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From Michael Dürig (JIRA) <j...@apache.org>
Subject [jira] [Commented] (OAK-3348) Cross gc sessions might introduce references to pre-compacted segments
Date Mon, 21 Mar 2016 12:36:25 GMT

    [ https://issues.apache.org/jira/browse/OAK-3348?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15204106#comment-15204106
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Michael Dürig commented on OAK-3348:
------------------------------------

I just [pushed|https://github.com/mduerig/jackrabbit-oak/commit/d0fcbd7369f432c0afc3fef914a3feff0b6a17e8]
a new cleanup implementation that leverages the GC generations: instead of relying on reachability
this strategy assigns a live time to records based on its generation. The life time is currently
hard coded to two generations, so anything further back than the last generation will be removed.
Ultimately this threshold should be configurable and I left a comment in the code to this
respect. 
Note that bulk segments require some special treatment in the generation based cleanup strategy
as those don't have a generation assigned to them. (They actually can't as we share bulk records
across generations). For bulk segments the approach is to still use reachability. 

> 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, 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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