To ensure data integrity, the backup copy of a file must be an exact image of the original at a given point in time. However, copying a file is not an instantaneous process. Unless the file is very small, the backup client must read from the file and write to the backup media several times to make a complete copy. If the backup client cannot ensure that no other application modifies the file while it is being copied, you may have a problem with the integrity of the data being copied.
Example: Copy eight successive read and write operations
This example illustrates the data integrity problem, where a file is copied by eight successive read and write operations.
The backup client copies each block in turn. Halfway through the backup, while block 4 is being copied, an application makes small changes to blocks 2 and 6, which together form a single transaction (a debit and a credit, for example). The backup captures the change to block 6, but not the change to block 2, since it has already been copied. The backup copy, therefore, contains a partial transaction that may render the backup useless, because the application that created the file will probably consider the file corrupt.
This is a common problem in database applications. Many users require simultaneous access to a particular file, and at the same time, a single transaction is likely to make a number of small changes in different places within the file, or even to a group of files.
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