Server Hard Drives Hardware Guide

Server Hard Drives Hardware Guide

Servers are in an environment which demands multiple and quick file accesses.  Server hard drives are manufactured to meet those needs, yet also provide the highest reliability possible.  Unlike desktop consumer drives, server drives feature specialized protocols and hardware to deliver better performance under these conditions.  To keep up with all these data requests, server hard drives are somewhat different.  They spin at a much faster rate (15k or 10k compared to 7200) and are sometimes smaller so that disk head movement is kept to a minimum.  With these changes, server hard drives are usually more expensive and offer less data storage.  But they make up for it in longer life spans, less data corruption, and much faster random file accesses.  There are currently three standards for hard drives in the server market:  SATA, SAS, and SCSI.  Before we go into them, let us understand a few key terms. 

Technology and Key Terms

Native Command Queuing (NCQ) - is a technology designed to increase performance of SATA hard disks under certain situations by allowing the individual hard disk to internally optimize the order in which received read and write commands are executed. This can reduce the amount of unnecessary drive head movement, resulting in increased performance (and slightly decreased wear of the drive) for workloads where multiple simultaneous read/write requests are outstanding, most often occurring in server-type applications.

SATA

You will notice that SATA is also used in desktop systems.  This is due in fact that desktop drives are becoming more reliable and much cheaper to produce.  This in turn is perfect for inexpensive home or non-critical server computing needs.

The current SATA specification can support data transfer rates as high as 3.0 Gbit/s per device. SATA uses only 4 signal lines; cables are more compact and cheaper than for PATA. SATA supports hot-swapping and NCQ. There is a special connector (eSATA) specified for external devices, and an optionally implemented provision for clips to hold internal connectors firmly in place. SATA drives may be plugged into Serial Attached SCSI (SAS) controllers and communicate on the same physical cable as native SAS disks, but SATA controllers cannot handle SAS disks.  The SATA standard defines a data cable with seven conductors (3 grounds and 4 active data lines in two pairs) and 8 mm wide wafer connectors on each end. SATA cables can be up to 1 m (39 in) long, and connect one motherboard socket to one hard drive. Thus, SATA connectors and cables are easier to fit in closed spaces and reduce obstructions to air cooling. They are more susceptible to accidental unplugging and breakage than PATA, but cables can be purchased that have a 'locking' feature, whereby a small (usually metal) spring holds the plug in the socket.  SATA systems use differential signaling. In this system, it is easy to filter out the noise from the data signal at the receiving end. The higher noise rejection allows the SATA system to use only 500 mV peak-to-peak differential voltage to carry the signal at higher speeds without distortion or noise interference.

Small Computer System Interface (SCSI)

The oldest of the three standards, it is currently being replaced by the newer SAS.  SCSI is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it can connect a wide range of other devices, including scanners and CD drives. The SCSI standard defines command sets for specific peripheral device types; the presence of "unknown" as one of these types means that in theory it can be used as an interface to almost any device, but the standard is highly pragmatic and addressed toward commercial requirements.

• SCSI is an intelligent interface: it hides the complexity of physical format. Every device attaches to the SCSI bus in a similar manner.
  
• SCSI is a peripheral interface: up to 8 or 16 devices can be attached to a single bus. There can be any number of hosts and peripheral devices but there should be at least one host.
  
• SCSI is a buffered interface: it uses hand shake signals between devices, SCSI-1, SCSI-2 have the option of parity error checking. Starting with SCSI-U160 (part of SCSI-3) all commands and data is error checked by a CRC32 checksum.
  
• SCSI is a peer to peer interface: the SCSI protocol defines, communication from host to host, host to a peripheral device, peripheral device to a peripheral device. However most peripheral devices are exclusively SCSI targets, incapable of acting as SCSI initiators—unable to initiate SCSI transactions themselves. Therefore peripheral-to-peripheral communications are uncommon, but possible in most SCSI applications. The NCR 53C810 chip is an example of a PCI host interface that can act as a SCSI target.