Michael Dürig created OAK-4256:
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Summary: CLONE - Cross gc sessions might introduce references to
pre-compacted segments
Key: OAK-4256
URL: https://issues.apache.org/jira/browse/OAK-4256
Project: Jackrabbit Oak
Issue Type: Improvement
Components: segment-next
Reporter: Michael Dürig
Assignee: Michael Dürig
Priority: Critical
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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