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From "Tsz Wo (Nicholas), SZE (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3219) Disambiguate "visible length" in the code and docs
Date Tue, 23 Oct 2012 19:01:12 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3219?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13482574#comment-13482574

Tsz Wo (Nicholas), SZE commented on HDFS-3219:

Hi Yanbo,

First of all, visible length is not the same the data length in datanodes.  One nice property
of visible length is that if the visible length in one datanode is N, then all datanodes with
the block has data length >= N.  So that clients can start with any datanode and fail over
to any other datanode.

A little bit more details: in #2 of your example, we have a the write pipeline  

Client -> Datanode_1 -> Datanode_2 -> Datanode_3

Then, we have

BR_1 >= BR_2 >= BR_3 >= BA_3 >= BA_2 >= BA_1

where BR is "block received" and BA is "block acked".  Datanode_i takes BA_i as the visible
length since BR_i >= BA_j for any i and j.

For more details, please see Section 3.2 in the HDFS-265 design doc (appendDesign3.pdf).
> 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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