[jira] [Commented] (HDFS-404) Why open method in class DFSClient would compare old LocatedBlocks and new LocatedBlocks?

["Li Junjun (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HDFS-404?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13263460#comment-13263460 ] 

Li Junjun commented on HDFS-404:
--------------------------------

I think there are two situations.
1,the file has been swapped with another file,we check the blockId,that's correct we throw Exceptions!
2,but if  the file has not been swapped but being appending,we should just check the blockId ,and should not care about the block's stamp , because in fact we got the right and updated block list , cause file in hdfs can't be truncate .

so how about we  do it like this ?

if ( oldIter.next().getBlock().getBlockId() != newIter.next().getBlock().getBlockId() ) {
            throw new IOException("Blocklist for " + src + " has changed!");
}



                
> Why open method in class DFSClient would compare old LocatedBlocks and new LocatedBlocks?
> -----------------------------------------------------------------------------------------
>
>                 Key: HDFS-404
>                 URL: https://issues.apache.org/jira/browse/HDFS-404
>             Project: Hadoop HDFS
>          Issue Type: Wish
>            Reporter: qianyu
>            Assignee: Todd Lipcon
>   Original Estimate: 168h
>  Remaining Estimate: 168h
>
> This is in the package of org.apache.hadoop.hdfs, DFSClient.openInfo():
> if (locatedBlocks != null) {
>         Iterator<LocatedBlock> oldIter = locatedBlocks.getLocatedBlocks().iterator();
>         Iterator<LocatedBlock> newIter = newInfo.getLocatedBlocks().iterator();
>         while (oldIter.hasNext() && newIter.hasNext()) {
>           if (! oldIter.next().getBlock().equals(newIter.next().getBlock())) {
>             throw new IOException("Blocklist for " + src + " has changed!");
>           }
>         }
>       }
> Why we need compare old LocatedBlocks and new LocatedBlocks, and in what case it happen?
> Why not "this.locatedBlocks = newInfo" directly?

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