The computer hardware industry is capitalizing on new bus and drive technologies aimed at faster data transfer, improved storage, and increased reliability and efficiency. These advances drive system prices down while simultaneously expanding the reaches of media storage and utilization across industry.
The latest buzz words in bus, protocol and disk storage include: SAS (serial attached small computer systems interface, or SCSI), SATA (serial advanced technology attachment, or ATA), PATA (parallel ATA), eSATA (extended SATA) and iSCSI (Internet SCSI). This installment aims to summarize these buzz words, helping to provide a framework for understanding where these technologies fit into your application or domain.
We’ll start with SAS, a computer bus technology designed for the transfer of data between storage devices, including hard drives and CD-ROMS. The thrust of SAS is to replace parallel SCSI—the mainstay legacy format for wired communications between storage device and computer buses—while maintaining the SCSI command set.
Besides reducing wires, serial communications dramatically improves data transfer speeds. SAS consists of four basic components: an initiator, the device that originates service and task requests; a target, the device that receives the requests; a service delivery subsystem, which carries the information between the initiator and the target; and expanders, which connect SAS end devices with the initiator port.
During the development of SAS, the deficiencies of parallel SCSI were recognized and a goal to improve performance was established. SAS uses its serial transfer protocol for interfacing between multiple devices. This improves signaling, lowers overhead and results in higher speed connectivity.
Each device is connected by a dedicated bus to the initiator in a point-to-point mode, thus bus contention is mitigated compared to SCSI, which uses a multidrop principle that shares bandwidth across all devices.
SAS supports at least 16,384 devices compared to the legacy parallel-SCSI limitations of 32 (note that early SCSI only supported eight or 16 devices).SAS transfer speeds for each initiator-target connection can be 1.5 Gbps, 3 Gbps or 6 Gbps and SAS also supports serial-ATA devices.
MORE ON ATA
The successor to ATA is serial-ATA, again a bus technology designed to facilitate the transfer of data between storage devices. The parallel legacy format was recently tagged with a “p” to indicate parallel-ATA, or PATA, so as to differentiate the two.
Communications between devices in the serial domain rely upon two pairs of unidirectional signal wires that comprise a high-speed data link. Low-voltage differential signaling (LVDS) enables much greater speeds per wire, beginning at 1.5 Gbps.
Data encoding has reduced link data communications rate to 1.2 Gbps, or around 150 MBps. First-generation interfaces were known as SATA 1 and SATA/150. Early SATA implementation offered a bridge chip that converts PATA/133 to SATA/150, essentially making performance similar.
SATA specifies forward and backward compatibility. However, the second-generation SATA/300 (3 Gbps), aka SATA II, was plagued by the incomplete documentation of early SATA/150 interfaces, forcing the addition of a user-configurable selector jumper. Six Gbps implementations for SATA are on the roadmap and will be applicable to port multipliers enabling higher shared bandwidth across many drives.
SATA drives may be used with SAS controllers, and can communicate on the same physical cable as native SAS drives; however, SAS drives are not compatible with a SATA controller. Connectors physically differentiate the drive interfaces, with the larger triplet sets of pins supplying a mixture of 3.3, 5 and 12 volt power. Adaptor cables are available, but limit the power supply interface voltages to 5 and 12 volts.
The standardization process for eSATA began in mid-2004 and focused on coexisting with FireWire 400 (IEEE 1394a) and USB 2.0. eSATA takes direct aim at USB from a performance perspective, targeting the excessive overhead for USB attached drives. While the effective transfer rate is faster, eSATA may still be a bottleneck for RAID sets or faster disk drives.
Internet SCSI, or iSCSI, is a transport layer network protocol (as opposed to a bus technology) that implements the SCSI-3 framework over TCP/IP networks per the February 2003 IETF ratification. From a cost perspective, iSCSI is useful in building storage area networks (SAN) when compared with Fibre Channel SANs. The protocol uses a simple Ethernet interface, but in turn adds a level of overhead due to the requirements of TCP/IP. Certain implementations have mitigated the overhead issue through the use of TCP offload engines and host bus adaptors. iSCSI can also operate with standard Gigabit Ethernet network interface cards.
NOT TO BE CONFUSED
As with serial communications technologies, such as SAS, iSCSI allows a machine to use an initiator to connect to targets (i.e., disk drives) over an IP network. The initiator is like a client device that makes its connection to a service offered by a server. The target is the equivalent of the server, providing block-level access to the storage media. Do not confuse iSCSI devices with network attached storage (NAS) devices—which utilize a NAS-head and server software to arbitrate access requests between host and storage devices.
In concert with other SCSI-protocols, iSCSI does not mandate how devices are simultaneously shared between computers—that is the job of the operating system. In other words, the OS determines how iSCSI devices appear on the system.
Older storage systems or those with multiple mediums that might include SCSI, SAS and even Fibre Channel, can be attached to an IP-network via iSCSI. This provides extensibility and performance balancing across existing and future implementations.
It should be noted that high-end video server platforms—those targeting mission critical operations—generally employ protocols and bus technologies specifically designed for their applications. With that, dedicated graphics systems and simpler desktop editing platforms requiring extended storage can make use of these technologies to improve storage capacity and throughput without the cost of high-end RAID or Fibre Channel storage subsystems.
