1 - Intro
| in order to make optimal use of the processors’ computing power, it is essential that the Hard Drive provides the processor with data at a high speed. If the Hard Drive is slow in providing data to the processor, the processor remains unutilized for the period it waits for data to “arrive” from the hard drive. |
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This means that even though your processor is capable of processing the data faster, it cannot do so because your Hard Drive
is slow in supplying the processor with data to process. With increasing choice, comes increased complexity in decision
making and this is what this guide intends to mitigate and facilitate understanding of the buying process. Understanding
the specification of Hard Drives and their bearing on the performance enable you to make an informed decision when buying
a hard drive.
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2 - How it Works
Hard disks offer greater storage capacity than other storage mediums such as zip drives, tapes or CD ROMs. A typical
Hard Drive consists of a spindle upon which disks (platters) coated with magnetic material, spin at a constant rotational
velocity. Moving along and between the platters, on a common armature, are read-write heads, one head for each platter
surface. The armature moves the heads radially across the (spinning) platters, allowing each head access to the whole of
the platter. Information is written onto the disk by transmitting an electromagnetic flux through the read-write head.
The entire arrangement of disks and read/write heads is enclosed in a hard casing to protect it from damage.
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3 - Types of Hard Drives
Depending on where they are placed, Hard Drives are classified as Internal or External. Unlike internal Hard Drives,
external Hard Drives are placed outside the computer box (chassis/housing). This allows you to add additional Hard Drives
to your machine even if the drive bays are full. External Hard Drives offer high capacity and, easily removable form of
storage. The Hard Drives found in most computers are internal Hard Drives.
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4 - Speed
The speed of the Hard Drive indicates how fast the PC boots up and the speed at which applications load. The speed of
the Hard Drive also becomes crucial when multitasking or when large amount of data is being processed. e.g. editing graphics.
The specifications which indicate the speed of the Hard Drive are:
- Spindle Speed
- Access Time
- Internal Transfer Rate
- Interface Transfer Rate
Spindle Speed: To make optimum use of faster processors being developed, it is important that the Hard Drive is able to provide the processor with data at high speeds. If the Hard Drive cannot provide the processor with data fast enough, it results in the processor being underutilized (i.e. it remains idle, waiting to receive data to be processed). The spindle is the shaft on which the disks of the Hard Drive are mounted. The faster the spindle rotates the faster the head locates and reads from the sector containing the required data. The speed of Hard Drive ranges from 4200 rpm (Revolution per minute) to 15000 rpm. Hard Drives with spindle speed ranging from 5400 rpm to 10000 rpm are commonly used for desktop machines.
Access Time: Access time, is the time required for the hard disk to read data from the Hard Drive, after it has received the “command” to do so. The problem with using access time to compare performance of Hard Drives is that different manufacturers have a different definition for access time. Though access time is defined as a composite of Command Time, Seek Time, Settle time and Latency, not all manufacturers accept this definition. Quite a few of them, use the term access time to mean seek time, which is just one part of access time. Without going into the depth of these definitions, broadly, faster access time means faster access to the data. Access time is represented in milliseconds (ms).
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Interface Transfer Rate: Also called External transfer rate, it is the speed of transfer over the interface between the hard disk and the rest of the PC. i.e. the speed at which data can be exchanged between the system memory and the cache built into the drive. The faster the interface transfer rate higher the speeds at which data can be accessed from the hard drive. |
The Spindle Speed, Access Time, Interface Transfer Rate and the Internal Transfer Rate together control the speed at which the Hard Drive performs.
