block contains the same number of fixed-size data records depending on the applicationa record might be only few bytes -composed of little or nothing more than the key -or might be hundreds of bytes with relatively small key field records are assumed not to cross block boundaries these assumptions can be relaxed for special-purpose sorting applicationsbut ignoring such complications makes the principles clearer as explained in section sector is the basic unit of / in other wordsall disk reads and writes are for one or more complete sectors sector sizes are typically power of twoin the range to bytesdepending on the operating system and the size and speed of the disk drive the block size used for external sorting algorithms should be equal to or multiple of the sector size under this modela sorting algorithm reads block of data into buffer in main memoryperforms some processing on itand at some future time writes it back to disk from section we see that reading or writing block from disk takes on the order of one million times longer than memory access based on this factwe can reasonably expect that the records contained in single block can be sorted by an internal sorting algorithm such as quicksort in less time than is required to read or write the block under good conditionsreading from file in sequential order is more efficient than reading blocks in random order given the significant impact of seek time on disk accessit might seem obvious that sequential processing is faster howeverit is important to understand precisely under what circumstances sequential file processing is actually faster than random accessbecause it affects our approach to designing an external sorting algorithm efficient sequential access relies on seek time being kept to minimum the first requirement is that the blocks making up file are in fact stored on disk in sequential order and close togetherpreferably filling small number of contiguous tracks at the very leastthe number of extents making up the file should be small users typically do not have much control over the layout of their file on diskbut writing file all at once in sequential order to disk drive with high percentage of free space increases the likelihood of such an arrangement the second requirement is that the disk drive' / head remain positioned over the file throughout sequential processing this will not happen if there is competition of any kind for the / head for exampleon multi-user time-shared computer the sorting process might compete for the / head with the processes of other users even when the sorting process has sole control of the / headit is still likely that sequential processing will not be efficient imagine the situation where all processing is done on single disk drivewith the typical arrangement