[GitHub] [spark] gaborgsomogyi commented on a change in pull request #27398: [SPARK-30481][DOCS][FOLLOWUP] Document event log compaction into new section of monitoring.md

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gaborgsomogyi commented on a change in pull request #27398: [SPARK-30481][DOCS][FOLLOWUP] Document event log compaction into new section of monitoring.md
URL: https://github.com/apache/spark/pull/27398#discussion_r373985499
 
 

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 File path: docs/monitoring.md
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 @@ -95,6 +95,44 @@ The history server can be configured as follows:
   </tr>
 </table>
 
+### Applying compaction of old event log files
+
+A long-running streaming application can bring a huge single event log file which may cost a lot to maintain and
+also requires a bunch of resource to replay per each update in Spark History Server.
+
+Enabling <code>spark.eventLog.rolling.enabled</code> and <code>spark.eventLog.rolling.maxFileSize</code> would
+let you have multiple event log files instead of single huge event log file which may help some scenarios on its own,
+but it still doesn't help you reducing the overall size of logs.
+
+Spark History Server can apply 'compaction' on the rolling event log files to reduce the overall size of
+logs, via setting the configuration <code>spark.history.fs.eventLog.rolling.maxFilesToRetain</code> on the
+Spark History Server.
+
+When the compaction happens, History Server lists all the available event log files, and considers the event log files older than
+retained log files as a target of compaction. For example, if the application A has 5 event log files and
+<code>spark.history.fs.eventLog.rolling.maxFilesToRetain</code> is set to 2, first 3 log files will be selected to be compacted.
+
+Once it selects the files, it analyzes these files to figure out which events can be excluded, and rewrites these files
+into one compact file with discarding some events. Once rewriting is done, original log files will be deleted.
+
+The compaction tries to exclude the events which point to the outdated things like jobs, and so on. As of now, below describes
+the candidates of events to be excluded:
+
+* Events for the job which is finished, and related stage/tasks events
+* Events for the executor which is terminated
+* Events for the SQL execution which is finished, and related job/stage/tasks events
+
+but the details can be changed afterwards.
+
+Please note that Spark History Server may not compact the old event log files if figures out not a lot of space
 
 Review comment:
   This paragraph not really answers to me why not always compact.

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