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Hard Disk Drive Interfaces for Servers

The interface over which data is transferred to and from the hard disk drive and the server host controller or RAID controller is extremely important and has undergone many changes and improvements. Most motherboards now support different levels of RAID architecture an essential feature for many server systems where fault tolerance and redundancy are key. Many servers will also be attached to a RAID array directly, either through an add-in card or a standalone unit, which may also be accessed via the network.

Few home users or small offices have the need for a server and most of those which have a server will just use the motherboard interface rather than using more expensive enterprise level solutions. Where multiple users need to access a server, an enterprise solution is more likely to be required in order to serve the data properly. The use of a RAID solution featuring fault tolerance offers the best protection for your data and in the event of a failure taking it offline, will give our data recovery engineers the best chance of a successful recovery.

Early PC Disk Interfaces

Early PCs used the ST-506 interface, developed by Seagate, which used Modified Frequency Modulation (MFM) data encoding. Each individual drive required a dedicated controller board plugged into an expansion slot and provided a transfer speed of 5MB/s. The second generation of PC based interface developed by Seagate, the ST-412 made use of Run Length Limited (RLL) encoding to store the data. The stepper motor was controlled by a microprocessor, reducing the seek time, thereby increasing the data transfer speed.

Maxtor then developed the ESDI (Enhanced Small Disk Interface) drive, integrating some of the electronics onto the hard disk drive unit, increasing the data transfer speed to 20MB/s. Suitable server operating systems to run on a PC were yet to be developed, meaning servers using these types of drives were rare.

SCSI/SAS/Fibre Channel Hard Drives

The Small Computer system Interface (SCSI) was developed by Shugart Associates, integrating all the electronics onto the hard drive controller board. Although SCSI devices require a host adaptor card to be plugged into the computer, they can access multiple devices attached to the bus simultaneously. Although early SCSI drives were also only capable of 5MB/s data transfer speeds, they were considered more reliable and therefore more suited to server systems. By 2003 the SCSI interface was capable of 640MB/s data transfer speeds over a parallel interface.

The successors to SCSI are Serial Attached SCSI (SAS) and Fibre Channel (FC) both of which are extremely important for enterprise server storage systems. Fibre Channel often used for attaching large RAID arrays was capable of transfer 16Gbit/s in 2014, extremely important within a data centre environment.

IDE/EIDE/ATA Drive Interface

IDE (Integrated Drive Electronics) developed by Western Digital, like SCSI integrated the electronics on the drive controller board. These used the AT Attachment interface, often called Parallel ATA (PATA) became the de facto standard for the PC in the late 80s with most motherboards integrating the IDE interface. As the drive speed and capacity increased, limitations required the development of Extended IDE (EIDE) and later the Ultra ATA (UATA) interface.

These interfaces were soon found to limit the transfer speed due to the way the data was sent by the transfer protocol. The Direct Memory Access (DMA) interface allowed areas of memory to be sent directly, resulting in faster transfer speeds. By the time Ultra DMA (UDMA) using the ATA-7 protocol was developed it allowed a transfer speed of 133MB/s. The use of these drives in server systems started to take off when PC based Unix operating systems, such as BSD and Linux were developed. The use of a parallel interface requires the data to be synchronised between each data line. As the speeds increased, this was found to be difficult as interference or crosstalk between the data lines was found to limit the achievable data transfer speed.

Serial ATA Disk Interface

In 2003 the Serial ATA (SATA) interface was introduced as the successor to Parallel ATA, overcoming its disadvantages. The signal rate could be increased significantly and also resulted in a reduction of the wiring which was required.

The introduction in 2013 of the SATA 3 interface, has seen the data transfer speed increase to 600MB/s. The 3.2 revision of the SATA specification states that data transfer speeds of 1969MB/s should be possible through combining the SATA interface and PCI Express bus. These drives are usually never found within an enterprise server solution. For small and medium businesses and consumers requiring a server system it is much more likely for SATA drives to be used even within dedicated NAS or SAN RAID arrays.

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