Auxiliary LRU List for Asynchronous Data Replication
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Solution Overview
Problem
In asynchronous data replication environments, the inefficiency arises when primary storage systems destage and demote data elements before mirroring, requiring re-staging and mirroring the entire data element, which is less efficient than mirroring before destage and demotion, leading to suboptimal performance.
Innovation Solution
A method that monitors unmirrored data elements in the primary storage system's cache, using a regular LRU list to determine if data elements at the LRU end are unmirrored, and moves them to a transfer-pending LRU list to delay demotion, allowing for asynchronous mirroring without re-staging from backend storage drives.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the primary storage system uses a standard LRU cache demotion policy, then cache management is simple and fast, but unmirrored data elements may be demoted before mirroring occurs, requiring costly re-staging operations
Solution Approach 1:
The patent segments the single LRU cache into two distinct LRU lists: a first LRU list for data elements eligible for immediate demotion, and a second LRU list for unmirrored data elements that should be retained. This segmentation allows the system to apply different demotion policies to different types of data, preventing premature demotion of unmirrored elements while maintaining efficient cache management overall.
Solution Approach 2:
The patent introduces a migration mechanism that acts as an intermediary between the two LRU lists. When mirroring status changes, data elements are migrated between lists through a migration indicator and migration logic. This intermediary mechanism ensures that unmirrored data elements are properly identified and transferred to the retention list before demotion, resolving the contradiction between simple cache management and efficient data replication.
2Loss of time
If unmirrored data elements are retained in the cache longer, then mirroring efficiency improves by avoiding re-staging, but cache space is occupied longer by elements that may not be frequently accessed
Solution Approach 1:
The patent implements dynamic cache management where the retention period of unmirrored data elements is not fixed but adapts based on mirroring status. Elements in the second LRU list are retained dynamically until the mirroring operation completes, at which point they are migrated back to the first list for normal demotion. This dynamic approach balances the need to retain elements for efficient mirroring with the need to free cache space for active data.
Solution Approach 2:
The patent changes the demotion parameter (LRU evaluation) based on the mirroring status of data elements. By introducing a migration indicator that tracks whether elements are mirrored or unmirrored, the system applies different demotion parameters to different element types. This parameter change allows the cache to prioritize retention of unmirrored elements without permanently reducing overall cache utilization, as elements return to normal demotion scheduling after mirroring completes.
Data Source
AI summary
A method for improving asynchronous data replication between a primary storage system and a secondary storage system is disclosed. In one embodiment, such a method includes monitoring, in a cache of the primary storage system, unmirrored data elements needing to be mirrored, but that have not yet been mirrored, from the primary storage system to the secondary storage system. The method maintains a regular LRU list designating an order in which data elements are demoted from the cache. The method determines whether a data element at an LRU end of the regular LRU list is an unmirrored data element. In the event the data element at the LRU end of the regular LRU list is an unmirrored data element, the method moves the data element to a transfer-pending LRU list dedicated to unmirrored data elements in the cache. A corresponding system and computer program product are also disclosed.