A disk drive is an electromechanical device that govern the performance of the storage system environment.
Basic Disc Drive Components
- Platter – it is a circular disk that the data is recorded on in binary codes. The typical HDD cnsist of more than one platter
- Spindle – a spindle connects all the platters and is connected to a motor. The motor of the spindle rotates with a constant speed (revolutions per minute). The most common speeds are:
- 5400 rpm
- 7200 rpm
- 10000 rpm
- 15000 rpm
- Read/Write Head – Each platter have two R/W heads – one for each surface of the platter. The R/W head changes the magnetic polarization on the surface of the platter when writing data
- Actuator Arm Assembly – R/W heads are mounted to the actuator arm assembly, which position the R/W head for the location on the platter where the data needs to be written or read.
Disk Service Time
Also called access time. Seek Time describes the time taken to position the R/W head across the platter with a radial movement (moving along the radius of the platter). So to speak, seek time is the time taken to position and settle the arm and the head over the correct track.
The average seek time on a modern disk is typically in the range of 3 to 15 milliseconds. High seek time has a big impact on the read operation of random tracks. To minimize the seek time, data can be written to only a part of available space in cylinders. This results in lower usable capacity, and is known as short-stroking the drive.
To access data, the actuator arm moves the R/W head over the platter to a particular track while the platter spins to position the requested sector under the R/W head. The time taken by the platter to rotate and position the data under the R/W head is called rotational latency.
As you can notice, the average rotational latency depends on rpm of the disk. For example and average rotational latency for 5,400-rpm disk is about 5.5 ms, while for a 15,000-rpm disk is about 2.0 ms.
Data Transfer Rate
In a read operation the data first moves from the disk platters to R/W heads. Then it moves to the drive’s internal buffer. Finally data moves from the buffer thru the interface to the host HBA.
In a write operation the data moves from the HBA to the internal buffer of the disk thru the drive’s interface. The data then moves from the buffer to the R/W heads. Finally, it moves from the R/W heads to the platters. The data transfer rate is the average amonut of data per unit time that the drive can deliver to the HBA
Internal Transfer Rate
Internatl transfer rate is the speed at which data moves from platter’s surface to the internal buffer (cache) of the disk.
Check your knowledge
What describes a landing zone in a disk drive?
A. Area on which the read/write head rests
B. Area where the read/write head lands to access data
C. Area where the data is buffered before writing to platters
D. Area where sector-specific information is stored on the disk
What defines the time taken to position the read/write head across the platter with a radial movement in a disk drive?
A. Seek time
B. Rotational latency
C. Data transfer time
D. Service time
How is the internal transfer rate of disk drives defined?
A. Speed at which data moves from the read/write head to the platter
B. Speed at which data moves from a platter’s surface to the internal buffer
C. Speed at which data moves from internal buffer to the host interface
D. Speed at which data moves from the innermost cylinder to the read/write head