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https://issues.apache.org/jira/browse/CASSANDRA-19987?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18055159#comment-18055159
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Sam Lightfoot commented on CASSANDRA-19987:
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[~dnk] Thank you, and great question.
Currently Direct IO is only supported for iterator-based compaction (via the
scanner path). Cursor-based compaction does not yet use it, though the
underlying Direct IO infrastructure is already in place. Wiring Direct IO
through to cursor-based compaction would be a straightforward follow-up, which
I'd keen to do in the very near future.
> Direct IO support for compaction reads
> --------------------------------------
>
> Key: CASSANDRA-19987
> URL: https://issues.apache.org/jira/browse/CASSANDRA-19987
> Project: Apache Cassandra
> Issue Type: Improvement
> Components: Local/Compaction
> Reporter: Jon Haddad
> Assignee: Sam Lightfoot
> Priority: Normal
> Fix For: 5.x
>
> Attachments: buffered_during_compaction-reads_1m.txt,
> buffered_during_compaction-reads_5m.txt, buffered_page_cache_compaction,
> direct_during_compaction-reads_1m.txt, direct_during_compaction-reads_5m.txt,
> direct_page_cache_compaction, image-2025-12-24-10-20-44-947.png,
> image-2025-12-24-10-21-11-928.png, image-2025-12-24-10-34-10-834.png
>
> Time Spent: 4h
> Remaining Estimate: 0h
>
> If we use direct io to read SSTables during compaction, we can avoid
> polluting the page cache with data we're about to delete. As another side
> effect, we also evict pages to make room for whatever we're putting in. This
> unnecessary churn leads to higher CPU overhead and can cause dips in client
> read latency, as we're going to be evicting pages that could be used to serve
> those reads.
> This is most notable with STCS as the SSTables get larger, potentially
> evicting the entire hot dataset out of cache, but is affected by every
> compaction strategy.
> This is a follow up to be done after CASSANDRA-15452 since we will have an
> internal buffer.
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