Compaq ProLiant 1000 Drive technology overview - Page 5

Table 2. Strategies to improve single disk capacity, performance, and reliability Mechanical Magnetic Disk I/O Increase platter rotation rate Increase bit density per unit of track Write cache buffer data to be written to disk Increase areal density of data Queue read operations Reduce platter size Queue write operations Decrease seek times Reorder read and write operations to execute the next operation physically available on drive Several mechanical design strategies are used to reduce the physical distance that the read/write heads must travel to reach the target segment:  smaller diameter platters  multiple platters per drive  increased speed of platter rotation  increased areal density of data  decreased seek time per track Magnetic storage strategies are used to increase the amount of data in each track. Increasing the bit density per unit length of track also helps increase the rate at which data can be written to and read from the drive. Disk I/O strategies are used to reduce the time that a logical read/write spends waiting for the physical read/write operation. These strategies seek to effectively decouple the logical and physical operations of the disk. Increasingly sophisticated approaches become practical as the embedded processing power and memory incorporated into the drive increases. There are several approaches that can be used:  buffering the data to be written to disk (write caching)  queuing read operations  read-ahead caching  queuing write operations  write caching Reliability Disk drive reliability is measured in terms of Annual Failure Rates (AFR). The AFR is the percentage of disk drive failures occurring in a large population of drives in operation for one year. For example, a population of 100,000 drives with an AFR of 1.5% would experience approximately 1,500 failures per year. An AFR calculated from a small number of drives is subject to large statistical variations that render it inaccurate. Major factors in determining drive reliability are the duty cycle and the I/O workload to which the drives are subject. Duty cycle is simply power-on time, which is calculated as the number of hours that the disk drive is powered on, divided by the number of calendar hours. I/O workload is disk working time, which is calculated as the number of hours that the disk drive is aggressively reading and writing data, divided by the number of calendar hours. 5