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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-1497) Write pipeline sequence numbers should be sequential with no skips or duplicates
Date Tue, 07 Dec 2010 18:09:09 GMT

    [ https://issues.apache.org/jira/browse/HDFS-1497?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12968853#action_12968853
] 

Todd Lipcon commented on HDFS-1497:
-----------------------------------

Unfortunately it's not quite that simple, since you have to consider the multi-threaded case.
For example, if you have two threads call hflush() at the same time, the first will set {{lastFlushedOffset}}
in its synchronized section, after sending the data into the pipeline. It then drops the lock
to wait for the ack. If a second thread then calls hflush() it _also_ needs to wait for the
same ack - it can't just return, otherwise it's acting like data has been acknowledged that
is still somewhere in the pipeline.

> Write pipeline sequence numbers should be sequential with no skips or duplicates
> --------------------------------------------------------------------------------
>
>                 Key: HDFS-1497
>                 URL: https://issues.apache.org/jira/browse/HDFS-1497
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: hdfs client
>    Affects Versions: 0.20-append, 0.22.0
>            Reporter: Todd Lipcon
>            Assignee: Todd Lipcon
>         Attachments: hdfs-1497.txt, hdfs-1497.txt, hdfs-1497.txt
>
>
> In HDFS-895 we discovered that multiple hflush() calls in a row without intervening writes
could cause a skip in sequence number. This doesn't seem to have any direct consequences,
but we should maintain and assert the invariant that sequence numbers have no gaps or duplicates.

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