and large enough that they are used only for special purposes to store backup

And large enough that they are used only for special

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and large enough that they are used only for special purposes— to store backup copies of material stored on other devices, for example — arecalled tertiary storage. Each storage system provides the basic functions of storinga datum and holding that datum until it is retrieved at a later time. The maindifferences among the various storage systems lie in speed, size, and volatility.The wide variety of storage systems can be organized in a hierarchy (Figure 1.6)according to storage capacity and access time. As a general rule, there is astorage capacityaccess timesmallerlargerregisterscacheprimarystoragevolatilemain memorystorage-----------------------------------------------------------nonvolatilenonvolatile memorystoragesecondaryhard-disk drivesstorageoptical disktertiarymagnetic tapesstoragefasterslowerFigure 1.6Storage-device hierarchy.
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14Chapter 1 Introductiontrade-off between size and speed, with smaller and faster memory closer to theCPU. As shown in the figure, in addition to differing in speed and capacity, thevariousstorage systems are either volatile or nonvolatile. Volatile storage, as mentioned earlier,loses its contents when the power to the device is removed, so data must be written tononvolatile storage for safekeeping.The top four levels of memory in the figure are constructed using semi-conductor memory, which consists of semiconductor-based electronic circuits.NVM devices, at the fourth level, have several variants but in general are fasterthan harddisks. The most common form of NVMdevice is flash memory, which is popular inmobile devices such as smartphones and tablets. Increasingly, flash memory is beingused for long-term storage on laptops, desktops, and servers as well.Since storage plays an important role in operating-system structure, we will referto it frequently in the text. In general, we will use the following terminology:Volatile storage will be referred to simply as memory. If we need to empha-size a particular type of storage device (for example, a register),we will do soexplicitly.Nonvolatile storage retains its contents when power is lost. It will be referred toas NVS. The vast majority of the time we spend on NVSwill be on secondarystorage. This type of storage can be classified into two distinct types:Mechanical. A few examples of such storage systems areHDDs, opticaldisks, holographic storage, and magnetic tape. If we need to emphasize aparticular type of mechanical storage device (for example, magnetic tape),we will do so explicitly.Electrical. A few examples of such storage systems are flash memory,FRAM,NRAM, andSSD. Electrical storage will be referred to asNVM. Ifwe needto emphasize a particular type of electrical storage device (for example, SSD), wewill do so explicitly.
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