Data consistency

Data Consistency refers to the usability of data; we want data to be constant in time and for us to be capable of using and showing them in different ways without changing their structure.

Point-in-time consistency is an important property of backup files and a critical objective of software that creates backups. It is also relevant to the design of disk memory systems, specifically relating to what happens when they are unexpectedly shut down.

These large files - as with any database - contain numerous data structures which reference each other by location. For example, some structures are indexes which permit the database subsystem to quickly find search results. If the data structures cease to reference each other properly, then the database can be said to be corrupted.

The importance of point-in-time consistency can be illustrated with what would happen if a backup were made without it.

Because the backup is already halfway done and the index already copied, the backup will be written with the article data present, but with the index reference missing. As a result of the inconsistency, this file is considered corrupted.

Point-in-time consistency is also relevant to computer disk subsystems.

Specifically, operating systems and file systems are designed with the expectation that the computer system they are running on could lose power, crash, fail, or otherwise cease operating at any time. When properly designed, they ensure that data will not be unrecoverably corrupted if the power is lost. Operating systems and file systems do this by ensuring that data is written to a hard disk in a certain order, and rely on that in order to detect and recover from unexpected shutdowns.

On the other hand, rigorously writing data to disk in the order that maximizes data integrity also impacts performance. A process of write caching is used to consolidate and re-sequence write operations such that they can be done faster by minimizing the time spent moving disk heads.

Data consistency concerns arise when write caching changes the sequence in which writes are carried out, because it there exists the possibility of an unexpected shutdown that violates the operating system's expectation that all writes will be committed sequentially.

For example, in order to save a typical document or picture file, an operating system might write the following records to a disk in the following order:

The operating system relies on the assumption that if it sees item #1 is present (saying the file is about to be saved), but that item #4 is missing (confirming success), that the save operation was unsuccessful and so it should undo any incomplete steps already taken to save it (e.g. marking sector 123 free since it never was properly filled, and removing any record of XYZ from the file directory). It relies on these items being committed to disk in sequential order.

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Wikipedia