理解Computer中Disk buffer||Random access||disk array controller||disk seeks

• Random access

  wikipedia

  Random access(direct access) is the ability to access an arbitrary element of a sequence in equal time or any datum from a population of addressable elements roughtly as easily and efficiently as any other, no matter how many elements may be in the set.

• Sequential access

  From wikipedia, is a term describing a group of elements being accessed in a predetermined, ordered sequence.

• disk seek

  Higher performance in hard disk drives(HDD) comes from devices which have better performance characteristics.

  These performance characteristics can be grouped into two categories:

  • access time

    The access time (or response time) of a rotating drive is a measure of the time it takes before the drive can actually transfer data.

    The key components that are typically added together to obtain the access time are:

    1. Seek time

       With rotating drives, the seek time measures the time it takes the head assembly on the actuator arm to travel to the track of the disk where the data will be read or written.

       600ms – 25ms(1970s) – 20ms(1980s) – 4~9ms

       12-15ms(mobile devices)

    2. Rotational latency

    3. Command processing time

    4. Settle time

  • data transfer time (or rate)

• disk array controller

  From wikipedia, a disk array controller is a device that manages the physical disk drives and presents them to the computer as logical units.

• disk controller

  The disk controller is the controller circuit which enables the CPU to communicate with a hard disk, floppy disk or other kind of disk drive.

• disk buffer

  In computer storage, disk buffer(often ambiguously called disk cache or cache buffer) is the embedded memory in a hard disk drive(HDD) acting as a buffer between the rest of computer and the physical hard disk platter that is used for storage.

  Modern HDD come with 8–256MiB of such memory, and SSD 4GB.

  The disk buffer is controlled by the microcontroller in the hard disk drive, and the page cache is controlled by the computer to which that disk is attached.

  The disk buffer is usually quite small, and the page cache is generally all unused main memory.