Tail Latency Aware Foreground Garbage Collection for SSDs
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Solution Overview
Problem
Modern computer database systems face significant challenges with tail latency in Solid State Drives (SSDs), particularly due to garbage collection operations, which lead to poor performance and increased response times, especially when using log structured merge (LSM) trees.
Innovation Solution
Implementing a decision table within the SSD's flash translation layer to manage garbage collection by introducing additional threshold points for triggering foreground garbage collection, allowing for balanced input/output latency and distributing erase operations across multiple write commands to reduce worst-case scenarios.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If foreground garbage collection is performed to manage SSD storage, then storage reliability is improved, but query response time deteriorates due to operation delays
Solution Approach 1:
The patent performs garbage collection operations in advance before they are critically needed. By monitoring free block thresholds and proactively executing garbage collection when thresholds are approached, the system prepares storage resources ahead of time, preventing query delays while maintaining storage reliability.
Solution Approach 2:
The patent dynamically adjusts garbage collection behavior based on real-time system conditions. It monitors multiple thresholds (free blocks, query latency, write amplification) and adapts the timing and intensity of garbage collection operations accordingly, balancing storage management needs with query performance requirements.
2Quantity of substance
If garbage collection operations are executed to free storage blocks, then storage capacity is improved, but operational performance deteriorates due to concurrent query delays
Solution Approach 1:
The patent performs partial garbage collection operations rather than complete cleanup. By collecting garbage on only a portion of blocks at a time and stopping when threshold targets are reached, it frees sufficient storage capacity without executing full garbage collection that would cause excessive performance degradation.
Solution Approach 2:
The patent implements periodic garbage collection based on monitored thresholds rather than continuous operation. It checks free block counts and performance metrics at intervals, executing garbage collection only when thresholds indicate it is appropriate, thus balancing capacity management with performance maintenance.
3Device complexity
If traditional garbage collection thresholds are used to trigger operations, then storage management is simplified, but tail latency increases due to inadequate threshold granularity
Solution Approach 1:
The patent segments the garbage collection trigger mechanism into multiple threshold levels (e.g., different free block thresholds for different collection intensities). This segmentation allows nuanced control over when and how garbage collection occurs, reducing tail latency by preventing both premature and delayed collection while maintaining manageable complexity through structured threshold definitions.
Data Source
AI summary
A Solid State Drive (SSD) is disclosed. The SSD may include a host interface logic to receive a write command from a host and flash memory to store data. The SSD may also include an SSD controller, which may include storage for a just-in-time threshold and a tail latency threshold flash translation layer. The flash translation layer may invoke a just-in-time garbage collection strategy when the number of free pages on the SSD is less than the just-in-time threshold, and a tail latency-aware garbage collection strategy when the number of free pages is less than the tail latency threshold. The tail latency-aware garbage collection strategy may pair the write command with a garbage collection command.