What happens when #hadoop is in #safemode

 When Hadoop is in safe mode, it means that the Hadoop NameNode has started up and is running, but is not yet ready to serve client requests. Safe mode is a protective mode that the NameNode enters automatically when it detects certain conditions in the cluster that require administrator attention. In safe mode, the following things happen:

1. The NameNode does not allow any new file systems modifications, such as creating, deleting, or renaming files or directories.

2. The NameNode periodically checks the status of each DataNode to make sure that it has the minimum number of replicas for each block. If a DataNode is missing a replica, the NameNode will begin replicating the missing block to another DataNode.

3. The NameNode waits for a configurable threshold of DataNodes to report that they are alive and functioning properly. This threshold is configured using the dfs.safemode.min.datanodes property.

4. The NameNode also waits for a configurable threshold of blocks to be reported as available by DataNodes. This threshold is configured using the dfs.safemode.threshold.pct property.

Once the NameNode has verified that the cluster is in a healthy state, it will exit safe mode and begin serving client requests. If any problems are detected during the safe mode period, the NameNode will remain in safe mode until the problems are resolved.