Since it would be extraordinarily difficult to increase data speeds across a conventional ribbon cable, designers considered a serial approach that would pass one bit at a time between the drive and host. A serial interface can be longer and pass data at speeds far exceeding a parallel scheme. This is due to less crosstalk (from fewer signal wires), lower signaling voltages and better cable design.
Narrower cables and lengths of 39 inches allowed for drive connections in much larger enclosures -- even outside of the box. The 7-pin SATA interface first emerged around 2003 and was incorporated into standard NCITS 397-2005 in 2005. Older 4-pin Molex power connectors were also replaced by a low-profile 15-pin power connector supplying 3.3 volts, 5 volts and 12 volts. External SATA (eSATA) was standardized in 2004, which defined the cabling, connectors and signaling for external SATA cables reaching 78 inches while maintaining full disk speed. External SATA supports external drive boxes like RAID cabinets.
SATA data speeds
While it's more relevant to discuss a serial interface in bits per second, the need to compare SATA with its earlier parallel sibling means that SATA drives are often denoted in bytes per second. So, the initial release of SATA at 1.2 Gbps is almost always shown as 150 megabytes per second (MBps) (dubbed SATA/150). You may also see this noted as SATA 1.5 Gbps, which is the clock speed of the interface -- encoding overhead at the physical layer reduces the effective data transfer rate to 1.2 Gbps; 150 MBps may not seem faster than the 133 MBps of ATA/133, but the interface was poised for even faster performance.
Standards bodies quickly doubled the SATA clock speed to 3.0 Gbps, though encoding overhead again reduces the effective data transfer rate to 2.4 Gbps, or 300 MBps (SATA/300). There were no substantial changes to cabling. Most SATA drives and controllers now use SATA/300, but SATA/300 controllers should be backward compatible with SATA/150 drives -- though not all controllers can negotiate the speed difference between drives.
SATA/300 drives are often referred to as SATA II, though this is technically incorrect. SATA II was actually the name of the standards body that developed the SATA/300 specification. The standards body, now called "SATA-IO" strongly discourages the use of SATA II to mean SATA/300.
Faster SATA interface speeds are on the horizon. By 2007, the clock speed should double again to 6 Gbps, yielding an effective data transfer rate of 4.8 Gbps, or 600 MBps (SATA/600). However, it is unclear just how meaningful the advance will be, since no current drive technology can fill such bandwidth.
Notable SATA features
SATA technology introduced several important features that are particularly interesting to the enterprise. First, developers discarded the notion of primary/secondary drive relationships on the same controller. This caused a potential bottleneck with older PATA drives, because the primary and secondary drives had to share the ribbon cable and controller. With SATA, each drive has its own dedicated controller and bandwidth, so all SATA drives can simultaneously exchange data with their host controller. This is particularly noteworthy in high-I/O environments, such as striped RAID configurations.
The SATA interface also supports hot swapping; allowing a SATA drive to be physically unplugged or reconnected to a controller without powering down or rebooting the host system. This is a crucial feature for enterprise storage because new drives can be added, and failed drives can be replaced without taking the entire storage subsystem offline.
Native command queuing (NCQ) is another important feature to appear with SATA. Previously, a drive would receive one I/O request at a time and execute those requests in the precise order they were received. This was often inefficient since requests could send read/write heads back and forth across the drive. NCQ allows the drive to receive multiple I/O requests and decide on the most efficient order for executing those requests; often improving drive performance.
SATA drives are not compatible with existing PATA or SCSI interfaces. But SATA is compatible with serial attached SCSI (SAS) cables and controllers, and the two drive types can coexist in the same SAS storage subsystem. However, SAS disks are not compatible with SATA controllers.
Putting SATA to work
Seagate Technology LLC is noted for its enterprise disk drives, offering two families of nearline SATA products; the NL35 and the Barracuda. As one example, the NL35 family uses a SATA/300 interface, includes NCQ and provides 250, 400 and 500 GB drive capacities with burst data transfers of 300 MBps. Additional features, such as high tolerance against vibration, allow the 3.5-inch drives to be packed tightly together into drive arrays without fear of premature failure. A workload management feature monitors disk temperature and throttles activity to prevent excessive wear, helping to ensure long-term reliability. SATA disks, such as the NL35s, Barracudas and others, have found important roles in enterprise storage:
Replication. SATA drives have been applied in numerous enterprise storage tasks involving disk-to-disk (D2D) replication. For example, simple disk-to-disk backups might involve replicating the contents of mission-critical databases or other data to SATA disks in another storage array, such as the Axion from Avamar Technologies Inc. This is faster and more reliable than backing up to conventional tape. Replication can also be accomplished between disk systems at remote locations using a wide area network (WAN) connection.
VTL. A more formal backup scheme might deploy SATA drives in a virtual tape library (VTL), which is basically a storage array designed to "appear" like a tape library to backup software. This is another popular disk-to-disk scheme, but VTL maintains any existing backup processes and procedures used with tape platforms -- easing any transition to disk-to-disk storage. The NearStore from Network Appliance Inc. is one notable VTL platform.
CAS. Data that is only accessed infrequently is often not justified by the cost of FC drives, and that data is usually archived to low-cost, high-density storage like SATA disk arrays for long-term retention. Archival storage is often replaced by content addressed storage (CAS) disk systems that include storage features like data deduplication, security protection and retention/deletion controls. The Clariion Disk Library from EMC Corp. is just one example of archival disk storage.
Guarding against SATA failure
The benefits of SATA have long intrigued storage professionals, but the reliability of SATA drives has raised serious concerns. Even though SATA drives like Seagate's NL35 and Barracuda families offer a five-year warranty, that's only 43,800 hours -- a far cry from traditional enterprise-class FC drives touting MTBFs over 1 million hours. For example, Seagate's Cheetah 15K.5 family claims MTBFs of 1.4 million hours (almost 160 years of continuous operation).
RAID can certainly protect a SATA drive from failure, but the huge capacities in today's SATA drives result in rebuild times that can exceed six hours. While one SATA drive is rebuilding, there is a real statistical probability that a second drive in the RAID group might fail resulting in data loss. This concern has slowed the adoption of SATA early on, but RAID-6 has emerged to address reliability.
RAID-6 is a dual-parity protection scheme that protects against two simultaneous SATA drive failures by recording two independent sets of parity data across a RAID-6 disk group. If a second disk should fail while the first disk is being rebuilt, both failed disks can be reconstructed using parity data. There are disadvantages with RAID-6; two additional disks are needed with the RAID group, and there is a performance penalty when calculating and writing two sets of parity data to the disks (there is no performance penalty when reading). ***
Serial ATA (SATA) is a drive interface designed to replace the Parallel ATA physical storage interface. The storage world has been buzzing about SATA drives for years, debating how it stacks up against other technologies.
Users of the SATA interface are benefiting from greater speed, simpler upgradeable storage devices and easier configuration. While SATA drives don't match the performance of Fibre Channel (FC) hard drives, they provide the low cost per gigabyte and high storage densities crucial for "near-line" storage tasks such as performing backups and archiving.
This Fast Guide is a compilation of SATA-related tips that have appeared on SearchWinComputing.com. As our site devotes more coverage to SATA, expect to see more tips related to upgrades and configuration.
Fast Guide: Managing SATA drives
Balancing SATA and SCSI
Fixing conflicts between older and newer SATA drives
SATA technology advances and expands in the enterprise
Plugging into external SATA
Choose SCSI over SATA for enterprise servers
SATA can fill storage upgrade for older computers
This was first published in July 2006