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From "sam rash (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-1057) Concurrent readers hit ChecksumExceptions if following a writer to very end of file
Date Tue, 25 May 2010 18:48:37 GMT

    [ https://issues.apache.org/jira/browse/HDFS-1057?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12871288#action_12871288

sam rash commented on HDFS-1057:

hmm, in looking at the code more, I see that this depends on what # of bytes we want to make
available to readers.

-visible length (bytes acked) : needed for consistent view of data I think
-bytes on disk : this seems like we'll get inconsistent reads;  and in theory, acked data
may be *more* than on disk for a given node (if I read the doc + code right).  However, how
can a DN send data that's not on disk unless it's made available via memory? (very complex)

> Concurrent readers hit ChecksumExceptions if following a writer to very end of file
> -----------------------------------------------------------------------------------
>                 Key: HDFS-1057
>                 URL: https://issues.apache.org/jira/browse/HDFS-1057
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>          Components: data-node
>    Affects Versions: 0.21.0, 0.22.0
>            Reporter: Todd Lipcon
>            Assignee: sam rash
>            Priority: Blocker
>         Attachments: conurrent-reader-patch-1.txt, conurrent-reader-patch-2.txt, conurrent-reader-patch-3.txt
> In BlockReceiver.receivePacket, it calls replicaInfo.setBytesOnDisk before calling flush().
Therefore, if there is a concurrent reader, it's possible to race here - the reader will see
the new length while those bytes are still in the buffers of BlockReceiver. Thus the client
will potentially see checksum errors or EOFs. Additionally, the last checksum chunk of the
file is made accessible to readers even though it is not stable.

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