Upgrade is an important part of the lifecycle of any software system, especially a distributed multi-component system like Hadoop. This is a step-by-step procedure a Hadoop cluster administrator should follow in order to safely transition the cluster to a newer software version. This is a general procedure, for particular version specific instructions please additionally refer to the release notes and version change descriptions.
The purpose of the procedure is to minimize damage to the data stored in Hadoop during upgrades, which could be a result of the following three types of errors:
can lead to partial or complete data loss.
In our experience the worst damage to the system is incurred when as a result of a software or human mistake the name node decides that some blocks/files are redundant and issues a command for data nodes to remove the blocks. Although a lot has been done to prevent this behavior the scenario is still possible.
bin/stop-mapred.sh fsck command: bin/hadoop fsck / -files -blocks -locations > dfs-v-old-fsck-1.log fsck output is recommend for large clusters in order to avoid time consuming output to stdout. 3. Run lsr command: bin/hadoop dfs -lsr / > dfs-v-old-lsr-1.log report command to create a list of data nodes participating in the cluster. bin/hadoop dfsadmin -report > dfs-v-old-report-1.log 5. Optionally, copy all or unrecoverable only data stored in DFS to a local file system or a backup instance of DFS. 6. Optionally, stop and restart DFS cluster, in order to create an up-to-date namespace checkpoint of the old version. bin/stop-dfs.sh bin/start-dfs.sh 7. Optionally, repeat 3, 4, 5, and compare the results with the previous run to ensure the state of the file system remained unchanged. 8. Copy the following checkpoint files into a backup directory: dfs.name.dir/edits dfs.name.dir/image/fsimage 9. Stop DFS cluster. bin/stop-dfs.sh conf/slaves file before starting, to reflect the current set of active nodes. 12. Optionally, change the configuration of the name node’s and the job tracker’s port numbers, to ignore unreachable nodes that are running the old version, preventing them from connecting and disrupting system operation. fs.default.name mapred.job.tracker 13. Optionally, start name node only. bin/hadoop-daemon.sh start namenode -upgrade lsr command: bin/hadoop dfs -lsr / > dfs-v-new-lsr-0.log dfs-v-old-lsr-1.log 15. Start DFS cluster. bin/start-dfs.sh 16. Run report command: bin/hadoop dfsadmin -report > dfs-v-new-report-1.log dfs-v-old-report-1.log to ensure all data nodes previously belonging to the cluster are up and running. 17. Run lsr command: bin/hadoop dfs -lsr / > dfs-v-new-lsr-1.log dfs-v-old-lsr-1.log. These files should be identical unless the format of lsr reporting or the data structures have changed in the new version. 18. Run fsck command: bin/hadoop fsck / -files -blocks -locations > dfs-v-new-fsck-1.log dfs-v-old-fsck-1.log. These files should be identical, unless the fsck reporting format has changed in the new version. 19. Start map-reduce cluster bin/start-mapred.sh
In case of failure the administrator should have the checkpoint files in order to be able to repeat the procedure from the appropriate point or to restart the old version of Hadoop. The *.log files should help in investigating what went wrong during the upgrade.
This is a list of enhancements intended to simplify the upgrade procedure and to make the upgrade safer in general.
fsck processes 1-2 TB per minute. 4. Hadoop should provide a backup solution as a stand alone application. 5. Introduce an explicit -upgrade option for DFS (See below) and a related 6. finalize upgrade command.
During the shutdown the name node performs the following actions.
fsck, and optionally saves the result in a file provided.The main idea of upgrade is that each version that modifies data structures on disk has its own distinct working directory. For instance, we'd have a "v0.6" and a “v0.7” directory for the name node and for all data nodes. These version directories will be automatically created when a particular file system version is brought up for the first time. If DFS is started with the -upgrade option the new file system version will do the following:
fsck and then switch to the normal mode when it’s ready to serve clients’ requests.
This ensures that a snapshot of the old data is preserved until the new version is validated and tested to function properly. Following the upgrade the file system can be run for a week or so to gain confidence. It can be rolled back to the old snapshot if it breaks, or the upgrade can be “finalized” by admin using the “finalize upgrade” command, which would remove old version working directories.
Care must be taken to deal with data nodes that are missing during the upgrade stage. In order to deal with such nodes the name node should store the list of data nodes that have completed the upgrade, and reject data nodes that did not confirm the upgrade.
When DFS will allow modification of blocks, this will require copying blocks into the current version working directory before modifying them.
Linking allows the data from several versions of Hadoop to coexist and even evolve on the same hardware without duplicating common parts.
When the Hadoop administrator is convinced that the new version works properly he/she/it can issue a “finalize upgrade” request.
The finalize upgrade procedure can run in the background without disrupting the cluster performance. Being in finalize mode the name node will periodically verify confirmations from the data nodes and finalize itself when the load is light.
The new utilities will substantially simplify the upgrade procedure:
The procedure described here applies any Hadoop upgrade. In addition to this page, please read Hadoop-0.14 Upgrade when upgrading to Hadoop-0.14 (Check release notes). Hadoop-0.14 Upgrade describes how to follow the progress of upgrade that is specific to Hadoop-0.14.