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https://issues.apache.org/jira/browse/CASSANDRA-21134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18057927#comment-18057927
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Sam Lightfoot edited comment on CASSANDRA-21134 at 2/11/26 5:28 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
_Read Latency During Compaction_
|Percentile|Buffered #1 (ms)|Buffered #2 (ms)|Direct IO #1 (ms)|Direct IO #2
(ms)|
|—|—|—|—|—|
|p50|0.48|0.48|0.48|0.48|
|p90|1.36|1.34|0.98|0.97|
|p99|16.52|15.79|1.94|1.87|
|p99.9|141.6|59.5|8.19|7.60|
|p99.99|190.8|96.5|27.5|23.3|
|Mean|1.41|1.16|0.61|0.61|
|StdDev|7.74|3.94|0.67|0.61|
|Max|285.2|160.4|86.5|80.7|
|Total reads|5,730,193|5,730,138|5,730,235|5,730,288|
_Summary_
| 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
_Read Latency During Compaction_
|Percentile|Buffered #1 (ms)|Buffered #2 (ms)|Direct IO #1 (ms)|Direct IO #2
(ms)|
|—|—|—|—|—|
|p50|0.48|0.48|0.48|0.48|
|p90|1.36|1.34|0.98|0.97|
|p99|16.52|15.79|1.94|1.87|
|p99.9|141.6|59.5|8.19|7.60|
|p99.99|190.8|96.5|27.5|23.3|
|Mean|1.41|1.16|0.61|0.61|
|StdDev|7.74|3.94|0.67|0.61|
|Max|285.2|160.4|86.5|80.7|
|Total reads|5,730,193|5,730,138|5,730,235|5,730,288|
_Summary_
|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|—|
> 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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