The interface between the hard disk drive and the host controller has seen considerable development with improvements to data throughput, especially for RAID arrays of great importance. With most modern motherboards providing RAID capabilities it is not uncommon for hard disk drives to be attached directly to the motherboard, although for the fullest range of RAID architectures a dedicated RAID array or add-in controller is necessary.
For most home users and small offices attaching the hard disks to the on-board controller of the motherboard is generally sufficient for their requirements. Within the office environment, especially where the RAID array is access by multiple users, a dedicated controller board or purpose built RAID server will provide the best solution. This gives the best data protection and offers the best chance of retrieving your files through data recovery, should a failure occur which takes the RAID array offline.
Early Disk Interfaces
The earliest interface developed for the PC was the ST-506 developed by Seagate, which used Modified Frequency Modulation (MFM) data encoding. A dedicated controller board was required, which slotted into the motherboard, providing 5MB/s data transfer speeds.
Seagate developed a second generation of hard disk interface, the ST-412 which used Run Length Limited (RLL) encoding for the data stored on the platters. The use of a microprocessor controlled stepper motor allowed for faster seek times, resulting in faster data transfer speeds. The Enhanced Small Disk Interface (ESDI) was developed by Maxtor, which moved some of the electronics onto the hard disk drive, allowing data transfer speeds of 20MB/s. These types of drive which required a dedicated controller board were impractical for use within a RAID array.
SCSI/SAS/Fibre Channel Drive Interface
Shugart Associates developed the Small Computer System Interface (SCSI) which incorporated all of the electronics onto the hard disk drive. An additional host adaptor controller was required, but was not dedicated a single hard disk, enabling multiple drives to be attached. The SCSI protocol provides a common command interface, making a RAID array a practical proposition. Early data transfer speeds using SCSI drives were only 5MB/s, but by 2003 this had increased to 640MB/s over a parallel data interface.
SCSI and its successors Serial Attached SCSI (SAS) and Fibre Channel (FC) are most often used within enterprise storage solutions, ranging from simple servers through to complex RAID arrays such as SAN and NAS systems. The Fibre Channel interface has seen data transfer speeds increase to 16Gbit/s as of 2014. Apple Macintosh computers used SCSI hard disk drives until the development of the iMac, when these were replaced by the use of the ATA and then SATA interface.
IDE/EIDE/ATA Interface Types
The Integrated Drive Electronics (IDE) interface was developed by Western Digital, which evolved into the AT Attachment interface, better known as Parallel ATA (PATA) seeing large scale use in the PC market from the late 1980’s. Like SCSI drives the electronics were integrated as part of the hard disk, allowing it to be fully independent of the host computer. With increasing capacities and data transfer speeds, the Extended IDE (EIDE) and Ultra ATA (UATA) interfaces were developed.
To further increase the data transfer speeds the Direct Memory Access (DMA) interface was developed. When Ultra DMA (UDMA) was developed it allowed data to be transferred at 133MB/s using the ATA-7 protocol. These disks were rarely used for RAID arrays, with only a few low end solutions being developed for the home user and small office. A serious limitation of the PATA style of interface was the requirement synchronise a set of parallel communication lines, which were subject to crosstalk and other interference.
SATA Disk Interface
The Serial ATA (SATA) interface was introduced in 2003 superseding PATA, which had a number of disadvantages. Not only did it allow for a reduction in cabling, but higher signal rates could be used, allowing for a significant increase in data transfer speeds.
The SATA 3 interface, introduced in 2013 allows a data transfer speed of 600MB/s. Data transfer speeds of 1969MB/s have been outlined in the 3.2 revision of the specification by combining the SATA interface and PCI Express bus. While not used in large scale enterprise solutions, there are many low end mass storage devices available which allow multiple SATA drives to be placed into a dedicated NAS or SAN box.