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https://issues.apache.org/jira/browse/KAFKA-15609?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17782148#comment-17782148
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Divij Vaidya commented on KAFKA-15609:
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Hey folks
I did some more digging on this and I am not convinced that this is not a bug
but I don't have proof right now.
My understanding is as follow:
1. MMap will read/write from page cache in the OS.
2. MMap flushed data to page cache "eventually" after writes. Note that
MappedByteBuffer.force() method forces a flush of MMap to page cache.
3. In Kafka, we flush indexes leading to MappedByteBuffer.force() on segment
roll, but this is performed asynchronously in a separate thread.
Now consider the situation:
1. Data is written to segment and index.
2. Segment rolls and schedules a flush.
3. Since segment is closed, RSM will consider it for copying to remote.
4. RSM will read from page cache which is guaranteed to give correct (even
non-flushed / dirty) data for segment but it may give out of date data for
indexes.
Result: Indexes uploaded to remote tier will not contain all data that is
expected in it.
I will keep this ticket closed until I have some reproducer but I would solicit
your thoughts on this.
Proposal: To fix this, we should call AbstractIndex.flush() in RLM before
passing the index to RSM for upload.
> Corrupted index uploaded to remote tier
> ---------------------------------------
>
> Key: KAFKA-15609
> URL: https://issues.apache.org/jira/browse/KAFKA-15609
> Project: Kafka
> Issue Type: Bug
> Components: Tiered-Storage
> Affects Versions: 3.6.0
> Reporter: Divij Vaidya
> Priority: Minor
>
> While testing Tiered Storage, we have observed corrupt indexes being present
> in remote tier. One such situation is covered here at
> https://issues.apache.org/jira/browse/KAFKA-15401. This Jira presents another
> such possible case of corruption.
> Potential cause of index corruption:
> We want to ensure that the file we are passing to RSM plugin contains all the
> data which is present in MemoryByteBuffer i.e. we should have flushed the
> MemoryByteBuffer to the file using force(). In Kafka, when we close a
> segment, indexes are flushed asynchronously [1]. Hence, it might be possible
> that when we are passing the file to RSM, the file doesn't contain flushed
> data. Hence, we may end up uploading indexes which haven't been flushed yet.
> Ideally, the contract should enforce that we force flush the content of
> MemoryByteBuffer before we give the file for RSM. This will ensure that
> indexes are not corrupted/incomplete.
> [1]
> [https://github.com/apache/kafka/blob/4150595b0a2e0f45f2827cebc60bcb6f6558745d/core/src/main/scala/kafka/log/UnifiedLog.scala#L1613]
>
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