Optimization of Disk-Block Access 1. Data is transferred between disk and main memory in units called blocks . 2. A block is a contiguous sequence of bytes from a single track of one platter. 3. Block sizes range from 512 bytes to several thousand. 4. The lower levels of file system manager covert block addresses into the hardware-level cylinder, surface, and sector number. 5. Access to data on disk is several orders of magnitude slower than is access to data in main memory. Optimization techniques besides buffering of blocks in main memory. o Scheduling: If several blocks from a cylinder need to be transferred, we may save time by requesting them in the order in which they pass under the heads. A commonly used disk-arm scheduling algorithm is the elevator algorithm . o File organization . Organize blocks on disk in a way that corresponds closely to the manner that we expect data to be accessed. For example, store related information on the same track, or physically close tracks, or adjacent cylinders in order to minimize seek time. IBM mainframe OS's
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