VM Disk Synchronization via Write Mirroring and Flush Tracking

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Solution Overview

Problem

In virtual machine systems, the migration of storage from a source disk to a destination disk is often incomplete due to the virtual machine manager exiting before confirmation of successful migration, leading to potential data asymmetry and instability, especially in cases of power loss or system termination.

Innovation Solution

The virtual machine monitor employs a synchronization phase for mirroring disk writes and a steady state phase for synchronous disk flushes, ensuring that the contents of the source and destination disks are identical before switching, and tracks values like source-generation count, writes-mirrored, flush-request, and writes-flushed to manage the migration process.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Productivity

If the virtual machine manager exits before receiving confirmation from the monitor that migration was successfully completed, then the system can proceed with restart, but data consistency between source and destination disks cannot be guaranteed

Engineering Contradiction:

Improvesystem restart speedVSAvoiddata consistency

Core Design Contradiction:

ProductivityVSReliability

Solution Approach 1:

The patent applies preliminary action by having the virtual machine monitor perform migration verification and generate completion notifications before the virtual machine manager exits. The monitor tracks migration status and ensures data consistency is confirmed prior to allowing system restart, thus preventing data asymmetry issues.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The patent implements feedback mechanisms where the virtual machine monitor continuously reports migration status to the virtual machine manager. Completion notifications and status updates provide real-time feedback, allowing the manager to wait for confirmation before exiting, ensuring data consistency is verified.

Inventive Principle:

Principle #23Feedback

2Reliability

If the virtual machine monitor handles all disk operations synchronously, then data consistency is maintained, but migration performance and speed are reduced

Engineering Contradiction:

Improvedata consistencyVSAvoidmigration speed

Core Design Contradiction:

ReliabilityVSProductivity

Solution Approach 1:

The patent segments disk operations into two categories: migration-related operations (handled synchronously to ensure consistency) and normal I/O operations (handled asynchronously for performance). This segmentation allows the system to maintain data consistency during migration while preserving overall system performance.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent applies partial synchronous action by making only the critical migration-related disk operations synchronous, while allowing other operations to proceed asynchronously. This partial application of synchronous handling maintains data consistency for migration data without unnecessarily slowing down the entire system.

Inventive Principle:

Principle #16Partial or excessive action

3Reliability

If the virtual machine manager waits for confirmation before exiting, then data consistency is ensured, but system restart time increases

Engineering Contradiction:

Improvemigration completion assuranceVSAvoidrestart time

Core Design Contradiction:

ReliabilityVSLoss of time

Solution Approach 1:

The virtual machine monitor performs migration verification and prepares completion notifications in advance, before the manager needs to make exit decisions. This preliminary verification reduces the waiting time required for confirmation while ensuring data consistency is actually achieved.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The migration verification process runs continuously in the background without interrupting normal system operations. The manager can monitor progress without idle waiting, allowing the system to maintain productivity while ensuring migration completion before restart.

Inventive Principle:

Principle #20Continuity of useful action

Data Source

PatentUS10162873B2Synchronization of physical disks

Publication Date: 2018.12.25 RED HAT INC

AI summary

In a process for migrating a virtual machine's storage from a source disk to a destination disk, during a steady state (i.e., wherein the contents of the virtual machine stored on the source disk and the destination disk are equal), a virtual machine monitor receives a set of write requests from a guest operating system (“guest”) of the virtual machine, provides confirmation of the completion of the set of writes to the source disk, and asynchronously replicates the set of write requests to the destination disk. Upon receipt of a flush request from the guest, the virtual machine monitor confirms completion of the flushing of the destination disk following replication of the write requests to the destination disk. Upon receipt of a switch request from a virtual machine manager, the virtual machine monitor switches the virtual machine to the destination disk and issues subsequent write requests to the destination disk.