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https://issues.apache.org/jira/browse/OAK-5790?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15936387#comment-15936387
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Michael Dürig commented on OAK-5790:
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Initial WIP implementation:
https://github.com/mduerig/jackrabbit-oak/commit/5c52b9d6ab46b1eec525ed2d74b59f8eef013274
The core idea is to loop compaction through an {{ApplyDiff}} like mechanism
similar to what we do in {{Compactor}} for offline compaction. Non surprisingly
and somewhat unimaginative this class is currently called {{Compactor2}} in my
approach. {{Compactor2}} can, with the help of a {{SegmentWriter}} instance,
compact node states on top of already compacted node states by just applying
the differences. A bit care has to be taken here not to lose the stable id in
the process. {{FileStore}} uses a {{Compactor2}} to rebase the checkpoints and
the root inside the super root separately and in chronological order on top of
each other. The results are then reassembled into a super root.
Additionally, when compaction goes through multiple cycles each subsequent
cycle rebases its oldest checkpoint on top of the compacted root of the
previous cycle.
[~frm], could you have a look when time permits?
> Chronologically rebase checkpoints on top of each other during compaction
> -------------------------------------------------------------------------
>
> Key: OAK-5790
> URL: https://issues.apache.org/jira/browse/OAK-5790
> Project: Jackrabbit Oak
> Issue Type: Improvement
> Components: segment-tar
> Reporter: Michael Dürig
> Assignee: Michael Dürig
> Labels: compaction, gc, performance
> Fix For: 1.8, 1.7.1
>
>
> Currently the compactor does just a rewrite of the super root node without
> any special handling of the checkpoints. It just relies on the node
> de-duplication cache to avoid fully exploding the checkpoints.
> I think this can be improved by subsequently rebasing checkpoints on top of
> each other during compaction. (Very much like checkpoints are handled in
> migration).
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