ZNS SSD Super Block Allocation via Controlled Imbalance

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Solution Overview

Problem

Data storage devices with zoned namespace (ZNS) architecture face performance degradation due to super device imbalance caused by zone de-allocation and re-allocation, leading to inefficient write performance.

Innovation Solution

A controller is configured to set a free space threshold, identify super devices above this threshold, move cold zones to a designated super device, and allocate new super blocks across all super devices while excluding the device with excess free space, ensuring even distribution and maintaining a minimum free space level.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Productivity

If zones are randomly allocated to super devices to maximize write performance, then write performance is improved, but super device imbalance occurs causing performance degradation

Engineering Contradiction:

Improvewrite performanceVSAvoidsuper device balance

Core Design Contradiction:

ProductivityVSStability of the object's composition

Solution Approach 1:

The system changes the allocation parameter from random distribution to controlled imbalance. The controller intentionally creates an imbalance by consolidating cold zones into a designated super device while maintaining balanced allocation of new super blocks through round-robin distribution among active super devices. This parameter change resolves the contradiction by allowing one super device to have excess free space while ensuring fair distribution of allocation opportunities.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The system performs preliminary consolidation of cold zones into a designated super device before allocating new super blocks. This preliminary action ensures that when new zones are allocated, the controller can use round-robin distribution among active super devices, preventing any single super device from being overloaded. The preliminary consolidation maintains super device balance while allowing optimal allocation of new blocks.

Inventive Principle:

Principle #10Preliminary action

2Reliability

If cold zones are consolidated into one super device to maintain free space threshold, then free space distribution is improved, but allocation fairness deteriorates

Engineering Contradiction:

Improvefree space threshold maintenanceVSAvoidallocation fairness

Core Design Contradiction:

ReliabilityVSAdaptability or versatility

Solution Approach 1:

The system applies different allocation strategies to different types of zones. Cold zones are consolidated into a designated super device to maintain free space threshold and improve reliability. Meanwhile, new super blocks are allocated using round-robin distribution among active super devices to ensure allocation fairness. This local differentiation resolves the contradiction by treating cold and new zones differently based on their specific requirements.

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The system segments zone allocation into two distinct processes: consolidation of cold zones and allocation of new super blocks. By segmenting these operations, the controller can optimize each process independently - consolidating cold zones to maintain free space thresholds while using round-robin allocation for new blocks to ensure fairness. This segmentation allows both reliability and adaptability to be satisfied simultaneously.

Inventive Principle:

Principle #1Segmentation

3Adaptability or versatility

If round robin allocation is used for new super blocks, then allocation fairness is improved, but free space threshold control deteriorates

Engineering Contradiction:

Improveallocation fairnessVSAvoidfree space threshold control

Core Design Contradiction:

Adaptability or versatilityVSReliability

Solution Approach 1:

The system performs preliminary consolidation of cold zones into a designated super device before executing round-robin allocation of new super blocks. This preliminary action ensures that the free space distribution is optimized in advance, allowing the subsequent round-robin allocation to maintain fairness while respecting the free space threshold. The preliminary consolidation prevents any single super device from being overloaded during the fair distribution phase.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The system introduces asymmetry in the allocation process by designating a specific super device for cold zone consolidation while maintaining symmetric round-robin allocation for new super blocks. This asymmetric design allows one super device to absorb excess cold zones, maintaining free space thresholds, while the symmetric round-robin allocation ensures fair distribution of new blocks among all active super devices. The asymmetry resolves the contradiction by accommodating both threshold control and fairness requirements.

Inventive Principle:

Principle #4Asymmetry

Data Source

PatentUS11640254B2Controlled imbalance in super block allocation in ZNS SSD

Publication Date: 2023.05.02 SANDISK TECHNOLOGIES LLC

AI summary

A data storage device includes a memory device and a controller coupled to the memory device. The memory device includes a plurality of super devices. The controller is configured to set a free space threshold value for amount of free space that each super device of the plurality of super devices can have, determine that at least one super device of the plurality of super devices is at or above the free space threshold, determine that cold zones are disposed in more than one super device of the plurality of super devices, move data from the cold zones to a first super device of the plurality of super devices wherein after moving the data, all super devices are below the free space threshold, and allocate all new super blocks among the plurality of super devices without allocating any new super blocks to the first super device.