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From "Sanjay Radia (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-1073) Simpler model for Namenode's fs Image and edit Logs
Date Fri, 02 Apr 2010 01:43:27 GMT

    [ https://issues.apache.org/jira/browse/HDFS-1073?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12852652#action_12852652
] 

Sanjay Radia commented on HDFS-1073:
------------------------------------

This Jira proposes a simpler design for for managing fsimage and edit logs.
The edit logs and fsimage scheme in current Hadoop requires coordination and can lead to tricky
bugs  (HDFS-955).


Proposed design

#	All transactions have a transaction ID. A transaction ID is a number that  starts at zero
and incremented. Each journal record of the editLogs file has the transaction ID.
#	Fsimage file is identified by the transaction ID  of the last checkpointed transaction in
the file.
**  E.g. fsImage_<transactionIDofLastTranscationChekpointed>
#	An editsLog file is identified by the transaction ID  of the first recorded transaction
in the file.
** E.g. fsEditlogs<transactionIdofFirstTransaction> 
#	To start the name server,
** Load the fsImage with the greatest transactionID N. If no image exist, take N to be 0.
** Process all transactions >N from the editsLog: Find an editsLog that includes transaction
with IDs N+1. Process all transactions >= N+1 from that and all subsequent editLogs files.

Salient points
* This scheme does not require any synchronization between when fsImages are checkpointed
and editsLogs files are split (although it is convenient if when you checkpoint at transactionID
 N, then you also spilt your edits logs at N or slightly less).
* This means that the NameNode and BackupNode can share images and edits without coordination.
(This is very different from  the current design.). For example the primary NN can decided
that it wants a checkpoint and hence split the editLogs and ask the backup NN to do a checkpoint;
the checkpoint operation can succeed or fail without worries. (Btw if the split of the editLogs
is recorded as the last transaction in the edit logs then the backup NN will see that transaction
come across and realize that this is convenient time to checkpoint. 
* The scheme does not  require  coordination between checkpointing fsImages themselves! For
example, while the backup NN is doing a checkpoint, the NN could be asked to do a saveImage
by the admin.  
* Policies on how many edits and fsimages to keep is separable. 


> Simpler model for Namenode's fs Image and edit Logs 
> ----------------------------------------------------
>
>                 Key: HDFS-1073
>                 URL: https://issues.apache.org/jira/browse/HDFS-1073
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>            Reporter: Sanjay Radia
>
> The naming and handling of  NN's fsImage and edit logs can be significantly improved
resulting simpler and more robust code.

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