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From Boris Shkolnik <bo...@yahoo-inc.com>
Subject Re: What's different of 1)namenode metadata 2)image and 3)edit logs?
Date Mon, 16 Nov 2009 22:20:49 GMT
NameNode metadata - is in memory data structures describing file system
structure (directories, files, and file's blocks).

NameNode image is on-disk representation of the metadata. (fsimage file).

Edit logs - is a journal that keeps all the events that cause changes in the
metadata and allows restoration in case of NameNode failure.

Secondary Namenode - runs checkpoints from time to time. During a checkpoint
edit logs are merged into metadata and saved to fsimage, which is pushed to
the NameNode.


On 11/16/09 12:45 PM, "Steve Gao" <steve.gao@yahoo.com> wrote:

> What are the differences between the 3 concepts: (1) namenode metadata, (2)
> namenode image, and (3) edit logs?
> BTW,I know the secondary namenode backups namenode data.
> I was told that there is another daemon which also backups meta data . Is that
> true? What is the daemon? Thanks.

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