External Memory

4. Outline the disk performance parameters? When the disk drive is operating the disk is rotating at constant speed To read or write the head must be positioned at the desired track and at the beginning of the desired sector on the track Track selection involves moving the head in a movable-head system or electronically selecting one head on a fixed-head system Once the track is selected, the disk controller waits until the appropriate sector rotates to line up with the head Seek time On a movable-head system, the time it takes to position the head at the track Rotational delay (rotational latency) The time it takes for the beginning of the sector to reach the head Access time The sum of the seek time and the rotational delay The time it takes to get into position to read or write Transfer time Once the head is in position, the read or write operation is then performed as the sector moves under the head This is the data transfer portion of the operation 5. Describe the RAID? Redundant Array of Independent Disks Consists of 7 levels Levels do not imply a hierarchical relationship but designate different design architectures that share three common characteristics: 1) Set of physical disk drives viewed by the operating system as a single logical drive 2) Data are distributed across the physical drives of an array in a scheme known as striping 3) Redundant disk capacity is used to store parity information, which guarantees data recoverability in case of a disk failure