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From "Brandon Li (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-9092) Nfs silently drops overlapping write requests, thus data copying can't complete
Date Sat, 26 Sep 2015 04:48:04 GMT

    [ https://issues.apache.org/jira/browse/HDFS-9092?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14909083#comment-14909083
] 

Brandon Li commented on HDFS-9092:
----------------------------------

For category2, the assumption of the fix is that, the trimmed data is the same as what's already
written to HDFS.

So far we claim that NFS gateway supports the user cases of file uploading and file streaming.

For file uploading, the overlapped section is safe to drop since it will be the same as what's
already written to HDFS. It's the same case for file streaming. 

The only possible problem is this: before the patch, if users do random update to an HDFS
file, NFS gateway will report error. With this patch, there is a chance we wont's see the
error if it happens that the updated section is trimmed.

Since random write is not supported anyway, the possible nicer reaction to a random update
seems still acceptable to me.   

> Nfs silently drops overlapping write requests, thus data copying can't complete
> -------------------------------------------------------------------------------
>
>                 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
>         Attachments: HDFS-9092.001.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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