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Sam Lightfoot edited comment on CASSANDRA-21134 at 2/11/26 5:27 PM:
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Preliminary test results for unthrottled cursor compaction with direct IO for
compaction writes:
* Memory: 12GB
* Max Heap: 6GB
* Hot dataset size: 1GB
* Compaction reads: buffered
!image-2026-02-11-17-22-58-361.png|width=497,height=348!
!image-2026-02-11-17-25-58-329.png|width=291,height=182!
| Metric | Buffered (avg) | Direct IO (avg) | Improvement |
|---|---|---|---|
| **p99 read latency** | 16.2 ms | 1.9 ms | **8.5x** |
| **p99.9 read latency** | 100.6 ms | 7.9 ms | **12.7x** |
| **Mean read latency** | 1.29 ms | 0.61 ms | **2.1x** |
| **Stall_us/s** | 31,599 | 21,534 | **-32%** |
| **Active_File** | 471 MB | 488 MB | **+4%** |
| **Compaction throughput** | 246 MiB/s | 286 MiB/s | **+16%** |
| p50 read latency | 0.48 ms | 0.48 ms | — |
was (Author: JIRAUSER302824):
Preliminary test results for unthrottled cursor compaction with direct IO for
compaction writes:
* Memory: 12GB
* Max Heap: 6GB
* Hot dataset size: 1GB
* Compaction reads: buffered
!image-2026-02-11-17-22-58-361.png|width=497,height=348!
> Direct IO support for compaction writes
> ---------------------------------------
>
> Key: CASSANDRA-21134
> URL: https://issues.apache.org/jira/browse/CASSANDRA-21134
> Project: Apache Cassandra
> Issue Type: Improvement
> Components: Local/Compaction
> Reporter: Sam Lightfoot
> Assignee: Sam Lightfoot
> Priority: Normal
> Fix For: 5.x
>
> Attachments: image-2026-02-11-17-22-58-361.png,
> image-2026-02-11-17-25-58-329.png
>
>
> Follow-up from the implementation for compaction reads (CASSANDRA-19987)
> Notable points
> * Update the start-up check that impacts DIO writes
> ({_}checkKernelBug1057843{_})
> * RocksDB uses 1 MB flush buffer. This should be configurable and
> performance tested (256KB vs 1MB)
> * Introduce compaction_write_disk_access_mode
> * Support for the compressed path would be most beneficial
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