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The performance of a hard disk is very important
to the overall speed of the system - a slow hard disk having the potential to hinder a fast processor like no other
system component - and the effective speed of a hard disk is determined by a number of factors.
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My IBM 40GB 7200 rpm
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Quantum 30GB 7200 rpm
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Quantum 15GB 5400 rpm
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Seagate 10GB 5400 rpm
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Write Speed in MB/s.
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Disk Benchmark Result for C:\
Write Speed Average 30.51 MB/s.
Read Speed Average 27.94 MB/s. |
1. Rotational speed of the platters. Disk RPM is a critical component of hard drive performance because
it directly impacts the latency and the disk transfer rate. The faster the disk spins, the more data passes under
the magnetic heads that read the data; the slower the RPM, the higher the mechanical latencies. Hard drives only
spin at one constant speed.
2. Mechanical latencies, measured in milliseconds, include both seek time and rotational latency. "Seek
Time" is measured defines the amount of time it takes a hard drive's read/write head to find the physical
location of a piece of data on the disk. "Latency" is the average time for the sector being accessed
to rotate into position under a head, after a completed seek. It is easily calculated from the spindle speed, being
the time for half a rotation. A drive's "average access time" is the interval between the time a request
for data is made by the system and the time the data is available from the drive. Access time includes the actual
seek time, rotational latency, and command processing overhead time.
3. Disk transfer rate (sometimes called media rate) is the speed at which data is transferred to and from
the disk media (actual disk platter) and is a function of the recording frequency. It is generally described in
megabytes per second (MBps). Modern hard disks have an increasing range of disk transfer rates from the inner diameter
to the outer diameter of the disk. This is called a "zoned" recording technique. The key media recording
parameters relating to density per platter are Tracks Per Inch (TPI) and Bits Per Inch (BPI). A track is a circular
ring around the disk. TPI is the number of these tracks that can fit in a given area (inch). BPI defines how many
bits can be written onto one inch of a track on a disk surface.
Random access is the true measure of seek speed. Many drives advertise sub 10 millisecond seek speeds, but seek
speeds are misleading. Access time is the time it really takes to read data, not just the time it takes to move
the head to the proper cylinder. To calculate access time the software must read a single sector
off of the hard drive. By reading a sector the drive can not respond to the command until the sector is available,
so rotational latency + seek time = access time.
A 7200 rpm drive has a rotational latency of 4.15 ms on average. A 4500 rpm drive has a rotational latency of 6.67
ms on average. So a 7200 rpm drive with a seek of 12.5ms has an access time that is just as fast as a 4500 rpm
drive with a seek of 10.0 ms.
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Seek Time
Like latency, seek time is a measurement of the read/write performance of a disk drive. For a magnetic disk drive, seek time is the amount of time it takes to move the read/write head from its current
track
to the track where data is to be read from or written to. For a CD-ROM drive, seek time is the amount of time it
takes to position the laser beam over the portion of the CD’s single spiraling track that contains the data to
be read. Seek time is used to assess disk drive performance.
Latency
Like seek time, latency is a measurement of the read/write performance of a disk drive. Latency is the time it
takes for a particular sector to pass under the read/write head of a disk after the head is positioned over the
appropriate disk track. The maximum latency for a magnetic disk is the time it takes for a complete rotation of
a disk; the average latency of a magnetic disk is half the time of a complete rotation, assuming the sectors are
requested at random. For a CD-ROM, the latency is the time it takes for a particular sector to pass under the laser
beam after the photodetector is positioned an appropriate distance from the edge of the disk.
Read / Write Head
A component of a magnetic disk that reads data from and writes data to the disk surface. Most of the floppy disk
drives in use today have two read/write heads, one for the top surface of the disk and the other for the bottom.
Hard disk drives have many read/write heads, typically one for each side of each platter. There is a read/write
head for each side of each platter, mounted on arms which can move them towards the central spindle or towards
the edge. The arms are moved by the head actuator, which contains a voice-coil - an electromagnetic coil that can
move a magnet very rapidly. Loudspeaker cones are vibrated using a similar mechanism.
As the disk spins, magnetic fields on its surface representing
data bits pass under the read/write head. These fields generate minute electrical currents in the head that can
be interpreted by the computer as binary data. To write data, the process is reversed, and a current is sent to
the read/write heads. The current generates a magnetic field in the head which polarizes the magnetic particles
on the disk surface in one of two ways to represent either binary zeros or ones.
When the computer wants to read data, the operating system works out where the data is on the disk. To do this
it first reads the FAT (File Allocation Table) at the beginning of the partition. This tells the operating system
in which sector on which track to find the data. With this information, the head can then read the requested data.
The disk controller controls the drive's servo-motors and translates the fluctuating voltages from the head into
digital data for the CPU.
