unable to understand write policy in Cache memory

Question

I am studying write policies in cache memory ( for first time ). I am able to understand the 'write-through' but i am not able to understand 'write back' and the problems associated with it . Please can anyone explain me the 'write-back' concept clearly .

Answer 1

In a write-through cache every store operation from the processor simultaneously writes the new data into the cache-line and into the backing store (the next larger cache or the main memory).

In a write-back cache a store operation from the processor modifies only the cache-line, so the cache-line contains the most recent data while the data in the backing store is stale. The write to the backing store happens only when the cache line in question gets replaced because it is needed for some other line at a different address.

The pictures on the wikipedia page about caching are okay.

In a write-through cache every line is in only one of two states: valid or invalid. Thus when you need to fetch a line that is not in the cache you just throw out a line to make space for it

In a write-back cache every line can be in one of three states: valid, invalid, or dirty. When a read-miss occurs and you need to throw out a line to make space for the new line, the line you need to throw out may be dirty. If the line you need to throw out is dirty, you need to write it to the backing store before you can bring in the new line. This means that at the time you are processing a read-miss you may need to do two operations with the backing store instead of one.

If the same cache lines get written many times then write-back caches can dramatically reduce the number of times you need to send writes to the backing store. You just keep making modifications to the dirty cache line until the line needs to be replaced and then write back only the last values written to each location in the line.