A Practical Introduction to Data Structures and Algorithm Analysis

280 Chap. 8 File Processing and External SortingMedium 1996 1997 2000 2004 2006 2008RAM $45.00 7.00 1.500 0.3500 0.1500 0.0339Disk 0.25 0.10 0.010 0.0010 0.0005 0.0001Flash drive – – – 0.1000 0.0900 0.0029Floppy 0.50 0.36 0.250 0.2500 – –Tape 0.03 0.01 0.001 0.0003 – –Figure 8.1 Price comparison table for some writeable electronic data storagemedia in common use. Prices are in US Dollars/MB.8.1 Primary versus Secondary StorageComputer storage devices are typically classified into primary or main memoryand secondary or peripheral storage. Primary memory usually refers to RandomAccess Memory (RAM), while secondary storage refers to devices such as harddisk drives, removeable “flash” drives, floppy disks, CDs, DVDs, and tape drives.Primary memory also includes registers, cache, and video memories, but we willignore them for this discussion because their existence does not affect the principaldifferences between primary and secondary memory.Along with a faster CPU, every new model of computer seems to come withmore main memory. As memory size continues to increase, is it possible that relativelyslow disk storage will be unnecessary? Probably not, because the desire tostore and process larger files grows at least as fast as main memory size. Prices forboth main memory and peripheral storage devices have dropped dramatically in recentyears, as demonstrated by Figure 8.1. However, the cost for disk drive storageper megabyte is about two orders of magnitude less than RAM and has been formany years.There is now a wide range of removable media available for transferring dataor storing data offline in relative safety. These include floppy disks (now largelyobsolete), writable CDs and DVDs, “flash” drives, and magnetic tape. Opticalstorage such as CDs and DVDs costs roughly half the price of hard disk drivespace per megabyte, and have become practical for use as backup storage withinthe past few years. Tape used to be much cheaper than other media, and was thepreferred means of backup. “Flash” drives cost the most per megabyte, but due totheir storage capacity and flexibility, have now replaced floppy disks as the primarystorage device for transferring data between computer when direct network transferis not available.Secondary storage devices have at least two other advantages over RAM memory.Perhaps most importantly, disk and tape files are persistent, meaning that