In our earlier recipes, we have configured the Hadoop cluster and have gone through various concepts on cluster operations. We saw that Namenode stores the metadata, which is a combination of the fsimage file and edits file and these two images are never merged by Namenode, unless it is restarted or there is some other node, such as Secondary Namenode, which does this. We will be covering Secondary Namenode in this chapter at a later stage.
Whenever the Namenode is started, it applies all the changes in the edits file to the fsimage file and starts with a clean edits file. Depending upon the size of the edits log, it could take a long time to start up Namenode and this adds to the total time a Namenode stays in the safemode. Safemode, as discussed in the earlier chapters, is not just a factor of edits file size, but also of the time taken to build up the bitmap, which is a mapping of blocks to the Datanodes.