Write through is a storage method in which data is written into the cache and the corresponding main memory location at the same time. The cached data allows for fast retrieval on demand, while the same data in main memory ensures that nothing will get lost if a crash, power failure, or other system disruption occurs.
Although write through minimizes the risk of data loss, every write operation must be done twice, and this redundancy takes time. The active application program must wait until each block of data has been written into both the main memory and the cache before starting the next operation. The "data insurance" therefore comes at the expense of system speed.
Write through is the preferred method of data storage in applications where data loss cannot be tolerated, such as banking and medical device control. In less critical applications, and especially when data volume is large, an alternative method called write back accelerates system performance because updates are normally written exclusively to the cache, and are backed up in the main memory only at specified intervals or under certain conditions.
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