Specifications of a Hard Disk

Hard Disks have a few performance specifications that a buyer must always know accurately before purchasing the hard disk. Better specs will obviously make the system perform faster. The specifications are:

• Capacity: The amount of data that the disk can store. Usually 100's of GB's or TB's.
• Cache: A small memory chip that stores a fraction of frequently accessed data that can be accessed extremely fast. Usually 8 or 16 or 32 or 64 MB.
• Spin Speed: The speed at which the hard disk platter rotates. Usually, 5400 or 7200 or 10000 rpm.
• Average Seek Time: The average time in which the R/W head can be positioned over the requested track for random read/write requests. Usually, ~8-10 ms.
• Average Latency: The average time for the disk to rotate the platter and position the correct sector under the R/W head. Usually, ~3-5 ms.
• Interface: The interface that the hard disk uses and the port that it will use to connect to the system. The interfaces also have transfer speeds of their own. For example, a max. of 167 MBps for IDE, 600 MBps for SATA.

Ports for storage devices

Computer systems have a variety of secondary storage devices, some essential and the others optional. Some of these include hard-disk drives, optical drives, tape drives, floppy drives, etc.

For the transfer of data to and fro from the devices controllers are required to control the ports for connecting the devices. For the ports already present in the motherboard, usually the controller is on-board the motherboard too. The ports for hard-disk drives and optical drives include:

Parallel ATA (PATA/IDE):

• 40 pin cable and port
• max cable length: 45cm
• max number of hard drives: 4 (only in some motherboards, 2 per cable)
• max transfer speed: 133 MB/s

Serial ATA (SATA):

• 7 pin cable and port
• max cable length: 1m
• max number of hard drives: 8 (till now, one per cable)
• max transfer speed: 150-600 MB/s

SATA ports are fast replacing PATA ports. Controllers for extra ports like additional SATA or PATA ports, SCSI, USB, etc. that have been added via PCI card or similar are built on the PCI card itself.

There are ports for additional storage expansion as well like USB and SCSI. SCSI is mainly used with servers that have large arrays of hard-disk drives whereas USB controller is already present on the motherboard. The latest USB 3.0 specification is supposed to achieve 600 MB/s.

PCI-Express: why it is better

PCI Express or PCIe is an expansion card standard that was designed to replace the older AGP, PCI and PCIx bus standards. The PCI-Express standard offers higher system bus throughput and has lower I/O pin count.

PCIe has serial links between devices called Lanes, unlike the parallel connections present in PCI buses. PCIe can be a x1, x4, x8 or x16 type where the number equals the number of lanes. ( D,E : ports and sockets of a motherboard )

The PCIe (common revision 2.1) has a bandwidth of 16 Gbps for a 16 lane link (x16).

PCIe slot can be used for placing expansion port card, modems or sound cards, etc. But, Graphics performance can benefit the most with the use of PCIe because they require the maximum bandwidth for transfers. So, the graphic cards (or GPU) are built for and placed in PCIe slots.

PCI: Peripheral Component Interconnect

The PCI refers to the computer bus for attaching hardware devices in a computer. These devices can take either the form of an integrated circuit fitted onto the motherboard itself, called a planar device in the PCI specification, or an expansion card that fits into a PCI slot. ( A,B : ports and sockets of a motherboard )

Typically, PCI cards used in the expansion slots of a PC include network cards, sound cards, modems, extra ports such as USB or SATA, TV tuner and disk controllers.

The common specifications of PCI ports are:

• 32 or 64 bit bus-width
• 33.33 or 66.67 MHz synchronous clock
• transfer rate of 133 MB/s for 32-bit
• 32- or 64-bit memory address space (4 GB or 16 EB)
• 5-volt signaling