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From "Hairong Kuang (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-1057) Concurrent readers hit ChecksumExceptions if following a writer to very end of file
Date Wed, 26 May 2010 17:04:42 GMT

    [ https://issues.apache.org/jira/browse/HDFS-1057?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12871815#action_12871815

Hairong Kuang commented on HDFS-1057:

When I thought more about it, the replica length in my previous comment should be replica's
disk length. So after a chunk is flushed to disk, a block receiver updates its disk length
(as in trunk) and should also update its last chunk crc. When a read request comes in, a block
reader waits until its requested bytes are on disk and then return all the bytes up to the
replica's disk length.

> 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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