Data access time is time computer takes to position read/write heads on the required track and sector. Typical time required to access two default tracks is 8-12ms (milliseconds). In case of high-end hard drives this time is reduced to 5-7ms.
Required time to move a head from one track to the other is 0.5 – 1.5ms. Full rotation time of modern hard drives is 8 -16ms. Rotational latency is 3-8ms. The lesser the data access time is, the faster is a hard drive.
Read/write speed is data transfer rate from/to the disk drive, which depends on the disk controller, bus type and CPU speed.
Read/write speed varies from 1.5-3 Mbps in low-end HDDs to 4-5 Mbps in mid-range hard drives and to 20 Mbps in high-end HDDs.
SCSI hard drives have 10000 rpm spindle speed, 5ms – seek time and 40-80 Mbps data transfer rate.
3. HDD interfaces – a bus type via which a hard drive is connected to the PC. It is located on the motherboard. There are three main types of interfaces:
- IDE and its variations
IDE (Integrated Disk Electronic) or (ATA) Advanced Technology Attachment
Data transfer rate: 8.3, 16.7, 33.3, 66.6, 100 Mbps. The interface standard is supported by various devices: HDD, Super Floppy, magneto-optics, СD-ROM, СD-R, DVD-ROM, LS-120, ZIP, etc.
A major drawback of the IDE is a small number of devices which can be simultaneously connected to the PC (not more than 4). For modern PCs it is not enough.
Later, IDE were equipped with Ultra ATA moving protocols which considerably increased data transfer rate.
Mode 4 and DMA (Direсt Memory Aссess) Mode 2 run with a transfer rate of 16,6 Mbps.
Ultra DMA/33 and Ultra DMA/66 were developed by Quantum in 1998. Both interfaces have three modes of operation 1, 2 and 4. In the Mode 2, a hard drive runs with 33Mbps (Ultra DMA/33 Mode 2) but only by data transfer through a clipboard.
Basic requirements of Ultra DMA are
- Ultra DMA support by the motherboard and the HDD;
- A special driver which may be downloaded from the Internet.
Ultra DMA is compatible with earlier controllers. It has two variations: Ultra DMA/100 (developed in 2000) and Ultra DMA/133 (manufactured in 2001).
IDE (ATA) was replaced with Fireware (IEEE-1394) which is more reliable and runs at 100 Mbps.
SСSI (Small Computer System Interface). Devices with this interface are two times more expensive and require a special controller on the motherboard. Such interfaces are used in servers and PCs running publishing applications and CAD since they provide higher performance (up to 160Mbps) and the opportunity to connect a wide range of devices.
Data transfer rate of Ultra SCSI equals its frequency. In case of Ultra Wide data transfer rate is twice its frequency. For example Ultra2 Wide SCSI runs at 80 Mbps.
SCSI has two advantages over IDE: usability and performance.
SCSI can support 7-15 devices simultaneously, IDE only 4. At the same time SCSI allows connecting devices via longer cables than IDE.
Contrary to IDE, SCSI has higher data transfer rate and supports simultaneous transactions.
1. Ultra SCSI 2/3(Fast-20) runs at 40 Mbps while its 16-bit version Ultra2 SCSI – at up to 80 Mbps.
2. Another variety of SCSI is Fibre Channel Arbitrated Loop (FС-AL) which runs at 100 Mbps and allows connecting devices via a 30-meter cable. FС-AL supports HotPlug and additional error correction. FC-AL is more expensive than SCSI.
4. Other properties of modern HDDs
A large variety of HDDs makes the choice of a hard drive not an easy one. Besides storage capacity one should also pay attention to its performance. The latter depends on seek time, spindle speed, internal and external data transfer rate and disk cache size.
4.1. Seek Time
It takes a hard drive a certain amount of time to move a magnetic head from one place to another to read a portion of data. Seek time depends on the distance between the location of a magnetic head and the designated place. HDD manufacturers use different algorithms of the seek time calculation. That is why it is difficult to compare HDDs according to this property. For example Fujitsu and Western Digital calculate the seek time by a pair of tracks. Maxtor and Quantum use a method of default access time calculation. For writing, the seek time is higher than for reading. Some manufacturers indicate only seek time for reading. In any case it is advisable to take into account the maximum seek time (that is the time a hard drive takes to move a magnetic head between two mostly remote tracks) and the minimum seek time (time required to move a magnetic head to a neighboring track).
4.2. Spindle speed
Spindle speed determines the so called “hidden time”. The latter equals the time of one disk rotation divided by two. For example an HDD with 3600 RPM has 8.33ms of the hidden time, a 4500 RPM HDD – 6.67ms, a 5400 RPM HDD – 5.56ms, a 7200 RPM HDD – 4.17ms. In a variety of modes, the hidden time is as important as the seek time for HDD performance.
4.3. Internal data transfer rate
It is a property which determines how long it takes for an HDD to read or write data. Due to the zoned data recording, the internal data transfer rate is lower at remote tracks and higher at the tracks which are located closer to the disk centre. In most cases this very property determines the general data transfer rate.
4.4. External data transfer rate
It is the speed of data transfer via the interface. It depends on the type of interface and has a fixed value: 8.3, 11.1, 16.7Mbps for Enhanced IDE (PIO Mode2, 3, 4); 33.3 66.6 100 for Ultra DMA; 5, 10, 20, 40, 80, 160Mbps for synchronous SСSI, Fast SСSI-2, FastWide SСSI-2 Ultra SСSI (16-bit).
4.5. HDD cache size
HDD performance very much depends on the HDD cache size and type. The bigger cache size is the faster and more expensive is a hard drive. The value varies from 128 kb in earlier models to 64mb in modern ones.
Thus each of the mentioned properties is important when choosing a hard drive. However, to determine the extent to which each property contributes to a total performance of an HDD is not an easy task.