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Track
Part of the physical format of a disk’s surface. A disk’s storage area is organized into:
- sides of the disk surface (top and bottom)
A hard disk may have more than two sides if it consists of more than one platter.
- tracks, which are arranged as concentric rings on the surface of magnetic disks. CD-ROMs have a single
track, spiraling from the disk edge towards the center.
- sectors, which occupy arc-shaped portions of the tracks.
| When a disk undergoes a low-level format, it is divided
it into tracks and sectors. The tracks are concentric circles around the central spindle on either side of each
platter. |
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Transfer rate
The transfer rate specifies the number of bytes that can be read from a disk and placed into computer memory in
a second. This measurement is used to assess disk drive performance.
Magnetic disk
A disk that uses magnetic fields to store data. The most common types of magnetic disks are hard disks and floppy
disks. Although there are other types of disks used for data storage, such as CD-ROMs, magnetic disks are the type
most frequently referred to simply as "disks."
Platter
A rigid disk that serves as an individual component of a hard disk. Hard disks usually consist of several stacked
platters. The platters are coated with magnetic media, allowing them to store data as localized changes in magnetic
polarity. Typically two or three or more platters are stacked on top of each other with a common spindle that turns
the whole assembly at several thousand revolutions per minute. There's a gap between the platters, making room
for magnetic read/write head, mounted on the end of an actuator arm. This is so close to the platters that it's
only the rush of air pulled round by the rotation of the platters that keeps the head away from the surface of
the disk - it flies a fraction of a millimetre above the disk - 0.07mm or less.
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More about Hard Disk Performance
There are two ways to measure the performance of a hard disk:
1.1 Transfer (Data) rate - The data rate is the number of bytes per second that the drive can deliver to the CPU.
Rates between 5 and 40 megabytes per second are common.
1.2 Seek time - The seek time is the amount of time between when the CPU requests a file and when the first byte
of the file is sent to the CPU. Times between 10 and 20 milliseconds are common.
The other important parameter is the capacity of the drive, which is the number of bytes it can hold.
Performance in Details
When selecting a hard disk, an important feature to consider is the performance (speed) of the drive. Hard disks
come in a wide range of performance capabilities. As is true of many things, one of the best indicators of a drive's
relative performance is its price.
The Speed of a disk drive can be measured in two ways:
1. Average seek time
2. Transfer (Data) rate
1. Average seek time
Average seek time, normally measured in milliseconds (ms), is the average amount of time it takes to move
the heads from one cylinder to another cylinder a random distance away. One way to measure this specification is
to run many random track-seek operations and then divide the timed results by the number of seeks performed. This
method provides an average time for a single seek.
The standard way to measure average seek time used by many drive manufacturers involves measuring the time that
it takes the heads to move across one-third of the total cylinders. Average seek time depends only on the drive;
the type of interface or controller has little effect on this specification. The rating is a gauge of the capabilities
of the head actuator.
Note:
Be wary of benchmarks that claim to measure drive seek performance. Most IDE and SCSI drives use a scheme called
sector translation, so any commands to the drive to move the heads to a specific cylinder do not actually cause
the intended physical movement. This situation renders some benchmarks meaningless for those types of drives. SCSI
drives also require an additional command, because the commands first must be sent to the drive over the SCSI bus.
Even though these drives can have the fastest access times, because the command overhead is not factored in by
most benchmarks, the benchmark programs produce poor performance figures for these drives.
A slightly different measurement, called average access time, involves another element, called latency. Latency
is the average time (in milliseconds) that it takes for a sector to be available after the heads have reached a
track. On average, this figure is half the time that it takes for the disk to rotate one time, which is 8.33 ms
at 3,600 RPM. A drive that spins twice as fast would have half the latency. A measurement of average access time
is the sum of the average seek time and latency. This number provides the average amount of time required before
a randomly requested sector can be accessed.
Latency is a factor in disk read and write performance. Decreasing the latency increases the speed of access to
data or files, accomplished only by spinning the drive platters faster. I have a drive that spins at 4,318 RPM,
for a latency of 6.95 ms. Some drives spin at 7,200 RPM or faster, resulting in an even shorter latency time of
only 4.17 ms. In addition to increasing performance where real-world access to data is concerned, spinning the
platters faster also increases the data-transfer rate after the heads arrive at the desired sectors.
2. Transfer (Data) rate
The transfer rate probably is more important to overall system performance than any other specification.
Transfer rate is the rate at which the drive and controller can send data to the system. The transfer rate depends
primarily on the drive's HDA and secondarily on the controller. Transfer rate used to be more bound to the limits
of the controller, meaning that drives that were connected to newer controllers often outperformed those connected
to older controllers. This situation is where the concept of interleaving sectors came from. Interleaving refers
to the ordering of the sectors so that they are not sequential, enabling a slow controller to keep up without missing
the next sector.
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