Magnetic-disk storage. The primary medium for the long-term online storage
of data is the magnetic disk. Usually, the entire database is stored on
magnetic disk. The system must move the data from disk to main memory
so that they can be accessed. After the system has performed the designated
operations, the data that have been modified must be written to disk.
As of 2009, the size of magnetic disks ranges from 80 gigabytes to 1.5
terabytes, and a 1 terabyte disk costs about $100. Disk capacities have been
growing at about 50 percent per year, and we can expect disks of much larger
capacity every year. Disk storage survives power failures and system crashes.
Disk-storage devices themselves may sometimes fail and thus destroy data,
but such failures usually occur much less frequently than do system crashes.