once the read-write head is positioned as described earlierreading block is fairly fastrequiring only few milliseconds thusa disk access to read or write block is not very dependent on the size of the block it follows that the larger the blockthe more efficiently you can read or write single record (assuming you use all the records in the blocksequential ordering one way to arrange the phone book data in the disk file would be to order all the records according to some keysay alphabetically by last name the record for joseph aardvark would come firstand so on this is shown in figure aardvark (other dataaaron (other dataabbot (other data block figure able abrams (other data(other data abrell (other data block sequential ordering 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 an , -byte buffer in main memory if the keys of these records are too early in the alphabet (kellerfor example)you would 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 halfyou would eventually find the record you were looking for as we saw in binary search in main memory takes log comparisonswhich for , items would be 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