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From Michael Dürig (JIRA) <j...@apache.org>
Subject [jira] [Updated] (OAK-3348) Cross gc sessions might introduce references to pre-compacted segments
Date Tue, 19 Apr 2016 11:50:25 GMT

     [ https://issues.apache.org/jira/browse/OAK-3348?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Michael Dürig updated OAK-3348:
-------------------------------
    Labels: cleanup compaction gc  (was: candidate_oak_1_0 candidate_oak_1_2 candidate_oak_1_4
cleanup compaction gc)

> 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
>            Priority: Blocker
>              Labels: cleanup, compaction, gc
>             Fix For: 1.6, 1.5.2
>
>         Attachments: OAK-3348-1.patch, OAK-3348-2.patch, OAK-3348.patch, SCIT.patch,
cleanup-time.png, compaction-time.png, cross-gc-refs.pdf, image.png, repo-size.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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