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Error Correction and Fault Tolerance

In order to increase the reliability of the file system, and hence the operating system as a whole, NTFS includes several fault tolerance features. As the name suggests, these are capabilities that improve the ability of the file system to deal with error conditions that may arise when the system is in use. Fault tolerance is very important for business applications where a primary goal is to keep systems running smoothly with a minimum of downtime.

In no particular order, here are some of the fault tolerance and error-handling features that NTFS includes. Note that some of these capabilities are implemented through the use of the NTFS fault-tolerant disk driver, called "FTDISK":

• Transactional Operation: The way that NTFS handles transactions as atomic units, and allows transaction recovery, are key fault tolerance features that I have described elsewhere in this section. Recovery is performed automatically whenever the system is started.
• Software RAID Support: NTFS partitions can be set up to use software RAID if the appropriate version of Windows NT or 2000 is used. For more information, see the full discussion of RAID.
• Dynamic Bad Cluster Remapping: When the fault-tolerant disk driver is used, the file system has the ability to automatically detect when bad clusters have been encountered during read or write operations. When a bad cluster is found, the file system will automatically relocate the data from the bad location and mark the cluster bad so it will not be used in the future. Now, the FAT file system includes the ScanDisk utility that can do this as well, but you must run it manually--with NTFS this can be done automatically. Furthermore, ScanDisk can only identify clusters that have already gone bad, at which point, data may be lost. The FTDISK driver will actually read back data as it is written (sometimes called a "verify" operation) ensuring that data is unlikely to be lost due to a bad cluster at the time of a write. (Bear in mind, however, that it is possible for an area of the disk to "go bad" between the time that the data is written and the time that it is read back.)

These features all help to make NTFS a reliable file system. Of course, they only protect against certain types of problems, and only in certain ways. These capabilities should certainly not be considered replacements for proper backups and other system maintenance!

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