Database Reference
In-Depth Information
Secondary namenode
The
secondary namenode
daemon is responsible for performing periodic housekeeping
functions for namenode. It only creates checkpoints of the filesystem metadata (
fsimage
)
present in namenode by merging the
edits
logfile and the
fsimage
file from the na-
menode daemon. In case the namenode daemon fails, this checkpoint could be used to re-
build the filesystem metadata. However, it is important to note that checkpoints are done in
intervals and it is possible that the checkpoint data could be slightly outdated. Rebuilding
the
fsimage
file using such a checkpoint could lead to data loss. The secondary namen-
ode is not a failover node for the namenode daemon.
It is recommended that the secondary namenode daemon be hosted on a separate machine
for large clusters. The checkpoints are created by merging the
edits
logfiles and the
fsimage
file from the namenode daemon.
The following are the steps carried out by the secondary namenode daemon:
1. Get the
edits
logfile from the primary namenode daemon.
2. Get the
fsimage
file from the primary namenode daemon.
3. Apply all the actions present in the
edits
logs to the
fsimage
file.
4. Push the
fsimage
file back to the primary namenode.
This is done periodically and so whenever the namenode daemon is restarted, it would have
a relatively updated version of the
fsimage
file and the boot up time would be signific-
antly faster. The following diagram shows the communication between namenode and sec-
ondary namenode:
