SSD Cache System Sequential Write Wear Reduction
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Solution Overview
Problem
Flash memory in solid state drives (SSDs) faces limitations such as inefficient rewriting due to block erasure requirements and degradation after a number of program-erase cycles, affecting data integrity and storage performance.
Innovation Solution
A cache system that combines a solid state drive (SSD) and random access memory (RAM), where data is written sequentially to the SSD and non-sequentially to the RAM, with a cache control device managing data allocation and invalidation, and discarding oldest chunks in the SSD when free space is low to maintain performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If data is rewritten in flash memory by erasing entire blocks, then data can be updated, but storage efficiency deteriorates because many kilobytes of data must be erased to rewrite a single byte
Solution Approach 1:
The patent segments the flash memory into multiple blocks and introduces a separate log buffer area. Instead of erasing entire data blocks to update data, the system segments the update operation into: (1) writing new data to the log buffer, (2) marking the old block as invalid, and (3) later reclaiming space through garbage collection. This segmentation allows byte-level updates without erasing entire blocks, resolving the contradiction between data update capability and storage efficiency.
2Productivity
If flash memory undergoes multiple program-erase cycles, then data can be rewritten, but reliability deteriorates as wear deteriorates the integrity of the flash device
Solution Approach 1:
The patent implements a copy-on-write mechanism where new data is written to a log buffer (a copy) before the original data is invalidated. The system maintains multiple copies of data across different blocks and uses copy reconciliation to restore original data when needed. This copying approach allows data updates without repeatedly erasing and rewriting the same blocks, thereby maintaining reliability while enabling continuous data rewriting.
Solution Approach 2:
The system discards invalid block references and recovers space through garbage collection. When blocks become invalid due to updates, the system doesn't immediately erase them but marks them for later recovery. The garbage collection process periodically erases these marked blocks and redistributes their space. This approach separates the frequent update operations from the less frequent erasure operations, reducing P/E cycles on active data blocks and improving reliability.
3Duration of action of stationary object
If sequential writing is used in SSD to reduce wear, then flash memory longevity is improved, but data allocation complexity increases
Solution Approach 1:
The patent introduces a log buffer as an intermediary between the host system and the flash memory blocks. The log buffer absorbs write operations and manages the complexity of sequential allocation and garbage collection. Instead of directly managing block allocation for every write operation, the system uses the log buffer as a mediator that handles sequential writes and coordinates with the block management system. This intermediary layer simplifies the allocation logic while maintaining sequential writing benefits for longevity.
Data Source
AI summary
A cache system for a storage device includes a solid state drive (SSD), a random access memory (RAM), and a cache control device. The cache control device is configured to: retrieve data from the storage device in response to a request to read data from the storage device, store at least some of the data in one or both of (i) the SSD and (ii) the RAM, when storing the at least some of the data to the RAM, write to the RAM non-sequentially with respect to a memory space of the RAM, and when storing the at least some of the data in the SSD, write to the SSD sequentially with respect to a memory space of the SSD. The cache control device comprises an SSD interface device configured to allocate memory for storing data in the SSD sequentially with respect to the memory space of the SSD.