Write back cache is a popular caching technique used in most modern processor architectures since Intel introduced the 80486. Its unique feature is the ability to copy data to higher cache levels and back to main memory or storage.
Understanding Write Back Cache
Write back cache, also known as write cache, write behind cache, write deferred, or copy back cache, is designed to optimize write operations between the cache and the data source, typically Random Access Memory (RAM). It temporarily holds data until it can be permanently saved or modified.
In this method, new data is written to the cache when changes occur, but it's only written to the main memory under specific conditions or at set intervals. When a cache data location is updated, it's considered "fresh," while the corresponding main memory data becomes "stale."
The write back method significantly improves system speed by writing data only to the cache initially. However, this approach carries a risk of data loss during system crashes or adverse events. Despite this drawback, write back cache is preferred in applications where occasional data loss is tolerable.
For more critical applications like medical device control or banking, an alternative method called write through is often used. This method eliminates data loss risk by writing updates to both cache and main memory simultaneously.
Approaches to Write Back Cache
The write back cache process involves several steps:
- Data is initially written only to the cache.
- Writing to the backing store is postponed until modified content needs replacement.
- The cache tracks which locations have been written over.
- These locations are marked as "dirty" for easy identification when writing to the backing store later.
This approach, sometimes called "lazy write," can result in two memory accesses for read misses:
- Writing swapped data from cache back to storage
- Retrieving the required data
Two main approaches handle write misses:
- Write allocate (fetch on write): Loads missed-write location data into the cache, followed by a write-hit operation.
- No-write allocate (write around): Writes missed-write location data directly to the backing store, bypassing the cache.
Use Cases
Write back cache is commonly used by developers to manage peak workloads and by relational database storage engines. However, the risk of data loss during cache failure remains a concern.
Writing Strategies
Understanding write back cache strategies requires knowledge of caching basics. A cache stores frequently used data for quick access, acting as a buffer between the CPU and RAM.
When the processor needs to write data, it first checks if the desired address is in the cache (a write hit). If so, the cache value is updated, avoiding costly main memory access.
Write back cache uses dirty bits to indicate whether cached data has been modified. Clean bits don't require writing to memory, reducing write operations. However, modified data may be lost in case of cache failure, system crashes, or power outages.
Advantages of Write Back Cache
Write back cache offers several benefits:
- Low latency and high throughput for write-intensive applications
- Effective handling of mixed workloads when combined with read through
- Resilience to short database downtimes
- Potential reduction in overall database writes through coalescing or batching
The main disadvantage is the risk of data loss if the cache fails before data is moved to the backing store.
Conclusion
Write back cache is a powerful technique for optimizing data storage and retrieval. When implementing a caching strategy, it's crucial to evaluate your goals, consider the pros and cons of each approach, and understand your data access patterns. This will help you choose the best strategy or combination of strategies for your specific needs.
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For a deeper understanding of caching strategies, be sure to research other policies and compare them to write through cache. You may also want to explore the differences between write back and write through cache to make the most informed decision for your system architecture.