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From "Johannes Herr (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-7941) hsync() not working
Date Mon, 06 Jun 2016 18:35:21 GMT

    [ https://issues.apache.org/jira/browse/HDFS-7941?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15316948#comment-15316948
] 

Johannes Herr commented on HDFS-7941:
-------------------------------------

The file may not appear to grow because the new length is not sent to the Namenode on hsync
or because Block-compression is used and the data is still in the compress buffer.

See http://stackoverflow.com/a/37664114/136247 for more details.

> hsync() not working
> -------------------
>
>                 Key: HDFS-7941
>                 URL: https://issues.apache.org/jira/browse/HDFS-7941
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: hdfs-client
>    Affects Versions: 2.6.0
>         Environment: HDP 2.2 running on Redhat
>            Reporter: Sverre Bakke
>
> When using SequenceFile.Writer and appending+syncing to file repeatedly, the sync does
not appear to work other than:
> - once after writing headers
> - when closing.
> Imagine the following test case:
> http://pastebin.com/Y9xysCRX
> This code would append a new record every second and then immediately sync it. One would
also imagine that the file would grow for every append, however, this does not happen.
> After watching the behavior I have noticed that it only syncs the headers at the very
beginning (providing a file of 164 bytes) and then never again until its closed. This despite
it is asked to hsync() after every append.
> Looking into the debug logs, this also claims the same behavior (executed the provided
code example and grepped for "sync"):
> SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
> SLF4J: Defaulting to no-operation (NOP) logger implementation
> SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.
> 2015-03-17 15:55:14 DEBUG ProtobufRpcEngine:253 - Call: fsync took 11ms
> This was the only time the code ran fsync throughout the entire execution.
> This has been tested (with similar result) for the following deployments:
> - sequencefile with no compression
> - sequencefile with record compression
> - sequencefile with block compression
> - textfile with no compression



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