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https://issues.apache.org/jira/browse/HDFS-3219?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13248569#comment-13248569
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Tsz Wo (Nicholas), SZE commented on HDFS-3219:
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> ... It's a static property set on open, eg is not updated when a writer calls
> hflush. ...
The visible length is the length when the file is opened. It is not a dynamic
property. The design of hflush is to support new readers but the existing
readers.
> ... Also DFSInputStream#readBlockLength returns the 1st visible length of a
> replica it finds, so it seems possible (though unlikely) in a failure
> scenario it could return a length that was longer than what all replicas had.
This is not possible since the visible length returned by DN is the acked
length. All datanodes in the pipeline must have at lease that amount of data.
You may try to come up a failure scenario so that we can discuss it.
> Disambiguate "visible length" in the code and docs
> --------------------------------------------------
>
> Key: HDFS-3219
> URL: https://issues.apache.org/jira/browse/HDFS-3219
> Project: Hadoop HDFS
> Issue Type: Improvement
> Reporter: Eli Collins
> Priority: Minor
>
> HDFS-2288 there are two definition of visible length, or rather we're using
> the same name for two things:
> 1. The HDFS-265 design doc which defines it as property of the replica:
> {quote}
> visible length is the "number of bytes that have been acknowledged by the
> downstream DataNodes". It is replica (not block) specific, meaning it can be
> different for different replicas at a given time. In the document it is
> called BA (bytes acknowledged), compared to BR (bytes received).
> {quote}
> 2. The definition in HDFS-814 and DFSClient#getVisibleLength which defines it
> as a property of a file:
> {quote}
> The visible length is the length that *all* datanodes in the pipeline contain
> at least such amount of data. Therefore, these data are visible to the
> readers.
> According to this definition the visible length of a file is the floor of all
> visible lengths of all the replicas of the last block. It's a static property
> set on open, eg is not updated when a writer calls hflush. Also
> DFSInputStream#readBlockLength returns the 1st visible length of a replica it
> finds, so it seems possible (though unlikely) in a failure scenario it could
> return a length that was longer than what all replicas had.
> {quote}
> This has caused confusion in a number of other jiras. We should update the
> design doc, java doc, perhaps rename DFSClient#getVisibleLength etc to
> disambiguate this.
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