Difference Between Writeback and Writethrough Cache

Writeback cache temporarily stores data changes in the cache and updates the main memory later, boosting speed. On the other hand, Write Through cache writes data to both the cache and main memory simultaneously, ensuring data accuracy but with slightly lower performance. Let’s break down how both methods work and where each is preferred.

Write-through

When you update data, the system writes it to both the cache and the back-end storage. This mode makes operations easy, but it slows down data writing because the data has to be written to both the cache and the storage.

Write-back

When you update data, it is written only to the cache. The modified data is saved to the back-end storage only when the data is removed from the cache. This mode offers fast data write speed, but there is a risk of data loss if a power failure happens before the updated data is written to the storage.

Differences Between Write-Through and Write-Back Methods

1. Write-Through Method:
   • In this method, both the main memory and cache memory are updated simultaneously when a memory write operation occurs.
   • If the cache memory contains the data at the specified address, it is updated in parallel with the main memory.
2. Write-Back Method:
   • In this method, only the cache location is updated during a write operation.
   • The main memory and cache memory may have different data since the update is not immediately reflected in the main memory.
3. Frequency of Memory Write Operations:
   • The write-through method typically involves a higher number of memory write operations in a typical program.
   • The write-back method generally has fewer memory write operations in a typical program.
4. I/O Device Communication through DMA:
   • When an I/O device communicates through Direct Memory Access (DMA), it receives the most recent data in the write-through method.
   • In the write-back method, the I/O device communicating through DMA may not receive the most recent data since it is only updated in the cache.
5. Writing Process:
   • Write-through is a process of simultaneously writing data to both the cache and main memory.
   • Write-back is a process where data is written to the cache, and when it is removed from the cache, it is first copied to the main memory.