searching to search sequentially ordered file for particular last name such as smithyou could use binary search you would start by reading block of records from the middle of the file the records in the block are all read at once into , -byte buffer in main memory if the keys of these records are too early in the alphabet (kellerfor example)you' go to the / point in the file (princeand read block thereif the keys were too lateyou' go to the / point (deleonby continually dividing the range in half you' eventually find the record you were looking for as we saw in binary search in main memory takes log comparisonswhich for , items is about if every comparison tooksay microsecondsthis would be microsecondsor about / , of secondless than an eye blink howeverwe're now dealing with data stored on disk because each disk access is so time consumingit' more important to focus on how many disk accesses are necessary than on how many individual records there are the time to read block of records will be very much larger than the time to search the records in the block once they're in memory disk accesses are much slower than memory accessesbut on the other hand we access block at timeand there are far fewer blocks than records in our example there are , blocks log of this number is about so in theory we'll need about disk accesses to find the record we want in practice this number is reduced somewhat because we read records at once in the beginning stages of binary search it doesn' help to have multiple records in memory because the next access will be in distant part of the file howeverwhen we get close to the desired recordthe next record we want may already be in memory because it' part of the same block of this may reduce the number of comparisons by two or so thus we'll need about disk accesses ( )which at milliseconds per access requires about millisecondsor / second this is much slower than in-memory accessbut still not too bad insertion unfortunately the picture is much worse if we want to insert (or deletean item from sequentially ordered file because the data is orderedboth operations require moving half the records on the averageand therefore about half the blocks moving each block requires two disk accessesone read and one write once the insertion point is foundthe block containing it is read into memory buffer the last record in the block is savedand the appropriate number of records are shifted up to make room for the new onewhich is inserted then the buffer contents are written back to the disk file next the second block is read into the buffer its last record is savedall the other records are shifted upand the last record from the previous block is inserted at the external storagebeginning of the buffer then the buffer contents are again written back to disk this process continues until all the blocks beyond the insertion point have been rewritten