From cfcd17ffe7ccac843a65b1fe97d2c7290df6852b Mon Sep 17 00:00:00 2001 From: He Xiaoqiao Date: Sun, 3 Jan 2021 16:25:17 +0800 Subject: [PATCH] HDFS-15751. Add documentation for msync() API to filesystem.md. Contributed by Konstantin V Shvachko. Reviewed-by: He Xiaoqiao Reviewed-by: Chao Sun --- .../site/markdown/filesystem/filesystem.md | 30 +++++++++++++++++++ 1 file changed, 30 insertions(+) diff --git a/hadoop-common-project/hadoop-common/src/site/markdown/filesystem/filesystem.md b/hadoop-common-project/hadoop-common/src/site/markdown/filesystem/filesystem.md index 284a964f6e5..035c4d3b4b8 100644 --- a/hadoop-common-project/hadoop-common/src/site/markdown/filesystem/filesystem.md +++ b/hadoop-common-project/hadoop-common/src/site/markdown/filesystem/filesystem.md @@ -116,6 +116,36 @@ for both files and directories, MUST always return `true` to the `isEncrypted()` predicate. This can be done by setting the `encrypted` flag to true when creating the `FileStatus` instance. + +### `msync()` + +Synchronize metadata state of the client with the latest state of the metadata +service of the FileSystem. + +In highly available FileSystems standby service can be used as a read-only +metadata replica. This call is essential to guarantee consistency of +reads from the standby replica and to avoid stale reads. + +It is currently only implemented for HDFS and others will just throw +`UnsupportedOperationException`. + +#### Preconditions + + +#### Postconditions + +This call internally records the state of the metadata service at the time of +the call. This guarantees consistency of subsequent reads from any metadata +replica. It assures the client will never access the state of the metadata that +preceded the recorded state. + +#### HDFS implementation notes + +HDFS supports `msync()` in HA mode by calling the Active NameNode and requesting +its latest journal transaction ID. For more details see HDFS documentation +[Consistent Reads from HDFS Observer NameNode](https://hadoop.apache.org/docs/current/hadoop-project-dist/hadoop-hdfs/ObserverNameNode.html) + + ### `Path getHomeDirectory()` The function `getHomeDirectory` returns the home directory for the FileSystem