Write back is a storage method in which data is written into the cache every time a change occurs, but is written into the corresponding location in main memory only at specified intervals or under certain conditions.
When a data location is updated in write back mode, the data in cache is called fresh, and the corresponding data in main memory, which no longer matches the data in cache, is called stale. If a request for stale data in main memory arrives from another application program, the cache controller updates the data in main memory before the application accesses it.
Write back optimizes the system speed because it takes less time to write data into cache alone, as compared with writing the same data into both cache and main memory. However, this speed comes with the risk of data loss in case of a crash or other adverse event.
Write back is the preferred method of data storage in applications where occasional data loss events can be tolerated. In more critical applications such as banking and medical device control, an alternative method called write through practically eliminates the risk of data loss because every update gets written into both the main memory and the cache. In write through mode, the main memory data always stays fresh.
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