Write-back (versus write-through)
Beginner
A cache that keeps changes to itself and only saves them to main memory when the block is thrown out.
Novice
Write-back caches update only the cache on a store and mark the line dirty; the line is written to the next level when evicted. Write-through caches send every store on immediately.
Expert
Write-back cuts traffic for repeated stores and pairs naturally with invalidation coherence (the M state). Write-through simplifies coherence and error recovery but needs a write buffer and much more bandwidth. Most CPU caches are write-back with write-allocate.