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https://issues.apache.org/jira/browse/HDFS-1057?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12860074#action_12860074
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sam rash commented on HDFS-1057:
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sorry, as noted, that's an awful name: "metadata length which is the amount of
data for which we have meta data"
ie, the length of data for which we have valid crc data. data length is always
growing first so the solution here was to track the metadata length as 'visible
block length'. this doesn't cleanly solve the unstable last checksum problem
which is why we detect that change and recompute.
> 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
> Priority: Blocker
>
> 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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