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.
   • 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.
   • 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 provide programmers fine control on placement of files but increase programmer's burden.

     UNIX or PC OSs hide disk organizations from users. Over time, a sequential file may become fragmented. To reduce fragmentation, the system can make a back-up copy of the data on disk and restore the entire disk. The restore operation writes back the blocks of each file continuously (or nearly so). Some systems, such as MS-DOS, have utilities that scan the disk and then move blocks to decrease the fragmentation.

   • Nonvolatile write buffers. Use nonvolatile RAM (such as battery-back-up RAM) to speed up disk writes drastically (first write to nonvolatile RAM buffer and inform OS that writes completed).
   • Log disk. Another approach to reducing write latency is to use a log disk, a disk devoted to writing a sequential log. All access to the log disk is sequential, essentially eliminating seek time, and several consecutive blocks can be written at once, making writes to log disk several times faster than random writes.