Unified Write Filter for Persistent Memory State Recovery
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Solution Overview
Problem
Existing computing devices in public settings, such as kiosks and shared PCs, lack the ability to revert to a known state after user modifications, leading to instability and wear on storage media.
Innovation Solution
Implementing a Unified Write Filter (UWF) that redirects writes to a virtual overlay, allowing the device to discard changes upon reboot, thereby maintaining a known good state and reducing wear on storage media.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a write filter is implemented to redirect writes to an overlay, then the device can revert to a known good state upon reboot, but the overlay may fill up with redirected writes causing performance degradation
Solution Approach 1:
The patent applies local quality by differentiating the treatment of different disk blocks based on their allocation status. Instead of uniformly redirecting all writes to the overlay, the system redirects only writes to allocated blocks while allowing free blocks to be written directly to the protected volume. This selective approach reduces overlay occupancy and prevents performance degradation while maintaining the ability to revert to the known good state.
2Reliability
If all writes are redirected to the overlay, then the protected volume remains unchanged upon reboot, but storage space is wasted and wear is increased
Solution Approach 1:
The system implements local quality by selectively redirecting writes based on block allocation status. Writes to free blocks are allowed to go directly to the protected volume without overlay redirection, while only writes to allocated blocks are redirected to the overlay. This differentiation eliminates unnecessary overlay writes and reduces storage space consumption while maintaining protection of the known good state.
Solution Approach 2:
The patent converts the potential harm of free blocks being overwritten into a benefit by allowing such writes to proceed directly to the protected volume. This approach utilizes the free space in the protected volume efficiently, reducing the need for overlay redirection and minimizing wear on the overlay storage medium.
3Reliability
If the overlay is stored in RAM, then changes are lost upon reboot, but if stored in persistent memory, then changes persist requiring manual rollback
Solution Approach 1:
The system implements self-service by automatically detecting and redirecting writes to the overlay, which is then automatically discarded upon reboot to restore the known good state. This automatic behavior eliminates the need for manual rollback operations, as the system handles the state restoration autonomously through the write filter mechanism.
Data Source
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Figure 1B
Figure 2
AI summary
In a first area of a persistent memory, state data is stored that defines a known good state that is operable to launch a computing device to the known good state in response to a reboot. Allocation information indicative of free and allocated portions of the persistent memory is accessed. In response to a write request to the first area of persistent memory, when the write request is to an allocated portion of the persistent memory, the requested write is redirected to a second area of the persistent memory. A record of redirected writes to the second area of persistent memory is updated. When the write request is to a free portion of the persistent memory, the write request is allowed to be executed in the first area of persistent memory. The record of redirected writes is updated with the writes to the second area of persistent memory.