During start up the NameNode loads the file system state from the fsimage and the edits log file. It then waits for DataNodes to report their blocks so that it does not prematurely start replicating the blocks though enough replicas already exist in the cluster. During this time NameNode stays in Safemode. Safemode for the NameNode is essentially a read-only mode for the HDFS cluster, where it does not allow any modifications to file system or blocks.
Normally the NameNode leaves Safemode automatically after the DataNodes have reported that most file system blocks are available. NameNode front page shows whether Safemode is on or off. A more detailed description and configuration is maintained as JavaDoc for setSafeMode. HDFS supports the fsck command to check for various inconsistencies. It is designed for reporting problems with various files, for example, missing blocks for a file or under-replicated blocks.
Unlike a traditional fsck utility for native file systems, this command does not correct the errors it detects. Normally NameNode automatically corrects most of the recoverable failures. By default fsck ignores open files but provides an option to select all files during reporting. For command usage, see fsck. HDFS supports the fetchdt command to fetch Delegation Token and store it in a file on the local system.
This token can be later used to access secure server NameNode for example from a non secure client. For command usage, see fetchdt command. Typically, you will configure multiple metadata storage locations. Then, if one storage location is corrupt, you can read the metadata from one of the other storage locations. However, what can you do if the only storage locations available are corrupt?
In this case, there is a special NameNode startup mode called Recovery mode that may allow you to recover most of your data. When in recovery mode, the NameNode will interactively prompt you at the command line about possible courses of action you can take to recover your data.
This option will force recovery mode to always select the first choice. Normally, this will be the most reasonable choice. Because Recovery mode can cause you to lose data, you should always back up your edit log and fsimage before using it.
When Hadoop is upgraded on an existing cluster, as with any software upgrade, it is possible there are new bugs or incompatible changes that affect existing applications and were not discovered earlier. HDFS allows administrators to go back to earlier version of Hadoop and rollback the cluster to the state it was in before the upgrade.
HDFS can have one such backup at a time. The following briefly describes the typical upgrade procedure:. Most of the time, cluster works just fine. Once the new HDFS is considered working well may be after a few days of operation , finalize the upgrade. Note that until the cluster is finalized, deleting the files that existed before the upgrade does not free up real disk space on the DataNodes. If the NameNode encounters a reserved path during upgrade, it will print an error like the following:.
Please rollback and delete or rename this path, or upgrade with the -renameReserved [key-value pairs] option to automatically rename these paths during upgrade. Specifying -upgrade -renameReserved [optional key-value pairs] causes the NameNode to automatically rename any reserved paths found during startup.
For example, to rename all paths named. This is the second stable release of Apache Hadoop 3. This is the second stable release of Apache Hadoop 2. It contains bug fixes, improvements and enhancements since 2. Users are encouraged to read the overview of major changes since 2.
For details of bug fixes, improvements, and other enhancements since the previous 2. These advantages are especially significant when dealing with big data and were made possible with the particular way HDFS handles data. Note: Hadoop is just one solution to big data processing.
Another popular open-source framework is Spark. Want to learn more about the key differences between these big data processing solutions? Multiple DataNodes are linked to the master node in the cluster, the NameNode. The master node distributes replicas of these data blocks across the cluster.
It also instructs the user where to locate wanted information. However, before the NameNode can help you store and manage the data, it first needs to partition the file into smaller, manageable data blocks.
This process is called data block splitting. By default, a block can be no more than MB in size. The number of blocks depends on the initial size of the file. All but the last block are the same size MB , while the last one is what remains of the file.
For example, an MB file is broken up into seven data blocks. Six of the seven blocks are MB, while the seventh data block is the remaining 32 MB. It is recommended to have at least three replicas, which is also the default setting. The master node stores them onto separate DataNodes of the cluster. The state of nodes is closely monitored to ensure the data is always available. Options: -d : List the directories as plain files -h : Format the sizes of files to a human-readable manner instead of number of bytes -R : Recursively list the contents of directories.
Copy files from the local file system to HDFS, similar to -put command. This command will not work if the file already exists. To overwrite the destination if the file already exists, add -f flag to command.
Surya Surya 2, 5 5 gold badges 24 24 silver badges 34 34 bronze badges. Add a comment. Active Oldest Votes. VeLKerr 2, 2 2 gold badges 20 20 silver badges 41 41 bronze badges.
Tariq Tariq I see. You can actually use hdfs cat command if you wish to see the file's content or open the file on the webui. This will save you from downloading the file to your local fs. You are welcome. Not for just this one, but in general.
Just to add to my lat comment, if it is a binary file, cat won't show you the actual content. It seems to be a bug fixed.
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