Target-less Snapshot Creation Using Sequence Number Tracking
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Solution Overview
Problem
Previous data storage systems face significant challenges in creating target-less snapshots of a logical volume after it has been linked to a target-less snapshot, as all tracks of the target logical volume must be defined with data from the target-less snapshot, which can take a lengthy time, leading to potential snapshot creation failures due to expired timers or host requests.
Innovation Solution
The technology maintains a target device sequence number and target track sequence numbers for the target logical volume, allowing write operations to proceed by moving data from a target-less snapshot to the target logical volume and updating sequence numbers, enabling snapshot creation before all tracks are defined, and also uses source device and source track sequence numbers for data replication and storage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If all tracks of the target logical volume are defined with data from the target-less snapshot before creating a snapshot, then the snapshot creation is reliable, but the time required for snapshot creation increases significantly
Solution Approach 1:
The system performs preliminary actions by maintaining sequence numbers that track the state of track definitions before snapshot creation is requested. This allows the snapshot to be created immediately when needed, rather than waiting for all tracks to be pre-defined, thus reducing snapshot creation time while maintaining reliability through sequence number verification.
Solution Approach 2:
The system dynamically tracks the definition status of tracks using sequence numbers that are updated as tracks are defined. This dynamic tracking allows the snapshot creation process to proceed as soon as the necessary tracks are ready, rather than requiring all tracks to be statically pre-defined, thereby reducing the time loss without compromising reliability.
2Stability of the object's composition
If all tracks of the target logical volume are defined with data from the target-less snapshot, then data consistency is ensured, but the complexity of the snapshot creation process increases
Solution Approach 1:
The system changes the parameter tracking approach from monitoring the actual content of each track to monitoring sequence numbers that represent the state of track definitions. This parameter transformation simplifies the snapshot creation process by providing a straightforward numerical comparison method to verify data consistency, rather than requiring complex analysis of track contents.
Solution Approach 2:
The system uses sequence numbers as simplified copies or representations of the actual track definition states. Instead of directly managing and comparing the complex data structures of track definitions, the system copies the essential state information into sequence numbers, which can be easily compared and managed, thereby reducing process complexity while ensuring data consistency.
3Reliability
If the target logical volume is completely defined before snapshot creation, then snapshot integrity is maintained, but the productivity of the storage system decreases
Solution Approach 1:
The system prepares sequence number tracking infrastructure in advance, but does not require complete track definition before snapshot creation. This preliminary preparation of the tracking mechanism allows snapshots to be created immediately when needed, maintaining integrity through sequence number verification while avoiding the productivity loss associated with waiting for complete track definition.
Solution Approach 2:
The sequence number system automatically tracks and verifies track definition states without requiring manual intervention or complex validation processes. This self-service mechanism maintains snapshot integrity through automatic sequence number comparison, thereby preserving storage system productivity by eliminating time-consuming manual verification steps.
Data Source
AI summary
To allow prompt creation of a target-less snapshot of a target logical volume, after the target logical volume has been linked to a target-less snapshot to allow host access to the target-less snapshot, and prior to all the tracks of the target logical volume being defined with regard to data from the target-less snapshot, a target device sequence number is maintained for the target logical volume, and incremented each time a target-less snapshot is linked to the target logical volume. A target track sequence number is maintained for each track of the target logical volume, and updated each time target-less snapshot data is moved to the corresponding track. The target device sequence number and target track sequence numbers are compared prior to performing host I/O operations directed to the target logical volume to ensure data correctness on a track by track basis.