Flash Translation Layer Zone Allocation for SSD Die Capacity Balance
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Solution Overview
Problem
In Zoned Namespace (ZNS) solid-state drives, each NAND die has a different capacity due to varying numbers of bad blocks, leading to uneven die fullness and reduced system performance as dies reach capacity at different times, necessitating a method to allocate zones to prevent dies from reaching full capacity.
Innovation Solution
The solution involves a flash translation layer that classifies zones as 'hot' or 'cold' based on reset rates and allocates 'hot' zones to dies with fewer free blocks and 'cold' zones to dies with more free blocks, preventing dies from reaching full capacity by maintaining die capacity based on host behavior.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If zones are allocated to dies without considering zone reset rates, then allocation is simple and fast, but dies reach full capacity at different times reducing system performance
Solution Approach 1:
The patent applies local quality by differentiating zone allocation strategies based on zone characteristics. Hot zones (high reset rate) are allocated to dies with fewer free blocks, while cold zones (low reset rate) are allocated to dies with more free blocks. This localized differentiation in allocation policy prevents any single die from reaching full capacity, thereby maintaining system performance without requiring complex real-time monitoring.
Solution Approach 2:
The patent implements preliminary action by pre-classifying zones into hot, cold, and normal categories based on their reset rates before allocation. The flash translation layer maintains zone reset rate information and pre-determines which die should receive which zone type, preventing the problem of dies reaching capacity before the allocation decision is made. This proactive approach eliminates the need for complex real-time adjustments.
2Reliability
If all zones are treated equally in allocation, then allocation process is simple, but dies with different bad block counts reach full capacity at different times
Solution Approach 1:
The patent applies local quality by tailoring the allocation strategy to the specific characteristics of each zone. Instead of treating all zones uniformly, the system identifies hot zones (frequently reset) and cold zones (rarely reset) and allocates them to different dies based on their current block availability. This ensures that dies with fewer free blocks receive hot zones that will be quickly released, while dies with more free blocks receive cold zones that will occupy space longer, thereby balancing die capacity utilization.
Solution Approach 2:
The patent changes the allocation parameter from uniform distribution to distribution based on zone reset rate characteristics. By introducing the zone reset rate as a key parameter, the system dynamically adjusts which die receives which zone, transforming the allocation process from a simple round-robin or random approach to a more sophisticated method that considers both zone behavior and die status, thereby improving reliability without excessive complexity.
3Productivity
If hot zones are allocated to dies with most free blocks, then dies may reach full capacity faster, but cold zones allocated to dies with fewest free blocks would balance load
Solution Approach 1:
The patent applies local quality by inverting the intuitive allocation approach. Instead of allocating hot zones to dies with most free blocks, the system allocates hot zones to dies with fewest free blocks and cold zones to dies with most free blocks. This localized differentiation ensures that dies are prevented from reaching full capacity because hot zones are placed on dies that will quickly release blocks through frequent resets, while cold zones are placed on dies with capacity to accommodate longer-term storage.
Solution Approach 2:
The patent applies inversion by reversing the conventional wisdom of resource allocation. Typically, one might allocate actively used resources (hot zones) to systems with most available capacity. However, this patent inverts that logic by allocating hot zones to dies with fewest free blocks, counting on the frequent resets to quickly free up space. This inverted approach prevents die fullness issues while maintaining parallelism, as the system bets on the high turnover rate of hot zones to prevent capacity problems.
Data Source
AI summary
A memory controller includes, in one implementation, a host interface, a memory interface, and a flash translation layer (FTL). The FTL is configured to receive a request from a host device to store data in a zone of a solid-state memory. The FTL is also configured to determine a zone reset rate classification as one of a hot classification, a cold classification, and a normal classification. The FTL is further configured to allocate the zone to a memory die with the fewest free die blocks when the zone reset rate classification is the hot classification. The FTL is also configured to allocate the zone to a memory die with the most free die blocks when the zone reset rate classification is the cold classification. The FTL is further configured to send the data to the memory die for storage therein.