HDFS-17794 Documentation for dfs.datanode.synconclose

[Jimvin]

Description of PR

HDFS-17794. Documentation for dfs.datanode.synconclose.

The HDFS configuration property dfs.datanode.synconclose is absent from the hdfs-site.xml documentation. This PR adds and entry in hdfs-default.xml for this property along with a description of its purpose.

How was this patch tested?

Documentation only, no additional testing performed.

For code changes:

• Does the title or this PR starts with the corresponding JIRA issue id (e.g. 'HADOOP-17799. Your PR title ...')?
• Object storage: have the integration tests been executed and the endpoint declared according to the connector-specific documentation?
• If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0?
• If applicable, have you updated the LICENSE, LICENSE-binary, NOTICE-binary files?

[hadoop-yetus]

💔 -1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	25m 10s		Docker mode activated.
			_ Prechecks _
+1 💚	dupname	0m 0s		No case conflicting files found.
+0 🆗	codespell	0m 1s		codespell was not available.
+0 🆗	detsecrets	0m 1s		detect-secrets was not available.
+0 🆗	xmllint	0m 1s		xmllint was not available.
+1 💚	@author	0m 0s		The patch does not contain any @author tags.
-1 ❌	test4tests	0m 0s		The patch doesn't appear to include any new or modified tests. Please justify why no new tests are needed for this patch. Also please list what manual steps were performed to verify this patch.
			_ trunk Compile Tests _
+1 💚	mvninstall	33m 14s		trunk passed
+1 💚	compile	1m 27s		trunk passed with JDK Ubuntu-21.0.7+6-Ubuntu-0ubuntu120.04
+1 💚	compile	1m 26s		trunk passed with JDK Ubuntu-17.0.15+6-Ubuntu-0ubuntu120.04
+1 💚	mvnsite	1m 33s		trunk passed
+1 💚	javadoc	1m 11s		trunk passed with JDK Ubuntu-21.0.7+6-Ubuntu-0ubuntu120.04
+1 💚	javadoc	1m 8s		trunk passed with JDK Ubuntu-17.0.15+6-Ubuntu-0ubuntu120.04
+1 💚	shadedclient	65m 37s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	mvninstall	1m 19s		the patch passed
+1 💚	compile	1m 17s		the patch passed with JDK Ubuntu-21.0.7+6-Ubuntu-0ubuntu120.04
+1 💚	javac	1m 17s		the patch passed
+1 💚	compile	1m 16s		the patch passed with JDK Ubuntu-17.0.15+6-Ubuntu-0ubuntu120.04
+1 💚	javac	1m 16s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	mvnsite	1m 26s		the patch passed
+1 💚	javadoc	0m 59s		the patch passed with JDK Ubuntu-21.0.7+6-Ubuntu-0ubuntu120.04
+1 💚	javadoc	1m 2s		the patch passed with JDK Ubuntu-17.0.15+6-Ubuntu-0ubuntu120.04
+1 💚	shadedclient	29m 22s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
-1 ❌	unit	278m 30s	/patch-unit-hadoop-hdfs-project_hadoop-hdfs.txt	hadoop-hdfs in the patch passed.
+1 💚	asflicense	0m 45s		The patch does not generate ASF License warnings.
		406m 36s

Reason	Tests
Failed junit tests	hadoop.tools.TestHdfsConfigFields
	hadoop.hdfs.tools.TestDFSAdmin

Subsystem	Report/Notes
Docker	ClientAPI=1.51 ServerAPI=1.51 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8065/1/artifact/out/Dockerfile
GITHUB PR	#8065
Optional Tests	dupname asflicense compile javac javadoc mvninstall mvnsite unit shadedclient codespell detsecrets xmllint
uname	Linux e15108d6a1ad 5.15.0-156-generic #166-Ubuntu SMP Sat Aug 9 00:02:46 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	trunk / 5617e8d
Default Java	Ubuntu-17.0.15+6-Ubuntu-0ubuntu120.04
Multi-JDK versions	/usr/lib/jvm/java-21-openjdk-amd64:Ubuntu-21.0.7+6-Ubuntu-0ubuntu120.04 /usr/lib/jvm/java-17-openjdk-amd64:Ubuntu-17.0.15+6-Ubuntu-0ubuntu120.04
Test Results	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8065/1/testReport/
Max. process+thread count	2960 (vs. ulimit of 5500)
modules	C: hadoop-hdfs-project/hadoop-hdfs U: hadoop-hdfs-project/hadoop-hdfs
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8065/1/console
versions	git=2.25.1 maven=3.9.11
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[ayushtkn]

There is an entry of this conf in TestHdfsConfigFields, since we are adding it here, can we remove it from there?
https://github.com/apache/hadoop/blob/trunk/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/tools/TestHdfsConfigFields.java#L81-L82

[Jimvin]

Yes, as far as I can see the property dfs.datanode.synconclose is not deprecated and remains useful under some circumstances (notably HBase). I believe it can be removed as suggested.