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https://issues.apache.org/jira/browse/HDFS-3219?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13482182#comment-13482182
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Yanbo Liang commented on HDFS-3219:
-----------------------------------
There is a scenario:
1) Dfs client writes to a file on HDFS, then the client exit with exception.
2) All the replicas of the last block on the DataNodes are in RBW state and may
be with different length.
We assume the last block has three replicas that the visible length is R1 < R2
< R3.
3) When we open the file for read, DFSInputStream construction with invoking
openInfo().
At this moment we connect to the DataNode with replica R2 and set the parameter
lastBlockBeingWrittenLength with R2.
4) When we read from the stream with fail over to other DataNode such as the
DataNode with replica R1,
we can not get the last data of R2-R1.
I wonder whether this scenario is make sense?
> 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.
> {quote}
> 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.
> 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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