My Profile
Active Members
TodayLast 7 Days
more...
Awards & Gifts
Online Exams
Fresher Jobs
Our fresher job section is exclusively for fresh graduates! Find jobs for freshers in major Indian
cities including Bangalore, Chennai, Hyderabad, Pune or Kochi
Resources
Find educational articles, blogs, discussion threads and other resources.
Colleges
Find details about any college in India or search for courses.
|
The Harddisk
Posted Date: 22 Feb 2008 Resource Type: Articles/Knowledge Sharing Category: Computer & Technology
|
Posted By: syam s kurup Member Level: Silver
Rating:  Points: 5
|
|
|
The Harddisk
The hard disk can have a huge impact on the performance of your PC: The fact is that the rotating magnetic media of the hard disk is one of the severest performance bottlenecks, causing second-long delays while fat programs spin off the disk and into RAM. Whereas disk access times are measured in miliseconds, system RAM performance is counted in nanoseconds. Understanding hard disk operation - and optimizing - can eliminate teeth-grinding delays.
The factors that affect the speed of a harddisk:
Rotation speed Number of sectors per track Seek time / head switch time / cylinder switch time Rotational latency Data access time Cache on the HD How data is organized on the disks Transfer rates Interface (EIDE / SCSI)
On a harddisk, data is stored in the magnetic coating of the disk. The so called head, held by an actor arm, is used to write and read data. This disk rotates with a constant turn time, measured in revolutions per minute (rpm). Data is organized on a disk in cylinders, tracks and sectors. Cylinders are concentric tracks on the surface of the disk. A track is divided into sectors. A harddisk has a head on each side of a disk. Nowadays, the actuator arm is moved by a servo-motor (not a step-motor which needs more time while swinging in after moving over the desired track). All harddisks have reserved sectors, which are used automatically by the drive logic if there is a defect in the media.
Typical harddisks have a rotation speed from 4,500 to 7,200 rpm, a 10,000 rpm drive just hit the market. The faster the rotation, the higher the transfer rate, but also the louder and hotter the HD. You may need to cool a 7200 rpm disk with an extra fan, or its life would be much shorter. Modern HD’s read all sectors of a track in one turn (Interleave 1:1). The rotation speed is constant.
Modern harddisks use different track sizes. The outer parts of a disk have more space for sectors than the inner parts. Usually, HD’s begin to write from the outside to the inside of a disk. Hence, data written or read at the beginning of a HD is accessed and transferred faster rate.
The fastest seek time occurs when moving from one track directly to the next. The slowest seek time is the so called full-stroke between the outer and inner tracks. Some harddisks (especially SCSI drives) don't execute the seek command correctly. These drives position the head somewhere close to the desired track or leave the head where it was. The seek time everyone is interested in is the average seek time, defined as the time it takes to position the drive's heads for a randomly located request. Yes, you are correct: seek time should be smaller if the disk is smaller (5 1/4", 3 1/2" etc.).
All heads of a harddisk are carried on one actuator arm, so all heads are on the same cylinder. Head switch time measures the average time the drive takes to switch between two of the heads when reading or writing data. Cylinder switch time is the average time it takes to move the heads to the next track when reading or writing data. All these times are measured in milliseconds (ms).
After the head is positioned over the desired track, it has to wait for the right sector. This time is called rotational latency and is measured in ms. The faster the drives spins, the shorter the rotational latency time. The average time is the time the disk needs to turn half way around, usually about 4ms (7200rpm) to 6ms (5400rpm).
Data access time is the combination of seek time, head switch time and rotational latency and is measured in ms.
As you now know, the seek time only tells you about how fast the head is positioned over a wanted cylinder. Until data is read or written you will have to add the head switch time for finding the track and also the rotational latency time for finding the wanted sector.
All modern HD’s have their own cache varying in size and organization. The cache is normally used for writing and reading. On SCSI HD’s you may have to enable write caching, because often it is disabled by default. This varies from drive to drive. You will have to check the cache status with a program like ASPIID from Seagate.
With most EIDE drives, the PC’s system memory is also used for storing the HD’s firmware (e.g. software or "BIOS"). When the drive powers up, it reads the firmware from special sectors. By doing this, manufacturers save money by eliminating the need for ROM chips, but also give you the ability to easily update your drives "BIOS" if it is necessary (Like for the WD drives which had problems with some motherboard BIOS' resulting in head crashes!).
a harddisk has cylinders, heads and sectors. If you look in your BIOS you will find these 3 values listed for each harddisk in your computer. You learned that a harddisk don’t have a fixed sector size as they had in earlier days.
Today, these values are only used for compatibility with DOS, as they have nothing to do with the physical geometry of the drive. The harddisk calculates these values into a logical block address (LBA) and then this LBA value is converted into the real cylinder, head and sector values. Modern BIOS’ are able to use LBA, so limitations like the 504 MB barrier are now gone.
Cylinder, heads and sectors are still used in DOS environments. SCSI drives have always used LBA to access data on the harddisk. Modern operating systems access data via LBA directly without using the BIOS.
In the pictures you can see the several ways how data can be stored physically on the harddisk. With a benchmark program that calculates the transfer rate or seek time of the whole harddisk you can see if your drive is using a 'vertical' or a 'horizontal' mapping. Depending on what kind of read/write heads and servo-motors (for positioning the actuator arm) are used it is faster to switch heads or to change tracks.
|
Responses
|
| Author: Moncy 24 Feb 2008 | Member Level: Silver Points : 1 | Thanks Mr.Syam S Kurup..That was very infomative. Can you say why sata is much faster than pata HDs
|
|
Watch TV Channels
|