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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 2/13/26 8:04 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|Notes|
|—|—|—|—|—|
|*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%*|Time app blocks waiting for kernel page
reclaim|
|*Active_File*|471 MB|488 MB|+4%|Page cache pages actively referenced by reads|
|*Compaction throughput*|246 MiB/s|286 MiB/s|*+16%*| |
|p50 read latency|0.48 ms|0.48 ms|—| |
|Cache Hit Ratio|16.1%|17.6%|+9%|Similar — compaction reads dominate misses|
|*Cache Dirty Writes*|42,862|1,038|*-97.6%* |Page cache dirty writes/s|
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|Notes|
|—|—|—|—|—|
|*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%*|Time app blocks waiting for kernel page
reclaim|
|*Active_File*|471 MB|488 MB|+4%|Page cache pages actively referenced by reads|
|*Compaction throughput*|246 MiB/s|286 MiB/s|*+16%*| |
|p50 read latency|0.48 ms|0.48 ms|—| |
|Cache Hit Ratio|16.1%|17.6%|+9%|Similar — compaction reads dominate misses|
|*Cache Dirty Writes*|*42,862*|*1,038*|*-97.6% (41x)*|Page cache dirty writes/s|
> 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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