Host-Managed Garbage Collection for NAND Flash Write Amplification
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Solution Overview
Problem
Flash memory systems experience write amplification due to the inability to overwrite pages directly, leading to multiple invalid versions of data, increased device space consumption, and reduced efficiency in garbage collection, which affects both flash storage and database applications.
Innovation Solution
Implementing a simplified SSD controller with a logical-to-physical block mapping and a free block pool that manages write operations by aggregating writes to fill entire blocks and eliminating garbage collection from the SSD controller, allowing for efficient block reuse and reduced write amplification.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If flash page overwrite operation is performed by writing a new flash page and invalidating the old flash page, then the write operation can be completed, but multiple invalid copies of the same page increase device space consumption and write amplification
Solution Approach 1:
The patent extracts the garbage collection function from the SSD controller and relocates it to the host system. This allows the SSD controller to focus solely on managing valid data blocks, while the host application is responsible for tracking and eliminating invalid page copies, thereby reducing device space consumption without compromising write operation completion
Solution Approach 2:
The patent introduces a mapping structure that acts as an intermediary between the host and the flash memory. This mapping tracks the validity of each flash page, allowing the host to identify and eliminate invalid copies before they consume device space, thus resolving the contradiction between completing writes and reducing space consumption
2Quantity of substance
If device garbage collection is performed frequently to reclaim invalid pages, then device space is recovered, but the GC process interferes with device throughput
Solution Approach 1:
The patent removes the garbage collection function from the SSD controller entirely, extracting this burden from the device. By relocating GC responsibilities to the host application, the device can maintain high throughput while space management is handled by the host, which has better visibility into data validity and can perform space reclamation without interfering with device operations
Solution Approach 2:
The patent enables the host application to perform self-service garbage collection by providing it with the necessary mapping information. The host can independently identify and eliminate invalid page copies without requiring the device to interrupt its normal operations, thus maintaining device throughput while recovering space
3Quantity of substance
If blocks with many invalid pages are kept for potential reuse, then device space is preserved, but valid pages cannot be erased and blocks become unsuitable for reuse
Solution Approach 1:
The patent implements preliminary action by having the host application identify and eliminate invalid page copies before blocks become saturated with invalid pages. By proactively managing invalid page removal through the mapping structure, the system ensures blocks remain suitable for reuse while preserving device space, preventing the contradiction from arising in the first place
Data Source
AI summary
A storage device, such as a NAND flash device, avoids the need for garbage collection. An application executing on a host system tracks data objects that are marked as invalid and maintains an association between data objects and logical blocks, each logical block corresponding to a physical block of memory in the NAND flash device. Upon determining that the logical block contains no valid objects, the application instructs an SSD to trim the physical block of memory corresponding to the logical block. The application also aggregates write commands until a full block of data is ready to be written, at which point the application transmits a write command to the SSD.