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5 - Purchasing Factors
Storage Capacity
A proper assessment of storage requirements is required before buying a hard drive. Storage capacity is the most important
specification to consider while buying a Hard Drive. Hard Drives available in the market have a storage capacity ranging form
20 GB to 1000 GB. Buying a Hard Drive which has a lesser capacity may later force you to upgrade or add additional Hard Drives,
if the drive is unable to meet your growing data storage requirements. It is always a safer bet to opt for a Hard Drive with
storage capacity slightly in excess of your projected (future) data storage requirements. A 20 GB Hard Drive will suffice if
you use the computer for simple applications such as word processing. If you are using a large number of applications or if
you are using your computer for image/video/animation applications, go for a 250 GB to 300 GB drives. Remember, the cost of
going for a higher capacity hard drive, most of the time, works out to be lesser than having to go in for an upgrade later.
Cache Memory
Cache memory is crucial for the performance (speed) of the hard drive. The cache memory, also called buffer, is used to hold
parts of the recent reads from the hard disk and to “pre-fetch” data that may be required in the near future.
For example, parts of the (current) application which may be called on in the near future. Why use the cache memory when this
data can be accessed directly from the hard drive? Storing such data in the cache improves the performance of the Hard Drive
by reducing the physical access to the drive for repeated reads. Also by “pre-fetching” the data it allows
streaming of data from the drive even when the bus is busy. “Pre-fetching” helps in improving the response
time to get data. Hard disks have cache memory generally ranging from 512 Kb to 2 Mb. Greater the amount of cache memory
better the hard disk performance in terms of faster processing of applications due to “pre-fetching” data and
lesser wear of the Hard Drive due to “repeated physical access” of data. When using software applications for
graphics, like 3D Studio or Photoshop, a higher cache memory can significantly improve the processing speed by providing
faster access to data on the hard drive.
Interface Type
Besides speed, the interface type also plays a critical role in the performance of the Hard Drive. The two major interface
types are SCSI (Small Computer Systems Interface) and IDE/ATA (Integrated Drive Electronics)/(AT Attachment). IDE/ATA is by
far the most popular in today's market, and is the default standard for desktop computers of all types and brands. The main
reason behind this is that, the cost of the IDE/ATA drive is cheaper than comparable SCSI solutions. However, SCSI is more
robust, and powerful than an IDE interface.
While most modern systems have an IDE controller, in order to run SCSI drives, you'll need a controller card that supports
the standard your chosen drive uses, as well as special cables to hook the controller card and drive together. This of course,
adds to the cost of the drive.
Form Factor
Hard disks are manufactured in standard sizes. These sizes are called Hard Drive form factors. Form factor refers to the
external dimensions of Hard Drives (generally the width). The two most common form factors are 2.5 inch and 3.5 inch. The
physical size and lower power dissipation of a 2.5 inch drive allows up to four drives to be installed in the same physical
space which one 3.5 inch drive occupies. This offers advantages in situations where multiple Hard Drives need to be used.
2.5 inch drives are also designed to be quieter; this means a quieter home or office environment.
Reliability
It is the Hard Drive that stores all your critical data; hence a Hard Drive failure would imply a huge loss. Therefore, it is
vital to evaluate the reliability of Hard Drives. Unfortunately there is no standard specification which can accurately predict
when a Hard Drive will fail. Measures such as Mean Time before failure (MTBF) and warranty period only offer an indication
about the reliability of the drive. The mean time before failure, is the average number of hours a particular model of drive
will last before it “dies”. A longer period indicates a robust design, which is more reliable. Manufacturers offer
warranty only for a period during which, they believe, the product will not fail. Hence a longer warranty period points to a
Hard Drive that is considered more reliable (less prone to failure). Manufacturers offer warranty on Hard Drives for a period
ranging from 1 to 3 years.
SMART drives
SMART is an acronym for Self-Monitoring, Analysis, and Reporting. SMART compliant drives monitor themselves to ensure their
proper functioning. If it senses a problem, it notifies the user or system administrator. Such a system can alert users in
advance about impending drive failures. SMART drives and drives with built in Error Correction Code make your Hard Drive more
reliable. Understanding your information storage needs & mapping them to the specifications of a Hard Drive is the key
towards selecting the right hard drive.
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