Cloud Standards: Agreements That Hold Together Clouds by Marvin Waschke

Author:Marvin Waschke
Language: eng
Format: epub, pdf
Publisher: Apress®
Published: 2012-10-16T16:00:00+00:00

22 www.intel.com/content/www/us/en/io/serial-ata/ahci.html

23 www.serialata.org/technology/esata.asp provides a fairly detailed description of eSATA.

Figure 8-3. Connect multiple SATA hard disk drives to a single SATA port.

SATA 6Gb/s

SATA 6Gb/s is the latest version of SATA from SATA-IO. SATA-IO has released several versions. The earliest version was SATA 1.1Gb/s, followed SATA 3Gb/s. With SATA 6Gb/s, SATA is comparable to SCSI in several ways, both in throughput and in other ways.

SATA SCSI Tunneling

One of the most important aspects of SATA 6Gb/s is a feature of Serial SCSI (SAS). SATA devices can be plugged into a Serial SCSI domain by tunneling the ATA command set through the SCSI protocol. This makes it easy to plug relatively inexpensive SATA disks into a datacenter SCSI SAN.

Native Command Queuing (NCQ)

Native Command Queuing is also part of SATA 6Gb/s. NCQ is important when data transfer rates are high. The total throughput and performance of a disk depends on more than the theoretical number of bits that can be transferred in a millisecond. The data must be available to be sent. When data is received, it has to be used. If those conditions are not present, the line sits idle and data does not flow, no matter how high the transfer rate. This is one reason users are disappointed when a disk that doubles the transfer rate for short bursts of data only marginally improves performance. As the “Tagged Command Queuing” section described, time is lost while the heads shuffle around finding data. Reducing the shuffle time becomes a larger and larger factor as the transmission speed increases. TCQ attempted to solve the problem but was not satisfactory. NCQ is designed to address the problem with the aid of AHCI controllers. With AHCI controllers on the host side, the SATA NCQ commands give SATA an important boost in overall performance in many circumstances.

SATA 6Gb/s also provides improved power management. In large datacenters, optimized power consumption is important both by reducing power consumption and by cutting down the heat radiated from the drive.

Now it is time to look at the workhorse of datacenter storage.

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