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Disk (disc) mirroring

Disk mirroring protects data against hardware failure. In its simplest form, a twodisk subsystem would be attached to a host controller. One disk serves as the mirror image of the other. When data is written to it, it is also written to the other disk. Both disks will contain exactly the same information. If one fails, the other can supply the user data without problem.


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Mirror image backups (also referred to as bitstream backups) involve the backup of all areas of a computer hard disk drive or another type of storage media (e. g. , Zip disks, floppy disks, Jazz disks, etc. ). Such mirror image backups exactly replicate all sectors on a given storage device. Thus, all files and ambient data storage areas are copied. Such backups are sometimes referred to as “evidencegrade” backups and they differ substantially from standard file backups and network server backups. The making of a mirror image backup is simple in theory, but the accuracy of the backup must meet evidence standards. Accuracy is essential and to guarantee accuracy, mirror image backup programs typically rely on mathematical CRC computations in the validation process. These mathematical validation processes compare the original source data with the restored data. When computer evidence is involved, accuracy is extremely important, and the making of a mirror image backup is typically described as the preservation of the “electronic crime scene. ”
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A temporary storage area, usually in RAM. The purpose of most buffers is to act as a holding area, enabling the CPU to manipulate data before transferring it to a device. Because the processes of reading and writing data to a disk are relatively slow, many programs keep track of data changes in a buffer and then copy the buffer to a disk. For example, word processors employ a buffer to keep track of changes to files. Then when you save the file, the word processor updates the disk file with the contents of the buffer. This is much more efficient than accessing the file on the disk each time you make a change to the file. Note that because your changes are initially stored in a buffer, not on the disk, all of them will be lost if the computer fails during an editing session. For this reason, it is a good idea to save your file periodically. Most word processors automatically save files at regular intervals. Another common use of buffers is for printing documents. When you enter a PRINT command, the operating system copies your document to a print buffer (a free area in memory or on a disk) from which the printer can draw characters at its own pace. This frees the computer to perform other tasks while the printer is running in the background. Print buffering is called spooling. Most keyboard drivers also contain a buffer so that you can edit typing mistakes before sending your command to a program. Many operating systems, including DOS, also use a disk buffer to temporarily hold data that they have read from a disk. The disk buffer is really a cache.
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A collection of procedures and data objects that is protected in a domain of its own so that the internal structure of a data object is accessible only to the procedures of the encapsulated subsystem and that those procedures may be called only at designated domain entry points. Encapsulated subsystem, protected subsystem and protected mechanisms of the TCB are terms that may be used interchangeably.
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Instead of using one large disk to store data, one can use many smaller disks (because they are cheaper). See disk mirroring and duplexing. An approach to using many lowcost drives as a group to improve performance, yet also provides a degree of redundancy that makes the chance of data loss remote.
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Bitstreams backups (also referred to as mirror image backups) involve all areas of a computer hard disk drive or another type of storage media. Such backups exactly replicate all sectors on a given storage device. Thus, all files and ambient data storage areas are copied.
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