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https://issues.apache.org/jira/browse/OAK-3348?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15190730#comment-15190730
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Michael Dürig commented on OAK-3348:
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Scrape this one. It turns out to add yet another layer of complexity and still
not fully getting hold of the back references. After much research I came up
with another approach. I'll share it in a moment.
> Cross gc sessions might introduce references to pre-compacted segments
> ----------------------------------------------------------------------
>
> Key: OAK-3348
> URL: https://issues.apache.org/jira/browse/OAK-3348
> Project: Jackrabbit Oak
> Issue Type: Improvement
> Components: segmentmk
> Reporter: Michael Dürig
> Assignee: Michael Dürig
> Labels: candidate_oak_1_0, candidate_oak_1_2, cleanup,
> compaction, gc
> Fix For: 1.6
>
> Attachments: OAK-3348-1.patch, OAK-3348-2.patch, OAK-3348.patch,
> cross-gc-refs.pdf, image.png
>
>
> 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.
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