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https://issues.apache.org/jira/browse/OAK-3348?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14944704#comment-14944704
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Alex Parvulescu commented on OAK-3348:
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bq. Running SegmentCompactionIT with 8 concurrent writers comparing
UseCompactionLock == false vs. UseCompactionLock == true shows the former (in
red) to be way less efficient in terms of repository growth (GB on Y axis).
I can certainly attest to the fact that not using the lock will introduce a lot
of back references to old generations of segments. My local tests showed that a
generation 6 compaction would still reference stuff from generation 3, even 2
sometimes so there's definitely something happening.
> 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: Sub-task
> Components: segmentmk
> Reporter: Michael Dürig
> Assignee: Michael Dürig
> Labels: cleanup, compaction, gc
> Fix For: 1.3.9
>
> Attachments: OAK-3348-1.patch, OAK-3348-2.patch, OAK-3348.patch,
> 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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