[OAK-4256] CLONE - Cross gc sessions might introduce references to pre-compacted segments

• Type: Improvement
• Status: Resolved
• Priority: Critical
• Resolution: Won't Fix
• Component/s: segmentmk
• Labels: cleanup compaction gc
• Epic Link: SegmentMK revision GC
• Assignee: Michael Dürig
• Reporter: Michael Dürig
• Created: 20/Apr/16 12:47
• Updated: 20/Apr/16 12:47
• Resolved: 20/Apr/16 12:47

Description

I suspect that certain write operations during compaction can cause references from compacted segments to pre-compacted ones. This would effectively prevent the pre-compacted segments from getting evicted in subsequent cleanup phases.

The scenario is as follows:

• A session is opened and a lot of content is written to it such that the update limit is exceeded. This causes the changes to be written to disk.
• Revision gc runs causing a new, compacted root node state to be written to disk.
• The session saves its changes. This causes rebasing of its changes onto the current root (the compacted one). At this point any node that has been added will be added again in the sub-tree rooted at the current root. Such nodes however might have been written to disk before revision gc ran and might thus be contained in pre-compacted segments. As I suspect the node-add operation in the rebasing process not to create a deep copy of such nodes but to rather create a reference to them, a reference to a pre-compacted segment is introduced here.

Going forward we need to validate above hypothesis, assess its impact if necessary come up with a solution.

Issue Links

• blocks: OAK-4054 FileStore.containsSegment returns alway true (almost)
• blocks: OAK-4121 CompactionMap#get not transitive across compaction map generations
• blocks: OAK-4255 CLONE - FileStore.containsSegment returns alway true (almost)
• blocks: OAK-3668 Potential test failure: CompactionAndCleanupIT#testMixedSegments
• blocks: OAK-4243 Oak Segment Tar Module
• blocks: OAK-4015 Expedite commits from the compactor
• is a clone of: OAK-3348 Cross gc sessions might introduce references to pre-compacted segments
• is blocked by: OAK-3864 Filestore cleanup removes referenced segments
• is blocked by: OAK-3550 Add meta data to segments
• is blocked by: OAK-3560 Tooling for writing segment graphs to a file