address the correct PM port. Legacy BIOS systems do not contain that information. However it is possible to perform a BIOS boot from a SATA device, but BIOS will only address port 0. Once the boot is complete, a PM driver can be loaded that will address the other ports.

Port Selector

SATA can only have one path active at a time, which is a single point of failure. A port selector is basically a failover switch that will switch in a fail-over path to storage devices if the primary path fails. Port selectors are small and inexpensive and are manufactured within the SATA hard disk caddy or actually on the disk. Initially they will normally be found in high-end disk subsystems like NAS, RAID or disk-to-disk backup devices, where fail-over with no loss of service is important, but they are cheap enough for more general use.

Command queuing

The command queuing extensions are mainly aimed at enterprise storage applications.

Disks essentially contain a sequential series of concentric circles of data tracks. The data tracks are accessed by read write heads that move across the disk to access different tracks. Data within each track is accessed as the disk spins under the heads. Applications rarely request data in sequential order, but usually request data randomly from different parts of the disk. It takes a considerable time to move the heads, or wait for the disk to spin (in CPU terms) so if the heads have to traverse back and forward over the disks, performance will suffer. When several applications are multi-tasking, and all accessing the same disk, the problem is exacerbated.

Native Command Queuing (NCQ) is intended to fix this problem. Commands are queued within the drive’s internal command queue, and can be reordered to minimize head movement. NCQ is internal to the drive, and allows several commands to be active and queued internally in the drive at the same time. The commands can be re-ordered in the queue to minimise the amount of head movement needed to read all the data in the queue.

This process is sometimes called elevator seeking. Consider an elevator in a tall building. It will be given commands to proceed to a floor, depending on which button is pressed. Someone enters at the ground floor, and presses the button to floor 21, then someone else at floor 10 presses the up button to request the elevator to stop. It is inefficient for the elevator to go straight to floor 21, then back down to floor 10 to pick up the next customer. The ‘button’ commands will be sorted in order, and the elevator will stop at all required floors on the way up. Disk elevator seeking, or NCQ, works on exactly the same principle.

NCQ also allows commands to be outstanding in the drive, while passing command complete back to the application so processing can continue, and more commands passed to the drive. Applications and operating systems are increasingly taking advantage this facility.

SATA can be considered equivalent to Serial Attached SCSI, see the SAS section for details

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