What is write-through in memory caches?

When a datum is written to the cache, it must at some point be written to the backing store as well. The timing of this write is controlled by what is known as the write policy. In a write-through cache, every write to the cache causes a write to the backing store. Alternatively, in a write-back cache, writes are not immediately mirrored to the store. Instead, the cache tracks which of its locations have been written over (these locations are marked dirty). The data in these locations is written back to the backing store when those data are evicted from the cache. For this reason, a miss in a write-back cache will often require two memory accesses to service: one to retrieve the needed datum, and one to write replaced data from the cache to the store.

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