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From "Andrew Wang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-12222) Add EC information to BlockLocation
Date Fri, 01 Sep 2017 00:23:02 GMT

    [ https://issues.apache.org/jira/browse/HDFS-12222?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16149832#comment-16149832
] 

Andrew Wang commented on HDFS-12222:
------------------------------------

Hi Huafeng, thanks for the rev, looks good. Please proceed with a full patch.

> Add EC information to BlockLocation
> -----------------------------------
>
>                 Key: HDFS-12222
>                 URL: https://issues.apache.org/jira/browse/HDFS-12222
>             Project: Hadoop HDFS
>          Issue Type: Bug
>    Affects Versions: 3.0.0-alpha1
>            Reporter: Andrew Wang
>            Assignee: Huafeng Wang
>              Labels: hdfs-ec-3.0-nice-to-have
>         Attachments: HDFS-12222.001.patch, HDFS-12222.002.patch
>
>
> HDFS applications query block location information to compute splits. One example of
this is FileInputFormat:
> https://github.com/apache/hadoop/blob/d4015f8628dd973c7433639451a9acc3e741d2a2/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapred/FileInputFormat.java#L346
> You see bits of code like this that calculate offsets as follows:
> {noformat}
>     long bytesInThisBlock = blkLocations[startIndex].getOffset() + 
>                           blkLocations[startIndex].getLength() - offset;
> {noformat}
> EC confuses this since the block locations include parity block locations as well, which
are not part of the logical file length. This messes up the offset calculation and thus topology/caching
information too.
> Applications can figure out what's a parity block by reading the EC policy and then parsing
the schema, but it'd be a lot better if we exposed this more generically in BlockLocation
instead.



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