Re: Will Backup Node download image and edits log from NameNode?

[Gurmukh Singh <gu...@yahoo.com.INVALID>]

your observation is correct. backup node will also download.

If you look at the journey/evolution of hadoop, we had primary, backup 
only, checkpointing node and then a generic secondary node.

checking node will do the merge of fsimage and edits


On 25/9/17 5:57 pm, Chang.Wu wrote:
>
> From the official document of Backup Node 
> <https://hadoop.apache.org/docs/r2.7.2/hadoop-project-dist/hadoop-hdfs/HdfsUserGuide.html#Backup_Node>， 
> it Says:
>
> The Backup node does not need to download fsimage and edits files from 
> the active NameNode in order to create a checkpoint, as would be 
> required with a Checkpoint node or Secondary NameNode, since it 
> already has an up-to-date state of the namespace state in memory.
>
> I noticed the keyword \'*does not need*\' ，it gives no clear 
> information about whether or not the Backup Node will download fsimage 
> and edits files from the active NameNode like checkpoint node.
>
> I checked the source code , namely 
> |org.apache.hadoop.hdfs.server.namenode.Checkpointer|， I find that 
> Both Backup Node and Checkpoint Node will download fsimage and edits 
> files from active NameNode，no difference.
>
> So , although it is unnecessary for Backup Node to download downlaod 
> fsimage and edits files from the active NameNode because it keeps a 
> journal stream from the active name node, but in fact Backup Node 
> still do so. Right?Thanks.
>