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https://issues.apache.org/jira/browse/HDFS-9092?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16351338#comment-16351338
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caixiaofeng commented on HDFS-9092:
-----------------------------------
mark this.
> 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
> Priority: Major
> Fix For: 2.8.0, 3.0.0-alpha1
>
> 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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