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https://issues.apache.org/jira/browse/CASSANDRA-21486?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Sebastian Marsching updated CASSANDRA-21486:
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Attachment: image-2026-07-03-22-55-11-607.png
> Possible NIO mapped byte buffer leak
> ------------------------------------
>
> Key: CASSANDRA-21486
> URL: https://issues.apache.org/jira/browse/CASSANDRA-21486
> Project: Apache Cassandra
> Issue Type: Bug
> Reporter: Sebastian Marsching
> Priority: Normal
> Attachments: image-2026-07-03-22-53-29-737.png,
> image-2026-07-03-22-53-37-284.png, image-2026-07-03-22-55-11-607.png
>
>
> After upgrading two clusters from C* 5.0.5 to 5.0.7, we have noticed the
> following behavior:
> Where previously, memory (RAM) usage was mostly stable over time, it is now
> increasing constantly over time, with the consequence that we have hit alarm
> limits on some of the nodes. A restart of the Cassandra daemon decreases the
> usage only slightly. It is still much higher than before.
> This behavior seems to correlate with the NIO mapped byte buffers. On one of
> the nodes, we noticed that the estimated number of these buffers sharply
> increased (about fourfold from about 4000 to about 16000) about three weeks
> ago. This was accompanied by a sharp increase of the estimated capacity and
> used bytes (from about 9 GiB to about 12 GiB). While the number of buffers
> dropped to about 900 after restarting the Cassandra daemon, the capacity and
> used bytes did not drop at all and are steadily increasing.
> This sharp rise happened around a time where the number of live SSTables for
> our main keyspace rose from about 290 to about 320, and the live disk space
> bytes dropped slightly. This might hint at the fact that the sharp increase
> in the number of NIO buffers might be associated with compaction.
> For context, I should add that a while ago, we switched from STCS to UCS, but
> this was rather long ago (in October 2025), while the problem with memory
> usage only started in early April 2026, after upgrading from 5.0.5 to 5.0.7.
> The problem seems somewhat similar to CASSANDRA-20753, but as this has been
> fixed in 5.0.7, it cannot be the cause. I wonder whether the fix could
> possibly have introduced a new problem, which would explain why we only
> started seeing the problem after upgrading to 5.0.7.
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