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https://issues.apache.org/jira/browse/HDDS-14982?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Wei-Chiu Chuang reassigned HDDS-14982:
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Assignee: Wei-Chiu Chuang
> [Docs] Snapshot deletion lifecycle
> ----------------------------------
>
> Key: HDDS-14982
> URL: https://issues.apache.org/jira/browse/HDDS-14982
> Project: Apache Ozone
> Issue Type: Improvement
> Components: documentation, Snapshot
> Reporter: Wei-Chiu Chuang
> Assignee: Wei-Chiu Chuang
> Priority: Major
> Labels: pull-request-available
> Attachments: Gemini_Generated_Image_bbm7jtbbm7jtbbm7.png
>
>
> Add a system internal doc on Snapshot deletion lifecycle:
> 1. Request Phase: OMSnapshotDeleteRequest
> * User Action: User runs ozone sh snapshot delete.
> * Operation: This request only marks the snapshot's status as
> SNAPSHOT_DELETED in the SnapshotInfoTable and sets the deletionTime.
> * Result: It does not adjust the chain or neighbors yet. The snapshot is now
> a candidate for the background service.
> 2. Reclamation Phase: SnapshotDeletingService
> * Operation: This background service identifies snapshots with the
> SNAPSHOT_DELETED status.
> * Key Moving: It moves entries from the deleted snapshot's deletedTable,
> deletedDirTable, and renamedTable to either:
> * The next active snapshot in the chain.
> * The Active Object Store (AOS) (the main bucket) if no subsequent snapshot
> exists.
> * Settling: Once all keys/dirs for that snapshot have been moved (i.e., the
> snapshot's deleted tables are empty), the service collects the snapshot's DB
> key into a "purge list".
> 3. Finalization Phase: OMSnapshotPurgeRequest
> * Trigger: Triggered by the SnapshotDeletingService once a snapshot is
> "empty" (all keys reclaimed).
> * Chain Adjustment: This is where the actual chain surgery happens. It
> updates the previousSnapshotId of the next snapshot in the chain to point to
> the
> deleted snapshot's predecessor, effectively "stitching" the chain back
> together.
> * Deep Clean Flag: It sets setDeepClean(false) on the next snapshots. This
> tells the KeyDeletingService that it can now perform a "deeper" cleanup
> because a snapshot in the middle of the chain has been removed, potentially
> uncovering more keys to delete.
> * Removal: Finally, it removes the snapshot from the SnapshotInfoTable cache
> and deletes the checkpoint directory on disk.
> 4. Persistence: OMSnapshotPurgeResponse
> * Operation: The Ozone Manager double-buffer flushes the transaction.
> * Result: The snapshot record is physically deleted from the RocksDB
> snapshotInfoTable.
> Summary of roles:
> * OMSnapshotDeleteRequest: The Labeler (marks for death).
> * SnapshotDeletingService: The Reclaimer (moves the data).
> * OMSnapshotPurgeRequest: The Surgeon & Janitor (re-links the chain and
> deletes the record).
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