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https://issues.apache.org/jira/browse/OAK-3348?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Michael Dürig updated OAK-3348:
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Attachment: cross-gc-refs.pdf
Attaching a visualisation of the cross gc references: the big circle shows the
segments before compaction. The small circle shows the segments created by
compaction and the medium circle shows the segments written after compaction.
Red edges show references out of a segment and blue ones references into a
segment. The trouble spot here are the references from the medium circle to the
big circle. Those shouldn't be there.
> 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: cleanup, compaction, gc
> Fix For: 1.4
>
> 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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