FlashCopy Cascade Reversal Using Fdisk Data Structures
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Reversing a FlashCopy process in cascaded implementations is complicated by the need for a disk to access all disks containing data grains, leading to reduced flexibility as intermediate images are lost when reversing the cascade.
Innovation Solution
The introduction of a data structure using fdisks to represent source or target disks in the cascade, allowing multiple representations of a VDisk across maps and cascades, enabling simultaneous map activation in opposite directions without losing intermediate images, and defining rules for IO command processing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If all target disks between source and target in the cascade are taken offline to reverse the FlashCopy process, then the reversal can be achieved, but the flexibility of the FlashCopy implementation is drastically restricted and intermediate images are lost
Solution Approach 1:
The patent segments the FlashCopy cascade into independent virtual disk (VDisk) units, each with its own primary and secondary fdisk representations. This segmentation allows individual VDisks to be reversed independently without affecting other VDisks in the cascade, enabling selective reversal while preserving intermediate images. Each VDisk can be manipulated separately through the fdisk mechanism, resolving the contradiction between achieving reversal and maintaining flexibility.
Solution Approach 2:
The patent introduces fdisk as an intermediary data structure that mediates between the physical disks and the FlashCopy cascade operations. The fdisk mechanism allows the system to track and manage multiple representations of VDisks, enabling reversal operations without directly manipulating the physical disk connections. This intermediary layer preserves intermediate images while enabling reversal, resolving the contradiction between these two requirements.
2Loss of information
If intermediate target disks are preserved during FlashCopy reversal, then image integrity is maintained, but the complexity of managing multiple disk representations increases
Solution Approach 1:
The patent creates copies of VDisk representations in the form of fdisk structures. Each VDisk can have multiple fdisk representations (primary and secondary) that track different states in the cascade. This copying mechanism allows the system to preserve intermediate images by maintaining multiple valid representations simultaneously, while the copying approach actually simplifies management compared to trying to track all disk relationships in a single complex structure.
Solution Approach 2:
The patent adds a new dimension to disk representation by introducing the fdisk layer above physical disks. Instead of managing disk relationships in a single dimensional space, the system creates a second dimensional layer (fdisk) that tracks cascade relationships. This dimensional change allows intermediate images to be preserved without increasing physical disk complexity, as the additional representation layer handles the complexity management.
3Stability of the object's composition
If a disk maintains access to all upstream disks containing data grains, then the disk can maintain its image, but the system flexibility is reduced due to complex inter-disk dependencies
Solution Approach 1:
The patent extracts the disk image maintenance logic from physical disk connections and relocates it to the fdisk data structure. Instead of requiring physical disks to maintain access to all upstream disks, the system extracts this functionality into the fdisk representation, which tracks the cascade relationships logically. This extraction allows disks to be physically independent while maintaining image integrity through the fdisk mechanism, resolving the contradiction between stability and flexibility.
Solution Approach 2:
The fdisk structure serves as an intermediary that mediates between physical disk independence and image integrity requirements. Rather than requiring direct physical access between upstream and downstream disks, the fdisk intermediary tracks the cascade relationships and maintains image consistency logically. This intermediary approach allows flexible physical configurations while preserving image integrity through logical tracking.
Data Source
AI summary
A technique for handling a FlashCopy® process includes receiving a FlashCopy® instruction for a source disk, performing a FlashCopy® point in time copy of the source disk on to a target disk, creating a map specifying the FlashCopy® point in time copy from the source disk to the target disk, creating a primary fdisk for the source disk, if one does not already exist, and creating a primary fdisk for the target disk, if one does not already exist, or, if one does already exist, converting the existing primary fdisk for the target disk into a secondary fdisk, and creating a new primary fdisk for the target disk.