From 8d242458e96c7a9bcd5390a32dc9324be93cf5fc Mon Sep 17 00:00:00 2001
From: zeekling <lingzhaohui@zeekling.cn>
Date: Wed, 27 Mar 2024 23:42:27 +0800
Subject: [PATCH] =?UTF-8?q?=20=E5=A2=9E=E5=8A=A0hdfs=E4=BB=A3=E7=A0=81?=
 =?UTF-8?q?=E6=B3=A8=E9=87=8A?=
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---
 .../org/apache/hadoop/hdfs/server/namenode/FSDirectory.java | 6 ++++++
 .../apache/hadoop/hdfs/server/namenode/FSNamesystem.java    | 5 +++--
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSDirectory.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSDirectory.java
index fc17eaebf7..37b368c31f 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSDirectory.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSDirectory.java
@@ -151,7 +151,10 @@ private static INodeDirectory createRoot(FSNamesystem namesystem) {
         .isdir(true)
         .build();
 
+  // 整个文件系统目录树的根节点， 是INodeDirectory类型的 。
   INodeDirectory rootDir;
+
+  // Namenode的核心类， 这个类主要支持对数据块进行操作的一些方法， 例如addBlock()。
   private final FSNamesystem namesystem;
   private volatile boolean skipQuotaCheck = false; //skip while consuming edits
   private final int maxComponentLength;
@@ -159,6 +162,7 @@ private static INodeDirectory createRoot(FSNamesystem namesystem) {
   private final int lsLimit;  // max list limit
   private final int contentCountLimit; // max content summary counts per run
   private final long contentSleepMicroSec;
+  // 记录根目录下所有的INode,并维护INodeId ->INode的映射关系。
   private final INodeMap inodeMap; // Synchronized by dirLock
   private long yieldCount = 0; // keep track of lock yield count.
   private int quotaInitThreads;
@@ -300,6 +304,7 @@ public int getListLimit() {
   /**
    * Caches frequently used file names used in {@link INode} to reuse 
    * byte[] objects and reduce heap usage.
+   * 将常用的name缓存下来， 以降低byte[]的使用， 并降低JVM heap的使用
    */
   private final NameCache<ByteArray> nameCache;
 
@@ -318,6 +323,7 @@ public enum DirOp {
     this.inodeId = new INodeId();
     rootDir = createRoot(ns);
     inodeMap = INodeMap.newInstance(rootDir);
+    // 是否开启权限管理
     this.isPermissionEnabled = conf.getBoolean(
       DFSConfigKeys.DFS_PERMISSIONS_ENABLED_KEY,
       DFSConfigKeys.DFS_PERMISSIONS_ENABLED_DEFAULT);
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSNamesystem.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSNamesystem.java
index b8e687085e..3267b6fc12 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSNamesystem.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSNamesystem.java
@@ -1218,12 +1218,13 @@ private List<AuditLogger> initAuditLoggers(Configuration conf) {
     return Collections.unmodifiableList(auditLoggers);
   }
 
+  // FSNamesystem在初始化完FSDirectory dir成员，会调用loadFSImage方法，从fsimage和edits加载元数据信息
   private void loadFSImage(StartupOption startOpt) throws IOException {
     final FSImage fsImage = getFSImage();
 
-    // format before starting up if requested
+    // format before starting up if requested // 如果启动选项类型为FORMAT（格式化），在启动之前需要进行格式化
     if (startOpt == StartupOption.FORMAT) {
-      // reuse current id
+      // reuse current id 对FSImage执行格式化操作
       fsImage.format(this, fsImage.getStorage().determineClusterId(), false);
 
       startOpt = StartupOption.REGULAR;