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https://issues.apache.org/jira/browse/HDFS-12222?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16109829#comment-16109829
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Andrew Wang commented on HDFS-12222:
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I like Ajay's idea, since it makes it very easy for current users to migrate
their current code. One downside compared to an iterator is that it allocates
another array of BlockLocation references, but since this array is small I
think it's not a big deal.
[~drankye] could you elaborate on your question? This new API would only be
used by schedulers or other locality-aware apps.
> 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
> Labels: hdfs-ec-3.0-nice-to-have
>
> 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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