trees and external storage in this figure the new nodes created as the result of split are shadedand new connections are shown as wiggly lines external storage trees are examples of multiway treeswhich have more than two children and more than one data item another kind of multiway treethe -treeis useful when data resides in external storage external storage typically refers to some kind of disk systemsuch as the hard disk found in most desktop computers or servers in this section we'll begin by describing various aspects of external file handling we'll talk about simple approach to organizing external datasequential ordering finallywe'll discuss -trees and explain why they work so well with disk files we'll finish with another approach to external storageindexingwhich can be used alone or with -tree we'll also touch on other aspects of external storagesuch as searching techniques in the next we'll mention different approach to external storagehashing the details of external storage techniques are dependent on the operating systemlanguageand even the hardware used in particular installation as consequenceour discussion in this section will be considerably more general than for most topics in this book accessing external data the data structures we've discussed so far are all based on the assumption that data is stored entirely in main memory (often called ramfor random access memoryhoweverin many situations the amount of data to be processed is too large to fit in main memory all at once in this case different kind of storage is necessary disk files generally have much larger capacity than main memorythis is made possible by their lower cost per byte of storage of coursedisk files have another advantagetheir permanence when you turn off your computer (or the power fails)the data in main memory is lost disk files can retain data indefinitely with the power off howeverit' mostly the size difference that we'll be involved with here the disadvantage of external storage is that it' much slower than main memory this speed difference means that different techniques must be used to handle it efficiently as an example of external storageimagine that you're writing database program to handle the data found in the phone book for medium-sized city--perhaps , entries each entry includes nameaddressphone numberand various other data