Revolutions per minute (abbreviated rpm, RPM, r/min, or r·min−1) is a measure of the frequency of a rotation. It annotates the number of full rotations completed in one minute around a fixed axis. It is used as a measure of rotational speed of a mechanical component.
Standards organizations generally recommend the symbol r/min, which is more consistent with the general use of unit symbols. This is not enforced as an international standard. In French for example, tr/mn (tours par minute) is commonly used, and the German equivalent reads U/min (Umdrehungen pro Minute).
According to the International System of Units (SI), rpm is not a unit. This is because the Revolution is a semantic annotation rather than a unit. The annotation is instead done in the subscript of the formula sign if needed. Because of the measured physical quantity, the formula sign has to be f for (rotational) frequency and ω or Ω for angular velocity. The corresponding basic SI unit is s−1 or Hz. When measuring angular speed, rad·s−1 can also be used as unit.
Even though angular velocity, angular frequency and hertz all have the dimensions of 1/s, angular velocity and angular frequency are not expressed in hertz, but rather in an appropriate angular unit such as radians per second. Thus a disc rotating at 60 revolutions per minute (rpm) is said to be rotating at either 2π rad/s or 1 Hz, where the former measures the angular velocity and latter reflects the number of complete revolutions per second. The conversion between a frequency f measured in hertz and an angular velocity ω measured in radians per second are:
A typical desktop hard disk rotate at 7,200 revolutions per minute (RPM). A typical server hard disk spin at 10,000 or 15,000 rpm to achieve sequential media transfer speeds. You can use hard disk model number to obtain disk RPM. For example, a typical Seagat disk Model # ST373455SS can provide following information:
- ST - Brand identity
- 3 - Form Factor (3 = 3.5")
- 73 - Disk size / Capacity in GB i.e. 73GB
- 4 - Reserved for future use
- 5 - RPM ( 5 = 15k and 0 = 10K)
- 5 - Generation
- SS - Indicates interface i.e Serial Attached SCSI
1 arcminute = 4.62962963 × 10-5 revolutions
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.
Number Of Sectors Per Track
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.
Seek Time / Head Switch Time / Cylinder Switch Time
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 (5.25", 3.5" 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).
Rotational Latency
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
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.
Cache
I guess you already know about cache. 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.
You may be surprized that it is not the cache size that is important, but the organization of the cache itself (write / read cache or look ahead cache).
With most EIDE drives, the cache memory of the harddisk 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!).
Organization Of The Data On The Disks
You now know, 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.
Transfer Rates / Mappings
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.
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