pagesallocation unitsor some other namedepending on the system we'll call them blocks the disk drive always reads or writes minimum of one block of data at time block size variesdepending on the operating systemthe size of the disk driveand other factorsbut it is usually power of for our phone book examplelet' assume block size of , bytes ( thus our phone book database will require , , bytes divided by , bytes per blockwhich is , blocks your software is most efficient when it specifies read or write operation that' multiple of the block size if you ask to read bytesthe system will read one block , bytesand throw away all but or if you ask to read , bytesit will read two blocksor , bytesand throw away almost half of them by organizing your software so that it works with block of data at time you can optimize its performance assuming our phone book record size of bytesyou can store records in block ( , divided by )as shown in figure thus for maximum efficiency it' important to read records at time (or multiples of this numberfigure blocks and records notice that it' also useful to make your record size multiple of that way an integral number of them will always fit in block of course the sizes shown in our phone book example for recordsblocksand so on are only illustrativethey will vary widely depending on the number and size of records and other software and hardware constraints blocks containing hundreds of records are commonand records may be much larger or smaller than bytes once the read-write head is positioned as described earlierreading block is fairly fastrequiring only few milliseconds thus 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