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Frequently Asked Questions About Hardisks


  • MR (Magneto-Resistive) Heads

    MR heads allow for an increased areal density (1-2 billion BPSI) and hence storage capacity of hardisks without sacrificing performance. It works, when reading, by passing a constant flow of current through the MR read element of the head. When the head passes over a magnetic field on the media, the head changes its resistance, which is then detected by the change in amperage of the current. This mechanism cannot be used for writing, and so a conventional thin film inductive write element is used for that.
  • PRML Read Channels

    Read channels perform the data conversion from digital data to analog signals required to write data to a hardisk. At high data densities, the analog signal peaks start to overlap, causing data degradation. By using digital coding and filtering techniques, PRML read channels sample the analog signal at a few points, allowing for the accurate determination of the shape of the readback signal. This allows for higher areal densities to be used.
  • Areal Density

    This refers to how densely information is recorded on the disk media. It is measured in Bits Per Square Inch (BPSI) and is the product of the disk media's Bits Per Inch (BPI) and Tracks Per Inch (TPI) quantities.
  • Data Caching

    Access to a hardisk is usually performed in a predictable manner. Hence, performance can be increased by first reading anticipated data to be requested later into a memory cache before the computer asks for it; and then retrieving it from the memory as and when needed (data retrieval from memory is much faster). This memory cache is called a Read Look-Ahead Buffer. A well implemented cache design can increase hardisk performance significantly.
  • Segmented Caching

    This refers to the partitioning of a hardisk's Read Look-Ahead Buffer into several parts, allowing the drive to behave as if it had two seperate caches (but of smaller size). This can improve the efficiency of the Read Look-Ahead Buffer.
  • Adaptive Caching

    Adaptive segmentation of the hardisk's Read Look-Ahead Buffer allows the drive to allows the drive to control the number of segments in the cache.
  • Bad Sectors / Defective Sectors

    These are areas of defect on the disk media and cannot be used for reliable storage and retrieval of data. A defect management system is built into the drive's disk controller to deal with these defective parts of disk media and provide defect-free operation. Defect management is done by setting aside some spare sectors during manufacture and then transparently mapping a defective sector onto one of the spares when the operating system requests that information be written to that defective sector.
  • Interfaces

    A drive interface is a standardized combination of connector configuration signal levels, functions, commands, and data transfer protocols. There are 2 main interfaces for hardisks: IDE and SCSI.

  • IDE (ATA)

    Short of Integrated Drive Electronics. Has 3 PIO modes.

    Mode 0: 3.3Mb/s
    Mode 1: 5.2Mb/s
    Mode 2: 8.3Mb/s

  • EIDE (Fast ATA & Fast ATA-2)

    An Enhanced version of IDE, this interface has a primary and a secondary channel that can be connected to two devices each, one as master and one as slave, for a total of four. EIDE controllers are integrated with the motherboard. ATAPI CD-ROMs can also use the EIDE interface. EIDE controllers are backward compatible with normal IDE devices. Has an additional 2 PIO modes over IDE.

    Mode 3: 11.1Mb/s (Fast ATA)
    Mode 4: 16.6Mb/s (Fast ATA-2)
    Mode 5: 20.0Mb/s [Proposed by some peripheral vendors, but never really took off]

    Fast ATA (Advanced Technology Attachment) is the implementation of the ANSI standardized Programmed Input/Output (PIO) Mode 3 and multi-word Direct Memory Access (DMA) Mode 1 protocols. This allows for a a maximum data transfer rate of 11.1Mb/s (Mode 3) or 13.3Mb/s (Multiword DMA mode).

    Fast ATA-2 is the implementation of ANSI PIO Mode 4 and multi-word DMA Mode 2 protocols. This allows for a maximum data transfer rate of 16.7Mb/s.

    Systems that do not support the Fast ATA and/or Fast ATA-2 standards can be made to support these standards by upgrading through flashing of the BIOS, if equipped, or by using Fast ATA or Fast ATA-2 host adapters equipped with their own BIOS.

  • Ultra DMA/33

    This is a new mode introduced by Quantum with a maximum transfer rate of 33.3Mb/s. It uses the IDE interface. Ultra DMA/33 hardisks can be used on normal EIDE controllers, but will run only in mode 4. To gain the full benifits of Ultra DMA/33, a Ultra DMA/33 controller is needed. Not only is this mode faster, but it also supports error checking.

