Open NAND Block Sealing in Open-Channel SSDs
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Solution Overview
Problem
In open-channel SSDs, host-side FTL failures lead to open NAND blocks remaining unsealed for extended periods, increasing data loss risk due to lower noise immunity and inefficiencies in rebuilding the FTL mapping table.
Innovation Solution
A system where the SSD controller monitors the time an open superblock remains unsealed and seals it by writing a pseudo-random number sequence and logical block address directly to a free page, bypassing data-processing modules, to prevent data loss and enhance system efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the host-side FTL module is used to manage open NAND blocks, then the host can maintain the FTL mapping table, but open blocks may stay open for longer than desired when the host-side FTL crashes or experiences a failure, increasing the risk of data loss
Solution Approach 1:
The SSD controller performs self-service by autonomously detecting FTL failures and sealing open NAND blocks without requiring host intervention. The controller monitors the open block status and automatically initiates the sealing process by writing dummy data and updating the FTL mapping table, enabling the system to recover from FTL failures independently.
Solution Approach 2:
The system implements feedback mechanisms where the SSD controller continuously monitors the status of open NAND blocks and detects FTL failure conditions. Based on this feedback, the controller dynamically adjusts its behavior by initiating the sealing process when failures are detected, creating a closed-loop control system that responds to system state changes.
2Ease of repair
If metadata is read out from SSDs to rebuild the FTL mapping table in host memory, then the FTL can be recovered, but there exists a gap in time between FTL crash and subsequent rebuilding, increasing the risk of data loss
Solution Approach 1:
The SSD controller performs preliminary actions by maintaining monitoring of open NAND blocks even during normal operation. When an FTL failure is detected, the controller is already prepared to immediately seal open blocks without waiting for the host to initiate recovery, effectively performing the sealing action in advance of what would otherwise be a delayed host-driven recovery process.
3Reliability
If open NAND blocks are monitored and sealed by the host-side FTL, then data reliability is improved, but the device complexity increases due to additional monitoring and sealing operations
Solution Approach 1:
The SSD controller performs self-service by autonomously detecting FTL failures and sealing open NAND blocks without requiring host intervention. The controller monitors the open block status and automatically initiates the sealing process by writing dummy data and updating the FTL mapping table, enabling the system to recover from FTL failures independently.
Data Source
AI summary
One embodiment provides a system which facilitates data management. The system allocates a superblock of a storage device, wherein the superblock is in an open state. The system writes data to the superblock. The system monitors, by a controller of the storage device, an amount of time that the superblock remains in the open state. Responsive to detecting a failure associated with a flash translation layer (FTL) module, the system determines that the monitored amount of time exceeds a predetermined threshold, and seals, by the controller, the superblock by writing directly to a respective free page in the superblock while bypassing one or more data-processing modules.