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From "Yongjun Zhang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-9092) Nfs silently drops overlapping write requests and causes data copying to fail
Date Thu, 15 Oct 2015 07:25:05 GMT

    [ https://issues.apache.org/jira/browse/HDFS-9092?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14958457#comment-14958457
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Yongjun Zhang commented on HDFS-9092:
-------------------------------------

Thanks [~liuml07]. It'd be nice to find out why jenkins job did not report the same. We should
look into HDFS-9245 too. 



> Nfs silently drops overlapping write requests and causes data copying to fail
> -----------------------------------------------------------------------------
>
>                 Key: HDFS-9092
>                 URL: https://issues.apache.org/jira/browse/HDFS-9092
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: nfs
>    Affects Versions: 2.7.1
>            Reporter: Yongjun Zhang
>            Assignee: Yongjun Zhang
>             Fix For: 2.8.0
>
>         Attachments: HDFS-9092.001.patch, HDFS-9092.002.patch
>
>
> When NOT using 'sync' option, the NFS writes may issue the following warning:
> org.apache.hadoop.hdfs.nfs.nfs3.OpenFileCtx: Got an overlapping write (1248751616, 1249677312),
nextOffset=1248752400. Silently drop it now
> and the size of data copied via NFS will stay at 1248752400.
> Found what happened is:
> 1. The write requests from client are sent asynchronously. 
> 2. The NFS gateway has handler to handle the incoming requests by creating an internal
write request structuire and put it into cache;
> 3. In parallel, a separate thread in NFS gateway takes requests out from the cache and
writes the data to HDFS.
> The current offset is how much data has been written by the write thread in 3. The detection
of overlapping write request happens in 2, but it only checks the write request against the
curent offset, and trim the request if necessary. Because the write requests are sent asynchronously,
if two requests are beyond the current offset, and they overlap, it's not detected and both
are put into the cache. This cause the symptom reported in this case at step 3.



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