  • SCSI (Small Computer System Interface)

    Very expensive compared to EIDE, it is used in high performance setups and is able to daisy chain up to 7 devices on the same channel (8bit) or up to 15 devices on the same channel (16bit), the SCSI interface has 2 variants: 8-bit (50 wire data cable) and 16-bit (68 wire data cable). To further complicate things, it comes in different clock speeds:

    Type Clock Speed Transfer Speed Bits Notes
    SCSI 1 5MHz 2-5Mb/s 8 Bits Async/Sync
    SCSI 2 (Fast SCSI) 10MHz 10-20Mb/s 16 Bits Differential
    SCSI 3 (Ultra SCSI) 20MHz 20-40Mb/s 16 Bits Ultra
    Ultra2 SCSI 40MHz 40-80Mb/s 16 Bits LVD

    Ultra2 SCSI increases bus data rates to 80 Mb/s, provides differential data integrity, extends the SCSI bus cable lengths to 12 meters, and supports up to 15 peripherals. This is compares to the Ultra SCSI single-ended cable restrictions of 3 meters with 3 peripherals and 1.5 meters for 7 peripherals. Ultra2 SCSI is a part of the overall SCSI 3 definition and Ultra2 products will work on the older SCSI bus designs and vice versa.
  • Measuring Hard Disk Drive Performance

    1. Average Seek Time - The amount of time taken for the actuator arm to move the read/write head to a randomly selected track.

    2. Head Switch Time - The average amount of time required for the drive to switch between two of the read/write heads while reading or writing data.

    3. Cylinder Switch Time - The average amount of time required for the drive to switch from one cylinder to another while reading or writing data.

    4. Rotational Latency - The average amount of time required for the drive to position the head over the correct sector once positioned on the proper track. This measurement is dependent on the drive's rotational speed.

    Rotational Speed Rotational Latency

    3,600rpm

    8.3ms

    4,500rpm

    6.7ms

    5,400rpm

    5.7ms

    7,200rpm

    4.2ms

    10,000rpm

    3.3ms

    5. Data Access Time - A combination of seek time, head switch time, and rotational latency. It is the amount of time required to position a read/write head over a particular track and find the sectors of interest within the track for reading or writing.

    6. Transfer Rates - This is the speed at which data transfer between the disk and the CPU occurs at. This is dependant on both the hardisk and the host controller.

    7. Data Throughput Rate - A combination of both data access time and data transfer rate. It is a measurement of the total amount of data that the processor can access per unit time. It is dependant also on the speed of the host computer.
    Benchmarking

    The hardisk is one of the weakest links in the computer and it has a great impact on overall system performance. Much time is wasted as the processor sits around waiting for the hardisk to feed it data. I don't have many hardisks and here are just a few benchmarks I have run on the hardisks that are avaliable to me. I use the CheckHD 0.99c programme to benchmark the hardisks.

    1. Quantum Fireball TM 3200AT
    2. Quantum Fireball 1280AT
    3. Seagate ST3145A

    The test system is as follows

    CPU: Cyrix 6x86 P200+ (75x2)
    Motherboard: Abit SM5
    Chipset: Intel Triton VX
    Level 2 Cache: 512Kb Pipeline Burst
    Main Memory: 32Mb 10ns Fujitsu SDRAM
    Video Card: Diamond Stealth 2000 3D
    Video Memory: 2MB EDO DRAM
    Monitor: Sony SFII 15" Trinitron
    CD-ROM: Optics Storage Maverick 16x


    Hardisk Size/Mb Rotational Speed/RPM Overall Score
    Quantum Fireball TM 3200AT 3200 4500 2.35
    Quantum Fireball 1280AT 1280 5400 2.65
    Seagate ST3145A 130 ? 1.00

    The 2.65 score of the Fireball 1280AT does not tally with the score published by the author of CheckHD (2.40). The reason I was given when I emailed the author was that the programme was "over compensating" for a certain factor. Even the 2.35 scored by the Fireball TM 3200AT is higher than the 2.25 reported by the author. The same goes for the Seagate ST3145A... something special in my system perhaps? Sadly, the author of CheckHD is now suffering from RSI (repetitive stree injury) and is not able to make any modifications to the CheckHD programme. I wish him a speedy recovery!

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    Last modified on September 29, 1997